Rust Compiler Error Index

E0001

Note: this error code is no longer emitted by the compiler.

This error suggests that the expression arm corresponding to the noted pattern will never be reached as for all possible values of the expression being matched, one of the preceding patterns will match.

This means that perhaps some of the preceding patterns are too general, this one is too specific or the ordering is incorrect.

For example, the following match block has too many arms:

match Some(0) {
    Some(bar) => {/* ... */}
    x => {/* ... */} // This handles the `None` case
    _ => {/* ... */} // All possible cases have already been handled
}Run

match blocks have their patterns matched in order, so, for example, putting a wildcard arm above a more specific arm will make the latter arm irrelevant.

Ensure the ordering of the match arm is correct and remove any superfluous arms.

E0002

Note: this error code is no longer emitted by the compiler.

This error indicates that an empty match expression is invalid because the type it is matching on is non-empty (there exist values of this type). In safe code it is impossible to create an instance of an empty type, so empty match expressions are almost never desired. This error is typically fixed by adding one or more cases to the match expression.

An example of an empty type is enum Empty { }. So, the following will work:

enum Empty {}

fn foo(x: Empty) {
    match x {
        // empty
    }
}Run

However, this won't:

This example deliberately fails to compile
fn foo(x: Option<String>) {
    match x {
        // empty
    }
}Run

E0004

This error indicates that the compiler cannot guarantee a matching pattern for one or more possible inputs to a match expression. Guaranteed matches are required in order to assign values to match expressions, or alternatively, determine the flow of execution. Erroneous code example:

This example deliberately fails to compile
enum Terminator {
    HastaLaVistaBaby,
    TalkToMyHand,
}

let x = Terminator::HastaLaVistaBaby;

match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered
    Terminator::TalkToMyHand => {}
}Run

If you encounter this error you must alter your patterns so that every possible value of the input type is matched. For types with a small number of variants (like enums) you should probably cover all cases explicitly. Alternatively, the underscore _ wildcard pattern can be added after all other patterns to match "anything else". Example:

enum Terminator {
    HastaLaVistaBaby,
    TalkToMyHand,
}

let x = Terminator::HastaLaVistaBaby;

match x {
    Terminator::TalkToMyHand => {}
    Terminator::HastaLaVistaBaby => {}
}

// or:

match x {
    Terminator::TalkToMyHand => {}
    _ => {}
}Run

E0005

Patterns used to bind names must be irrefutable, that is, they must guarantee that a name will be extracted in all cases. Erroneous code example:

This example deliberately fails to compile
let x = Some(1);
let Some(y) = x;
// error: refutable pattern in local binding: `None` not coveredRun

If you encounter this error you probably need to use a match or if let to deal with the possibility of failure. Example:

let x = Some(1);

match x {
    Some(y) => {
        // do something
    },
    None => {}
}

// or:

if let Some(y) = x {
    // do something
}Run

E0007

This error indicates that the bindings in a match arm would require a value to be moved into more than one location, thus violating unique ownership. Code like the following is invalid as it requires the entire Option<String> to be moved into a variable called op_string while simultaneously requiring the inner String to be moved into a variable called s.

This example deliberately fails to compile
let x = Some("s".to_string());

match x {
    op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
    None => {},
}Run

See also the error E0303.

E0008

Names bound in match arms retain their type in pattern guards. As such, if a name is bound by move in a pattern, it should also be moved to wherever it is referenced in the pattern guard code. Doing so however would prevent the name from being available in the body of the match arm. Consider the following:

This example deliberately fails to compile
match Some("hi".to_string()) {
    Some(s) if s.len() == 0 => {}, // use s.
    _ => {},
}Run

The variable s has type String, and its use in the guard is as a variable of type String. The guard code effectively executes in a separate scope to the body of the arm, so the value would be moved into this anonymous scope and therefore becomes unavailable in the body of the arm.

The problem above can be solved by using the ref keyword.

match Some("hi".to_string()) {
    Some(ref s) if s.len() == 0 => {},
    _ => {},
}Run

Though this example seems innocuous and easy to solve, the problem becomes clear when it encounters functions which consume the value:

This example deliberately fails to compile
struct A{}

impl A {
    fn consume(self) -> usize {
        0
    }
}

fn main() {
    let a = Some(A{});
    match a {
        Some(y) if y.consume() > 0 => {}
        _ => {}
    }
}Run

In this situation, even the ref keyword cannot solve it, since borrowed content cannot be moved. This problem cannot be solved generally. If the value can be cloned, here is a not-so-specific solution:

#[derive(Clone)]
struct A{}

impl A {
    fn consume(self) -> usize {
        0
    }
}

fn main() {
    let a = Some(A{});
    match a{
        Some(ref y) if y.clone().consume() > 0 => {}
        _ => {}
    }
}Run

If the value will be consumed in the pattern guard, using its clone will not move its ownership, so the code works.

E0009

In a pattern, all values that don't implement the Copy trait have to be bound the same way. The goal here is to avoid binding simultaneously by-move and by-ref.

This limitation may be removed in a future version of Rust.

Erroneous code example:

This example deliberately fails to compile
struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the
                            //        same pattern
    None => panic!()
}Run

You have two solutions:

Solution #1: Bind the pattern's values the same way.

struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((ref y, ref z)) => {},
    // or Some((y, z)) => {}
    None => panic!()
}Run

Solution #2: Implement the Copy trait for the X structure.

However, please keep in mind that the first solution should be preferred.

#[derive(Clone, Copy)]
struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((y, ref z)) => {},
    None => panic!()
}Run

E0010

The value of statics and constants must be known at compile time, and they live for the entire lifetime of a program. Creating a boxed value allocates memory on the heap at runtime, and therefore cannot be done at compile time. Erroneous code example:

This example deliberately fails to compile
#![feature(box_syntax)]

const CON : Box<i32> = box 0;Run

E0013

Static and const variables can refer to other const variables. But a const variable cannot refer to a static variable. For example, Y cannot refer to X here:

This example deliberately fails to compile
static X: i32 = 42;
const Y: i32 = X;Run

To fix this, the value can be extracted as a const and then used:

const A: i32 = 42;
static X: i32 = A;
const Y: i32 = A;Run

E0015

The only functions that can be called in static or constant expressions are const functions, and struct/enum constructors. const functions are only available on a nightly compiler. Rust currently does not support more general compile-time function execution.

const FOO: Option<u8> = Some(1); // enum constructor
struct Bar {x: u8}
const BAR: Bar = Bar {x: 1}; // struct constructorRun

See RFC 911 for more details on the design of const fns.

E0017

References in statics and constants may only refer to immutable values. Erroneous code example:

This example deliberately fails to compile
static X: i32 = 1;
const C: i32 = 2;

// these three are not allowed:
const CR: &'static mut i32 = &mut C;
static STATIC_REF: &'static mut i32 = &mut X;
static CONST_REF: &'static mut i32 = &mut C;Run

Statics are shared everywhere, and if they refer to mutable data one might violate memory safety since holding multiple mutable references to shared data is not allowed.

If you really want global mutable state, try using static mut or a global UnsafeCell.

E0018

The value of static and constant integers must be known at compile time. You can't cast a pointer to an integer because the address of a pointer can vary.

For example, if you write:

This example deliberately fails to compile
static MY_STATIC: u32 = 42;
static MY_STATIC_ADDR: usize = &MY_STATIC as *const _ as usize;
static WHAT: usize = (MY_STATIC_ADDR^17) + MY_STATIC_ADDR;Run

Then MY_STATIC_ADDR would contain the address of MY_STATIC. However, the address can change when the program is linked, as well as change between different executions due to ASLR, and many linkers would not be able to calculate the value of WHAT.

On the other hand, static and constant pointers can point either to a known numeric address or to the address of a symbol.

static MY_STATIC: u32 = 42;
static MY_STATIC_ADDR: &'static u32 = &MY_STATIC;
const CONST_ADDR: *const u8 = 0x5f3759df as *const u8;Run

This does not pose a problem by itself because they can't be accessed directly.

E0019

A function call isn't allowed in the const's initialization expression because the expression's value must be known at compile-time. Erroneous code example:

This example deliberately fails to compile
enum Test {
    V1
}

impl Test {
    fn test(&self) -> i32 {
        12
    }
}

fn main() {
    const FOO: Test = Test::V1;

    const A: i32 = FOO.test(); // You can't call Test::func() here!
}Run

Remember: you can't use a function call inside a const's initialization expression! However, you can totally use it anywhere else:

enum Test {
    V1
}

impl Test {
    fn func(&self) -> i32 {
        12
    }
}

fn main() {
    const FOO: Test = Test::V1;

    FOO.func(); // here is good
    let x = FOO.func(); // or even here!
}Run

E0022

Constant functions are not allowed to mutate anything. Thus, binding to an argument with a mutable pattern is not allowed. For example,

This example deliberately fails to compile
const fn foo(mut x: u8) {
    // do stuff
}Run

Is incorrect because the function body may not mutate x.

Remove any mutable bindings from the argument list to fix this error. In case you need to mutate the argument, try lazily initializing a global variable instead of using a const fn, or refactoring the code to a functional style to avoid mutation if possible.

E0023

A pattern used to match against an enum variant must provide a sub-pattern for each field of the enum variant. This error indicates that a pattern attempted to extract an incorrect number of fields from a variant.

enum Fruit {
    Apple(String, String),
    Pear(u32),
}Run

Here the Apple variant has two fields, and should be matched against like so:

enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

// Correct.
match x {
    Fruit::Apple(a, b) => {},
    _ => {}
}Run

Matching with the wrong number of fields has no sensible interpretation:

This example deliberately fails to compile
enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

// Incorrect.
match x {
    Fruit::Apple(a) => {},
    Fruit::Apple(a, b, c) => {},
}Run

Check how many fields the enum was declared with and ensure that your pattern uses the same number.

E0025

Each field of a struct can only be bound once in a pattern. Erroneous code example:

This example deliberately fails to compile
struct Foo {
    a: u8,
    b: u8,
}

fn main(){
    let x = Foo { a:1, b:2 };

    let Foo { a: x, a: y } = x;
    // error: field `a` bound multiple times in the pattern
}Run

Each occurrence of a field name binds the value of that field, so to fix this error you will have to remove or alter the duplicate uses of the field name. Perhaps you misspelled another field name? Example:

struct Foo {
    a: u8,
    b: u8,
}

fn main(){
    let x = Foo { a:1, b:2 };

    let Foo { a: x, b: y } = x; // ok!
}Run

E0026

This error indicates that a struct pattern attempted to extract a non-existent field from a struct. Struct fields are identified by the name used before the colon : so struct patterns should resemble the declaration of the struct type being matched.

// Correct matching.
struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 1, y: 2 };

match thing {
    Thing { x: xfield, y: yfield } => {}
}Run

If you are using shorthand field patterns but want to refer to the struct field by a different name, you should rename it explicitly.

Change this:

This example deliberately fails to compile
struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 0, y: 0 };

match thing {
    Thing { x, z } => {}
}Run

To this:

struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 0, y: 0 };

match thing {
    Thing { x, y: z } => {}
}Run

E0027

This error indicates that a pattern for a struct fails to specify a sub-pattern for every one of the struct's fields. Ensure that each field from the struct's definition is mentioned in the pattern, or use .. to ignore unwanted fields.

For example:

This example deliberately fails to compile
struct Dog {
    name: String,
    age: u32,
}

let d = Dog { name: "Rusty".to_string(), age: 8 };

// This is incorrect.
match d {
    Dog { age: x } => {}
}Run

This is correct (explicit):

struct Dog {
    name: String,
    age: u32,
}

let d = Dog { name: "Rusty".to_string(), age: 8 };

match d {
    Dog { name: ref n, age: x } => {}
}

// This is also correct (ignore unused fields).
match d {
    Dog { age: x, .. } => {}
}Run

E0029

In a match expression, only numbers and characters can be matched against a range. This is because the compiler checks that the range is non-empty at compile-time, and is unable to evaluate arbitrary comparison functions. If you want to capture values of an orderable type between two end-points, you can use a guard.

This example deliberately fails to compile
let string = "salutations !";

// The ordering relation for strings can't be evaluated at compile time,
// so this doesn't work:
match string {
    "hello" ..= "world" => {}
    _ => {}
}

// This is a more general version, using a guard:
match string {
    s if s >= "hello" && s <= "world" => {}
    _ => {}
}Run

E0030

When matching against a range, the compiler verifies that the range is non-empty. Range patterns include both end-points, so this is equivalent to requiring the start of the range to be less than or equal to the end of the range.

For example:

This example deliberately fails to compile
match 5u32 {
    // This range is ok, albeit pointless.
    1 ..= 1 => {}
    // This range is empty, and the compiler can tell.
    1000 ..= 5 => {}
}Run

E0033

This error indicates that a pointer to a trait type cannot be implicitly dereferenced by a pattern. Every trait defines a type, but because the size of trait implementors isn't fixed, this type has no compile-time size. Therefore, all accesses to trait types must be through pointers. If you encounter this error you should try to avoid dereferencing the pointer.

This example deliberately fails to compile
let trait_obj: &SomeTrait = &"some_value";

// This tries to implicitly dereference to create an unsized local variable.
let &invalid = trait_obj;

// You can call methods without binding to the value being pointed at.
trait_obj.method_one();
trait_obj.method_two();Run

You can read more about trait objects in the Trait Objects section of the Reference.

E0034

The compiler doesn't know what method to call because more than one method has the same prototype. Erroneous code example:

This example deliberately fails to compile
struct Test;

trait Trait1 {
    fn foo();
}

trait Trait2 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }
impl Trait2 for Test { fn foo() {} }

fn main() {
    Test::foo() // error, which foo() to call?
}Run

To avoid this error, you have to keep only one of them and remove the others. So let's take our example and fix it:

struct Test;

trait Trait1 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }

fn main() {
    Test::foo() // and now that's good!
}Run

However, a better solution would be using fully explicit naming of type and trait:

struct Test;

trait Trait1 {
    fn foo();
}

trait Trait2 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }
impl Trait2 for Test { fn foo() {} }

fn main() {
    <Test as Trait1>::foo()
}Run

One last example:

trait F {
    fn m(&self);
}

trait G {
    fn m(&self);
}

struct X;

impl F for X { fn m(&self) { println!("I am F"); } }
impl G for X { fn m(&self) { println!("I am G"); } }

fn main() {
    let f = X;

    F::m(&f); // it displays "I am F"
    G::m(&f); // it displays "I am G"
}Run

E0038

Trait objects like Box<Trait> can only be constructed when certain requirements are satisfied by the trait in question.

Trait objects are a form of dynamic dispatch and use a dynamically sized type for the inner type. So, for a given trait Trait, when Trait is treated as a type, as in Box<Trait>, the inner type is 'unsized'. In such cases the boxed pointer is a 'fat pointer' that contains an extra pointer to a table of methods (among other things) for dynamic dispatch. This design mandates some restrictions on the types of traits that are allowed to be used in trait objects, which are collectively termed as 'object safety' rules.

Attempting to create a trait object for a non object-safe trait will trigger this error.

There are various rules:

The trait cannot require Self: Sized

When Trait is treated as a type, the type does not implement the special Sized trait, because the type does not have a known size at compile time and can only be accessed behind a pointer. Thus, if we have a trait like the following:

trait Foo where Self: Sized {

}Run

We cannot create an object of type Box<Foo> or &Foo since in this case Self would not be Sized.

Generally, Self : Sized is used to indicate that the trait should not be used as a trait object. If the trait comes from your own crate, consider removing this restriction.

Method references the Self type in its arguments or return type

This happens when a trait has a method like the following:

trait Trait {
    fn foo(&self) -> Self;
}

impl Trait for String {
    fn foo(&self) -> Self {
        "hi".to_owned()
    }
}

impl Trait for u8 {
    fn foo(&self) -> Self {
        1
    }
}Run

(Note that &self and &mut self are okay, it's additional Self types which cause this problem.)

In such a case, the compiler cannot predict the return type of foo() in a situation like the following:

This example deliberately fails to compile
trait Trait {
    fn foo(&self) -> Self;
}

fn call_foo(x: Box<Trait>) {
    let y = x.foo(); // What type is y?
    // ...
}Run

If only some methods aren't object-safe, you can add a where Self: Sized bound on them to mark them as explicitly unavailable to trait objects. The functionality will still be available to all other implementers, including Box<Trait> which is itself sized (assuming you impl Trait for Box<Trait>).

trait Trait {
    fn foo(&self) -> Self where Self: Sized;
    // more functions
}Run

Now, foo() can no longer be called on a trait object, but you will now be allowed to make a trait object, and that will be able to call any object-safe methods. With such a bound, one can still call foo() on types implementing that trait that aren't behind trait objects.

Method has generic type parameters

As mentioned before, trait objects contain pointers to method tables. So, if we have:

trait Trait {
    fn foo(&self);
}

impl Trait for String {
    fn foo(&self) {
        // implementation 1
    }
}

impl Trait for u8 {
    fn foo(&self) {
        // implementation 2
    }
}
// ...Run

At compile time each implementation of Trait will produce a table containing the various methods (and other items) related to the implementation.

This works fine, but when the method gains generic parameters, we can have a problem.

Usually, generic parameters get monomorphized. For example, if I have

fn foo<T>(x: T) {
    // ...
}Run

The machine code for foo::<u8>(), foo::<bool>(), foo::<String>(), or any other type substitution is different. Hence the compiler generates the implementation on-demand. If you call foo() with a bool parameter, the compiler will only generate code for foo::<bool>(). When we have additional type parameters, the number of monomorphized implementations the compiler generates does not grow drastically, since the compiler will only generate an implementation if the function is called with unparametrized substitutions (i.e., substitutions where none of the substituted types are themselves parametrized).

However, with trait objects we have to make a table containing every object that implements the trait. Now, if it has type parameters, we need to add implementations for every type that implements the trait, and there could theoretically be an infinite number of types.

For example, with:

trait Trait {
    fn foo<T>(&self, on: T);
    // more methods
}

impl Trait for String {
    fn foo<T>(&self, on: T) {
        // implementation 1
    }
}

impl Trait for u8 {
    fn foo<T>(&self, on: T) {
        // implementation 2
    }
}

// 8 more implementationsRun

Now, if we have the following code:

This example deliberately fails to compile
fn call_foo(thing: Box<Trait>) {
    thing.foo(true); // this could be any one of the 8 types above
    thing.foo(1);
    thing.foo("hello");
}Run

We don't just need to create a table of all implementations of all methods of Trait, we need to create such a table, for each different type fed to foo(). In this case this turns out to be (10 types implementing Trait)*(3 types being fed to foo()) = 30 implementations!

With real world traits these numbers can grow drastically.

To fix this, it is suggested to use a where Self: Sized bound similar to the fix for the sub-error above if you do not intend to call the method with type parameters:

trait Trait {
    fn foo<T>(&self, on: T) where Self: Sized;
    // more methods
}Run

If this is not an option, consider replacing the type parameter with another trait object (e.g. if T: OtherTrait, use on: Box<OtherTrait>). If the number of types you intend to feed to this method is limited, consider manually listing out the methods of different types.

Method has no receiver

Methods that do not take a self parameter can't be called since there won't be a way to get a pointer to the method table for them.

trait Foo {
    fn foo() -> u8;
}Run

This could be called as <Foo as Foo>::foo(), which would not be able to pick an implementation.

Adding a Self: Sized bound to these methods will generally make this compile.

trait Foo {
    fn foo() -> u8 where Self: Sized;
}Run

The trait cannot contain associated constants

Just like static functions, associated constants aren't stored on the method table. If the trait or any subtrait contain an associated constant, they cannot be made into an object.

This example deliberately fails to compile
trait Foo {
    const X: i32;
}

impl Foo {}Run

A simple workaround is to use a helper method instead:

trait Foo {
    fn x(&self) -> i32;
}Run

The trait cannot use Self as a type parameter in the supertrait listing

This is similar to the second sub-error, but subtler. It happens in situations like the following:

This example deliberately fails to compile
trait Super<A> {}

trait Trait: Super<Self> {
}

struct Foo;

impl Super<Foo> for Foo{}

impl Trait for Foo {}Run

Here, the supertrait might have methods as follows:

trait Super<A> {
    fn get_a(&self) -> A; // note that this is object safe!
}Run

If the trait Foo was deriving from something like Super<String> or Super<T> (where Foo itself is Foo<T>), this is okay, because given a type get_a() will definitely return an object of that type.

However, if it derives from Super<Self>, even though Super is object safe, the method get_a() would return an object of unknown type when called on the function. Self type parameters let us make object safe traits no longer safe, so they are forbidden when specifying supertraits.

There's no easy fix for this, generally code will need to be refactored so that you no longer need to derive from Super<Self>.

E0040

It is not allowed to manually call destructors in Rust. It is also not necessary to do this since drop is called automatically whenever a value goes out of scope.

Here's an example of this error:

This example deliberately fails to compile
struct Foo {
    x: i32,
}

impl Drop for Foo {
    fn drop(&mut self) {
        println!("kaboom");
    }
}

fn main() {
    let mut x = Foo { x: -7 };
    x.drop(); // error: explicit use of destructor method
}Run

E0044

You can't use type parameters on foreign items. Example of erroneous code:

This example deliberately fails to compile
extern { fn some_func<T>(x: T); }Run

To fix this, replace the type parameter with the specializations that you need:

extern { fn some_func_i32(x: i32); }
extern { fn some_func_i64(x: i64); }Run

E0045

Rust only supports variadic parameters for interoperability with C code in its FFI. As such, variadic parameters can only be used with functions which are using the C ABI. Examples of erroneous code:

This example deliberately fails to compile
#![feature(unboxed_closures)]

extern "rust-call" { fn foo(x: u8, ...); }

// or

fn foo(x: u8, ...) {}Run

To fix such code, put them in an extern "C" block:

extern "C" {
    fn foo (x: u8, ...);
}Run

E0046

Items are missing in a trait implementation. Erroneous code example:

This example deliberately fails to compile
trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {}
// error: not all trait items implemented, missing: `foo`Run

When trying to make some type implement a trait Foo, you must, at minimum, provide implementations for all of Foo's required methods (meaning the methods that do not have default implementations), as well as any required trait items like associated types or constants. Example:

trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {
    fn foo() {} // ok!
}Run

E0049

This error indicates that an attempted implementation of a trait method has the wrong number of type parameters.

For example, the trait below has a method foo with a type parameter T, but the implementation of foo for the type Bar is missing this parameter:

This example deliberately fails to compile
trait Foo {
    fn foo<T: Default>(x: T) -> Self;
}

struct Bar;

// error: method `foo` has 0 type parameters but its trait declaration has 1
// type parameter
impl Foo for Bar {
    fn foo(x: bool) -> Self { Bar }
}Run

E0050

This error indicates that an attempted implementation of a trait method has the wrong number of function parameters.

For example, the trait below has a method foo with two function parameters (&self and u8), but the implementation of foo for the type Bar omits the u8 parameter:

This example deliberately fails to compile
trait Foo {
    fn foo(&self, x: u8) -> bool;
}

struct Bar;

// error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
// has 2
impl Foo for Bar {
    fn foo(&self) -> bool { true }
}Run

E0053

The parameters of any trait method must match between a trait implementation and the trait definition.

Here are a couple examples of this error:

This example deliberately fails to compile
trait Foo {
    fn foo(x: u16);
    fn bar(&self);
}

struct Bar;

impl Foo for Bar {
    // error, expected u16, found i16
    fn foo(x: i16) { }

    // error, types differ in mutability
    fn bar(&mut self) { }
}Run

E0054

It is not allowed to cast to a bool. If you are trying to cast a numeric type to a bool, you can compare it with zero instead:

This example deliberately fails to compile
let x = 5;

// Not allowed, won't compile
let x_is_nonzero = x as bool;Run
let x = 5;

// Ok
let x_is_nonzero = x != 0;Run

E0055

During a method call, a value is automatically dereferenced as many times as needed to make the value's type match the method's receiver. The catch is that the compiler will only attempt to dereference a number of times up to the recursion limit (which can be set via the recursion_limit attribute).

For a somewhat artificial example:

This example deliberately fails to compile
#![recursion_limit="2"]

struct Foo;

impl Foo {
    fn foo(&self) {}
}

fn main() {
    let foo = Foo;
    let ref_foo = &&Foo;

    // error, reached the recursion limit while auto-dereferencing &&Foo
    ref_foo.foo();
}Run

One fix may be to increase the recursion limit. Note that it is possible to create an infinite recursion of dereferencing, in which case the only fix is to somehow break the recursion.

E0057

When invoking closures or other implementations of the function traits Fn, FnMut or FnOnce using call notation, the number of parameters passed to the function must match its definition.

An example using a closure:

This example deliberately fails to compile
let f = |x| x * 3;
let a = f();        // invalid, too few parameters
let b = f(4);       // this works!
let c = f(2, 3);    // invalid, too many parametersRun

A generic function must be treated similarly:

fn foo<F: Fn()>(f: F) {
    f(); // this is valid, but f(3) would not work
}Run

E0059

The built-in function traits are generic over a tuple of the function arguments. If one uses angle-bracket notation (Fn<(T,), Output=U>) instead of parentheses (Fn(T) -> U) to denote the function trait, the type parameter should be a tuple. Otherwise function call notation cannot be used and the trait will not be implemented by closures.

The most likely source of this error is using angle-bracket notation without wrapping the function argument type into a tuple, for example:

This example deliberately fails to compile
#![feature(unboxed_closures)]

fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }Run

It can be fixed by adjusting the trait bound like this:

#![feature(unboxed_closures)]

fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }Run

Note that (T,) always denotes the type of a 1-tuple containing an element of type T. The comma is necessary for syntactic disambiguation.

E0060

External C functions are allowed to be variadic. However, a variadic function takes a minimum number of arguments. For example, consider C's variadic printf function:

use std::os::raw::{c_char, c_int};

extern "C" {
    fn printf(_: *const c_char, ...) -> c_int;
}Run

Using this declaration, it must be called with at least one argument, so simply calling printf() is invalid. But the following uses are allowed:

unsafe {
    use std::ffi::CString;

    let fmt = CString::new("test\n").unwrap();
    printf(fmt.as_ptr());

    let fmt = CString::new("number = %d\n").unwrap();
    printf(fmt.as_ptr(), 3);

    let fmt = CString::new("%d, %d\n").unwrap();
    printf(fmt.as_ptr(), 10, 5);
}Run

E0061

The number of arguments passed to a function must match the number of arguments specified in the function signature.

For example, a function like:

fn f(a: u16, b: &str) {}Run

Must always be called with exactly two arguments, e.g. f(2, "test").

Note that Rust does not have a notion of optional function arguments or variadic functions (except for its C-FFI).

E0062

This error indicates that during an attempt to build a struct or struct-like enum variant, one of the fields was specified more than once. Erroneous code example:

This example deliberately fails to compile
struct Foo {
    x: i32,
}

fn main() {
    let x = Foo {
                x: 0,
                x: 0, // error: field `x` specified more than once
            };
}Run

Each field should be specified exactly one time. Example:

struct Foo {
    x: i32,
}

fn main() {
    let x = Foo { x: 0 }; // ok!
}Run

E0063

This error indicates that during an attempt to build a struct or struct-like enum variant, one of the fields was not provided. Erroneous code example:

This example deliberately fails to compile
struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0 }; // error: missing field: `y`
}Run

Each field should be specified exactly once. Example:

struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0, y: 0 }; // ok!
}Run

E0067

The left-hand side of a compound assignment expression must be a place expression. A place expression represents a memory location and includes item paths (ie, namespaced variables), dereferences, indexing expressions, and field references.

Let's start with some erroneous code examples:

This example deliberately fails to compile
use std::collections::LinkedList;

// Bad: assignment to non-place expression
LinkedList::new() += 1;

// ...

fn some_func(i: &mut i32) {
    i += 12; // Error : '+=' operation cannot be applied on a reference !
}Run

And now some working examples:

let mut i : i32 = 0;

i += 12; // Good !

// ...

fn some_func(i: &mut i32) {
    *i += 12; // Good !
}Run

E0069

The compiler found a function whose body contains a return; statement but whose return type is not (). An example of this is:

This example deliberately fails to compile
// error
fn foo() -> u8 {
    return;
}Run

Since return; is just like return ();, there is a mismatch between the function's return type and the value being returned.

E0070

The left-hand side of an assignment operator must be a place expression. An place expression represents a memory location and can be a variable (with optional namespacing), a dereference, an indexing expression or a field reference.

More details can be found in the Expressions section of the Reference.

Now, we can go further. Here are some erroneous code examples:

This example deliberately fails to compile
struct SomeStruct {
    x: i32,
    y: i32
}

const SOME_CONST : i32 = 12;

fn some_other_func() {}

fn some_function() {
    SOME_CONST = 14; // error : a constant value cannot be changed!
    1 = 3; // error : 1 isn't a valid place!
    some_other_func() = 4; // error : we can't assign value to a function!
    SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
                       // like a variable!
}Run

And now let's give working examples:

struct SomeStruct {
    x: i32,
    y: i32
}
let mut s = SomeStruct {x: 0, y: 0};

s.x = 3; // that's good !

// ...

fn some_func(x: &mut i32) {
    *x = 12; // that's good !
}Run

E0071

You tried to use structure-literal syntax to create an item that is not a structure or enum variant.

Example of erroneous code:

This example deliberately fails to compile
type U32 = u32;
let t = U32 { value: 4 }; // error: expected struct, variant or union type,
                          // found builtin type `u32`Run

To fix this, ensure that the name was correctly spelled, and that the correct form of initializer was used.

For example, the code above can be fixed to:

enum Foo {
    FirstValue(i32)
}

fn main() {
    let u = Foo::FirstValue(0i32);

    let t = 4;
}Run

E0072

When defining a recursive struct or enum, any use of the type being defined from inside the definition must occur behind a pointer (like Box or &). This is because structs and enums must have a well-defined size, and without the pointer, the size of the type would need to be unbounded.

Consider the following erroneous definition of a type for a list of bytes:

This example deliberately fails to compile
// error, invalid recursive struct type
struct ListNode {
    head: u8,
    tail: Option<ListNode>,
}Run

This type cannot have a well-defined size, because it needs to be arbitrarily large (since we would be able to nest ListNodes to any depth). Specifically,

size of `ListNode` = 1 byte for `head`
                   + 1 byte for the discriminant of the `Option`
                   + size of `ListNode`

One way to fix this is by wrapping ListNode in a Box, like so:

struct ListNode {
    head: u8,
    tail: Option<Box<ListNode>>,
}Run

This works because Box is a pointer, so its size is well-known.

E0073

Note: this error code is no longer emitted by the compiler.

You cannot define a struct (or enum) Foo that requires an instance of Foo in order to make a new Foo value. This is because there would be no way a first instance of Foo could be made to initialize another instance!

Here's an example of a struct that has this problem:

struct Foo { x: Box<Foo> } // errorRun

One fix is to use Option, like so:

struct Foo { x: Option<Box<Foo>> }Run

Now it's possible to create at least one instance of Foo: Foo { x: None }.

E0074

Note: this error code is no longer emitted by the compiler.

When using the #[simd] attribute on a tuple struct, the components of the tuple struct must all be of a concrete, nongeneric type so the compiler can reason about how to use SIMD with them. This error will occur if the types are generic.

This will cause an error:

#![feature(repr_simd)]

#[repr(simd)]
struct Bad<T>(T, T, T);Run

This will not:

#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);Run

E0075

The #[simd] attribute can only be applied to non empty tuple structs, because it doesn't make sense to try to use SIMD operations when there are no values to operate on.

This will cause an error:

This example deliberately fails to compile
#![feature(repr_simd)]

#[repr(simd)]
struct Bad;Run

This will not:

#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32);Run

E0076

When using the #[simd] attribute to automatically use SIMD operations in tuple struct, the types in the struct must all be of the same type, or the compiler will trigger this error.

This will cause an error:

This example deliberately fails to compile
#![feature(repr_simd)]

#[repr(simd)]
struct Bad(u16, u32, u32);Run

This will not:

#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);Run

E0077

When using the #[simd] attribute on a tuple struct, the elements in the tuple must be machine types so SIMD operations can be applied to them.

This will cause an error:

This example deliberately fails to compile
#![feature(repr_simd)]

#[repr(simd)]
struct Bad(String);Run

This will not:

#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);Run

E0080

This error indicates that the compiler was unable to sensibly evaluate an constant expression that had to be evaluated. Attempting to divide by 0 or causing integer overflow are two ways to induce this error. For example:

This example deliberately fails to compile
enum Enum {
    X = (1 << 500),
    Y = (1 / 0)
}Run

Ensure that the expressions given can be evaluated as the desired integer type. See the FFI section of the Reference for more information about using a custom integer type:

https://doc.rust-lang.org/reference.html#ffi-attributes

E0081

Enum discriminants are used to differentiate enum variants stored in memory. This error indicates that the same value was used for two or more variants, making them impossible to tell apart.

This example deliberately fails to compile
// Bad.
enum Enum {
    P = 3,
    X = 3,
    Y = 5,
}Run
// Good.
enum Enum {
    P,
    X = 3,
    Y = 5,
}Run

Note that variants without a manually specified discriminant are numbered from top to bottom starting from 0, so clashes can occur with seemingly unrelated variants.

This example deliberately fails to compile
enum Bad {
    X,
    Y = 0
}Run

Here X will have already been specified the discriminant 0 by the time Y is encountered, so a conflict occurs.

E0084

An unsupported representation was attempted on a zero-variant enum.

Erroneous code example:

This example deliberately fails to compile
#[repr(i32)]
enum NightsWatch {} // error: unsupported representation for zero-variant enumRun

It is impossible to define an integer type to be used to represent zero-variant enum values because there are no zero-variant enum values. There is no way to construct an instance of the following type using only safe code. So you have two solutions. Either you add variants in your enum:

#[repr(i32)]
enum NightsWatch {
    JonSnow,
    Commander,
}Run

or you remove the integer represention of your enum:

enum NightsWatch {}Run

E0087

Too many type parameters were supplied for a function. For example:

This example deliberately fails to compile
fn foo<T>() {}

fn main() {
    foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
}Run

The number of supplied parameters must exactly match the number of defined type parameters.

E0088

You gave too many lifetime parameters. Erroneous code example:

This example deliberately fails to compile
fn f() {}

fn main() {
    f::<'static>() // error: too many lifetime parameters provided
}Run

Please check you give the right number of lifetime parameters. Example:

fn f() {}

fn main() {
    f() // ok!
}Run

It's also important to note that the Rust compiler can generally determine the lifetime by itself. Example:

struct Foo {
    value: String
}

impl Foo {
    // it can be written like this
    fn get_value<'a>(&'a self) -> &'a str { &self.value }
    // but the compiler works fine with this too:
    fn without_lifetime(&self) -> &str { &self.value }
}

fn main() {
    let f = Foo { value: "hello".to_owned() };

    println!("{}", f.get_value());
    println!("{}", f.without_lifetime());
}Run

E0089

Not enough type parameters were supplied for a function. For example:

This example deliberately fails to compile
fn foo<T, U>() {}

fn main() {
    foo::<f64>(); // error, expected 2 parameters, found 1 parameter
}Run

Note that if a function takes multiple type parameters but you want the compiler to infer some of them, you can use type placeholders:

This example deliberately fails to compile
fn foo<T, U>(x: T) {}

fn main() {
    let x: bool = true;
    foo::<f64>(x);    // error, expected 2 parameters, found 1 parameter
    foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
}Run

E0090

You gave too few lifetime parameters. Example:

This example deliberately fails to compile
fn foo<'a: 'b, 'b: 'a>() {}

fn main() {
    foo::<'static>(); // error, expected 2 lifetime parameters
}Run

Please check you give the right number of lifetime parameters. Example:

fn foo<'a: 'b, 'b: 'a>() {}

fn main() {
    foo::<'static, 'static>();
}Run

E0091

You gave an unnecessary type parameter in a type alias. Erroneous code example:

This example deliberately fails to compile
type Foo<T> = u32; // error: type parameter `T` is unused
// or:
type Foo<A,B> = Box<A>; // error: type parameter `B` is unusedRun

Please check you didn't write too many type parameters. Example:

type Foo = u32; // ok!
type Foo2<A> = Box<A>; // ok!Run

E0092

You tried to declare an undefined atomic operation function. Erroneous code example:

This example deliberately fails to compile
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_foo(); // error: unrecognized atomic operation
                     //        function
}Run

Please check you didn't make a mistake in the function's name. All intrinsic functions are defined in librustc_codegen_llvm/intrinsic.rs and in libcore/intrinsics.rs in the Rust source code. Example:

#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_fence(); // ok!
}Run

E0093

You declared an unknown intrinsic function. Erroneous code example:

This example deliberately fails to compile
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn foo(); // error: unrecognized intrinsic function: `foo`
}

fn main() {
    unsafe {
        foo();
    }
}Run

Please check you didn't make a mistake in the function's name. All intrinsic functions are defined in librustc_codegen_llvm/intrinsic.rs and in libcore/intrinsics.rs in the Rust source code. Example:

#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_fence(); // ok!
}

fn main() {
    unsafe {
        atomic_fence();
    }
}Run

E0094

You gave an invalid number of type parameters to an intrinsic function. Erroneous code example:

This example deliberately fails to compile
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
                                 //        of type parameters
}Run

Please check that you provided the right number of type parameters and verify with the function declaration in the Rust source code. Example:

#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn size_of<T>() -> usize; // ok!
}Run

E0106

This error indicates that a lifetime is missing from a type. If it is an error inside a function signature, the problem may be with failing to adhere to the lifetime elision rules (see below).

Here are some simple examples of where you'll run into this error:

This example deliberately fails to compile
struct Foo1 { x: &bool }
              // ^ expected lifetime parameter
struct Foo2<'a> { x: &'a bool } // correct

struct Bar1 { x: Foo2 }
              // ^^^^ expected lifetime parameter
struct Bar2<'a> { x: Foo2<'a> } // correct

enum Baz1 { A(u8), B(&bool), }
                  // ^ expected lifetime parameter
enum Baz2<'a> { A(u8), B(&'a bool), } // correct

type MyStr1 = &str;
           // ^ expected lifetime parameter
type MyStr2<'a> = &'a str; // correctRun

Lifetime elision is a special, limited kind of inference for lifetimes in function signatures which allows you to leave out lifetimes in certain cases. For more background on lifetime elision see the book.

The lifetime elision rules require that any function signature with an elided output lifetime must either have

In the first case, the output lifetime is inferred to be the same as the unique input lifetime. In the second case, the lifetime is instead inferred to be the same as the lifetime on &self or &mut self.

Here are some examples of elision errors:

This example deliberately fails to compile
// error, no input lifetimes
fn foo() -> &str { }

// error, `x` and `y` have distinct lifetimes inferred
fn bar(x: &str, y: &str) -> &str { }

// error, `y`'s lifetime is inferred to be distinct from `x`'s
fn baz<'a>(x: &'a str, y: &str) -> &str { }Run

Lifetime elision in implementation headers was part of the lifetime elision RFC. It is, however, currently unimplemented.

E0107

This error means that an incorrect number of lifetime parameters were provided for a type (like a struct or enum) or trait:

This example deliberately fails to compile
struct Foo<'a, 'b>(&'a str, &'b str);
enum Bar { A, B, C }

struct Baz<'a> {
    foo: Foo<'a>, // error: expected 2, found 1
    bar: Bar<'a>, // error: expected 0, found 1
}Run

E0109

You tried to give a type parameter to a type which doesn't need it. Erroneous code example:

This example deliberately fails to compile
type X = u32<i32>; // error: type parameters are not allowed on this typeRun

Please check that you used the correct type and recheck its definition. Perhaps it doesn't need the type parameter.

Example:

type X = u32; // this compilesRun

Note that type parameters for enum-variant constructors go after the variant, not after the enum (Option::None::<u32>, not Option::<u32>::None).

E0110

You tried to give a lifetime parameter to a type which doesn't need it. Erroneous code example:

This example deliberately fails to compile
type X = u32<'static>; // error: lifetime parameters are not allowed on
                       //        this typeRun

Please check that the correct type was used and recheck its definition; perhaps it doesn't need the lifetime parameter. Example:

type X = u32; // ok!Run

E0116

You can only define an inherent implementation for a type in the same crate where the type was defined. For example, an impl block as below is not allowed since Vec is defined in the standard library:

This example deliberately fails to compile
impl Vec<u8> { } // errorRun

To fix this problem, you can do either of these things:

Note that using the type keyword does not work here because type only introduces a type alias:

This example deliberately fails to compile
type Bytes = Vec<u8>;

impl Bytes { } // error, same as aboveRun

E0117

This error indicates a violation of one of Rust's orphan rules for trait implementations. The rule prohibits any implementation of a foreign trait (a trait defined in another crate) where

Here's one example of this error:

This example deliberately fails to compile
impl Drop for u32 {}Run

To avoid this kind of error, ensure that at least one local type is referenced by the impl:

pub struct Foo; // you define your type in your crate

impl Drop for Foo { // and you can implement the trait on it!
    // code of trait implementation here
}

impl From<Foo> for i32 { // or you use a type from your crate as
                         // a type parameter
    fn from(i: Foo) -> i32 {
        0
    }
}Run

Alternatively, define a trait locally and implement that instead:

trait Bar {
    fn get(&self) -> usize;
}

impl Bar for u32 {
    fn get(&self) -> usize { 0 }
}Run

For information on the design of the orphan rules, see RFC 1023.

E0118

You're trying to write an inherent implementation for something which isn't a struct nor an enum. Erroneous code example:

This example deliberately fails to compile
impl (u8, u8) { // error: no base type found for inherent implementation
    fn get_state(&self) -> String {
        // ...
    }
}Run

To fix this error, please implement a trait on the type or wrap it in a struct. Example:

// we create a trait here
trait LiveLongAndProsper {
    fn get_state(&self) -> String;
}

// and now you can implement it on (u8, u8)
impl LiveLongAndProsper for (u8, u8) {
    fn get_state(&self) -> String {
        "He's dead, Jim!".to_owned()
    }
}Run

Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct. For example, NewType is a newtype over Foo in struct NewType(Foo). Example:

struct TypeWrapper((u8, u8));

impl TypeWrapper {
    fn get_state(&self) -> String {
        "Fascinating!".to_owned()
    }
}Run

E0119

There are conflicting trait implementations for the same type. Example of erroneous code:

This example deliberately fails to compile
trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}

struct Foo {
    value: usize
}

impl MyTrait for Foo { // error: conflicting implementations of trait
                       //        `MyTrait` for type `Foo`
    fn get(&self) -> usize { self.value }
}Run

When looking for the implementation for the trait, the compiler finds both the impl<T> MyTrait for T where T is all types and the impl MyTrait for Foo. Since a trait cannot be implemented multiple times, this is an error. So, when you write:

trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}Run

This makes the trait implemented on all types in the scope. So if you try to implement it on another one after that, the implementations will conflict. Example:

trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}

struct Foo;

fn main() {
    let f = Foo;

    f.get(); // the trait is implemented so we can use it
}Run

E0120

An attempt was made to implement Drop on a trait, which is not allowed: only structs and enums can implement Drop. An example causing this error:

This example deliberately fails to compile
trait MyTrait {}

impl Drop for MyTrait {
    fn drop(&mut self) {}
}Run

A workaround for this problem is to wrap the trait up in a struct, and implement Drop on that. An example is shown below:

trait MyTrait {}
struct MyWrapper<T: MyTrait> { foo: T }

impl <T: MyTrait> Drop for MyWrapper<T> {
    fn drop(&mut self) {}
}
Run

Alternatively, wrapping trait objects requires something like the following:

trait MyTrait {}

//or Box<MyTrait>, if you wanted an owned trait object
struct MyWrapper<'a> { foo: &'a MyTrait }

impl <'a> Drop for MyWrapper<'a> {
    fn drop(&mut self) {}
}Run

E0121

In order to be consistent with Rust's lack of global type inference, type placeholders are disallowed by design in item signatures.

Examples of this error include:

This example deliberately fails to compile
fn foo() -> _ { 5 } // error, explicitly write out the return type instead

static BAR: _ = "test"; // error, explicitly write out the type insteadRun

E0124

You declared two fields of a struct with the same name. Erroneous code example:

This example deliberately fails to compile
struct Foo {
    field1: i32,
    field1: i32, // error: field is already declared
}Run

Please verify that the field names have been correctly spelled. Example:

struct Foo {
    field1: i32,
    field2: i32, // ok!
}Run

E0128

Type parameter defaults can only use parameters that occur before them. Erroneous code example:

This example deliberately fails to compile
struct Foo<T=U, U=()> {
    field1: T,
    filed2: U,
}
// error: type parameters with a default cannot use forward declared
// identifiersRun

Since type parameters are evaluated in-order, you may be able to fix this issue by doing:

struct Foo<U=(), T=U> {
    field1: T,
    filed2: U,
}Run

Please also verify that this wasn't because of a name-clash and rename the type parameter if so.

E0130

You declared a pattern as an argument in a foreign function declaration. Erroneous code example:

This example deliberately fails to compile
extern {
    fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
                                //        function declarations
}Run

Please replace the pattern argument with a regular one. Example:

struct SomeStruct {
    a: u32,
    b: u32,
}

extern {
    fn foo(s: SomeStruct); // ok!
}Run

Or:

extern {
    fn foo(a: (u32, u32)); // ok!
}Run

E0131

It is not possible to define main with generic parameters. When main is present, it must take no arguments and return (). Erroneous code example:

This example deliberately fails to compile
fn main<T>() { // error: main function is not allowed to have generic parameters
}Run

E0132

A function with the start attribute was declared with type parameters.

Erroneous code example:

This example deliberately fails to compile
#![feature(start)]

#[start]
fn f<T>() {}Run

It is not possible to declare type parameters on a function that has the start attribute. Such a function must have the following type signature (for more information: http://doc.rust-lang.org/stable/book/first-edition/no-stdlib.html):

fn(isize, *const *const u8) -> isize;Run

Example:

#![feature(start)]

#[start]
fn my_start(argc: isize, argv: *const *const u8) -> isize {
    0
}Run

E0133

Unsafe code was used outside of an unsafe function or block.

Erroneous code example:

This example deliberately fails to compile
unsafe fn f() { return; } // This is the unsafe code

fn main() {
    f(); // error: call to unsafe function requires unsafe function or block
}Run

Using unsafe functionality is potentially dangerous and disallowed by safety checks. Examples:

These safety checks can be relaxed for a section of the code by wrapping the unsafe instructions with an unsafe block. For instance:

unsafe fn f() { return; }

fn main() {
    unsafe { f(); } // ok!
}Run

See also https://doc.rust-lang.org/book/first-edition/unsafe.html

E0136

A binary can only have one entry point, and by default that entry point is the function main(). If there are multiple such functions, please rename one.

E0137

More than one function was declared with the #[main] attribute.

Erroneous code example:

This example deliberately fails to compile
#![feature(main)]

#[main]
fn foo() {}

#[main]
fn f() {} // error: multiple functions with a #[main] attributeRun

This error indicates that the compiler found multiple functions with the #[main] attribute. This is an error because there must be a unique entry point into a Rust program. Example:

#![feature(main)]

#[main]
fn f() {} // ok!Run

E0138

More than one function was declared with the #[start] attribute.

Erroneous code example:

This example deliberately fails to compile
#![feature(start)]

#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize {}

#[start]
fn f(argc: isize, argv: *const *const u8) -> isize {}
// error: multiple 'start' functionsRun

This error indicates that the compiler found multiple functions with the #[start] attribute. This is an error because there must be a unique entry point into a Rust program. Example:

#![feature(start)]

#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!Run

E0139

Note: this error code is no longer emitted by the compiler.

There are various restrictions on transmuting between types in Rust; for example types being transmuted must have the same size. To apply all these restrictions, the compiler must know the exact types that may be transmuted. When type parameters are involved, this cannot always be done.

So, for example, the following is not allowed:

use std::mem::transmute;

struct Foo<T>(Vec<T>);

fn foo<T>(x: Vec<T>) {
    // we are transmuting between Vec<T> and Foo<F> here
    let y: Foo<T> = unsafe { transmute(x) };
    // do something with y
}Run

In this specific case there's a good chance that the transmute is harmless (but this is not guaranteed by Rust). However, when alignment and enum optimizations come into the picture, it's quite likely that the sizes may or may not match with different type parameter substitutions. It's not possible to check this for all possible types, so transmute() simply only accepts types without any unsubstituted type parameters.

If you need this, there's a good chance you're doing something wrong. Keep in mind that Rust doesn't guarantee much about the layout of different structs (even two structs with identical declarations may have different layouts). If there is a solution that avoids the transmute entirely, try it instead.

If it's possible, hand-monomorphize the code by writing the function for each possible type substitution. It's possible to use traits to do this cleanly, for example:

use std::mem::transmute;

struct Foo<T>(Vec<T>);

trait MyTransmutableType: Sized {
    fn transmute(_: Vec<Self>) -> Foo<Self>;
}

impl MyTransmutableType for u8 {
    fn transmute(x: Vec<u8>) -> Foo<u8> {
        unsafe { transmute(x) }
    }
}

impl MyTransmutableType for String {
    fn transmute(x: Vec<String>) -> Foo<String> {
        unsafe { transmute(x) }
    }
}

// ... more impls for the types you intend to transmute

fn foo<T: MyTransmutableType>(x: Vec<T>) {
    let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
    // do something with y
}Run

Each impl will be checked for a size match in the transmute as usual, and since there are no unbound type parameters involved, this should compile unless there is a size mismatch in one of the impls.

It is also possible to manually transmute:

unsafe {
    ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
}Run

Note that this does not move v (unlike transmute), and may need a call to mem::forget(v) in case you want to avoid destructors being called.

E0152

A lang item was redefined.

Erroneous code example:

This example deliberately fails to compile
#![feature(lang_items)]

#[lang = "panic_impl"]
struct Foo; // error: duplicate lang item found: `panic_impl`Run

Lang items are already implemented in the standard library. Unless you are writing a free-standing application (e.g. a kernel), you do not need to provide them yourself.

You can build a free-standing crate by adding #![no_std] to the crate attributes:

This example is not tested
#![no_std]Run

See also https://doc.rust-lang.org/book/first-edition/no-stdlib.html

E0154

Note: this error code is no longer emitted by the compiler.

Imports (use statements) are not allowed after non-item statements, such as variable declarations and expression statements.

Here is an example that demonstrates the error:

fn f() {
    // Variable declaration before import
    let x = 0;
    use std::io::Read;
    // ...
}Run

The solution is to declare the imports at the top of the block, function, or file.

Here is the previous example again, with the correct order:

fn f() {
    use std::io::Read;
    let x = 0;
    // ...
}Run

See the Declaration Statements section of the reference for more information about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

E0158

const and static mean different things. A const is a compile-time constant, an alias for a literal value. This property means you can match it directly within a pattern.

The static keyword, on the other hand, guarantees a fixed location in memory. This does not always mean that the value is constant. For example, a global mutex can be declared static as well.

If you want to match against a static, consider using a guard instead:

static FORTY_TWO: i32 = 42;

match Some(42) {
    Some(x) if x == FORTY_TWO => {}
    _ => {}
}Run

E0161

A value was moved. However, its size was not known at compile time, and only values of a known size can be moved.

Erroneous code example:

This example deliberately fails to compile
#![feature(box_syntax)]

fn main() {
    let array: &[isize] = &[1, 2, 3];
    let _x: Box<[isize]> = box *array;
    // error: cannot move a value of type [isize]: the size of [isize] cannot
    //        be statically determined
}Run

In Rust, you can only move a value when its size is known at compile time.

To work around this restriction, consider "hiding" the value behind a reference: either &x or &mut x. Since a reference has a fixed size, this lets you move it around as usual. Example:

#![feature(box_syntax)]

fn main() {
    let array: &[isize] = &[1, 2, 3];
    let _x: Box<&[isize]> = box array; // ok!
}Run

E0162

An if-let pattern attempts to match the pattern, and enters the body if the match was successful. If the match is irrefutable (when it cannot fail to match), use a regular let-binding instead. For instance:

This example deliberately fails to compile
struct Irrefutable(i32);
let irr = Irrefutable(0);

// This fails to compile because the match is irrefutable.
if let Irrefutable(x) = irr {
    // This body will always be executed.
    // ...
}Run

Try this instead:

struct Irrefutable(i32);
let irr = Irrefutable(0);

let Irrefutable(x) = irr;
println!("{}", x);Run

E0164

This error means that an attempt was made to match a struct type enum variant as a non-struct type:

This example deliberately fails to compile
enum Foo { B { i: u32 } }

fn bar(foo: Foo) -> u32 {
    match foo {
        Foo::B(i) => i, // error E0164
    }
}Run

Try using {} instead:

enum Foo { B { i: u32 } }

fn bar(foo: Foo) -> u32 {
    match foo {
        Foo::B{i} => i,
    }
}Run

E0165

A while-let pattern attempts to match the pattern, and enters the body if the match was successful. If the match is irrefutable (when it cannot fail to match), use a regular let-binding inside a loop instead. For instance:

This example deliberately fails to compile
struct Irrefutable(i32);
let irr = Irrefutable(0);

// This fails to compile because the match is irrefutable.
while let Irrefutable(x) = irr {
    // ...
}Run

Try this instead:

struct Irrefutable(i32);
let irr = Irrefutable(0);

loop {
    let Irrefutable(x) = irr;
    // ...
}Run

E0170

Enum variants are qualified by default. For example, given this type:

enum Method {
    GET,
    POST,
}Run

You would match it using:

enum Method {
    GET,
    POST,
}

let m = Method::GET;

match m {
    Method::GET => {},
    Method::POST => {},
}Run

If you don't qualify the names, the code will bind new variables named "GET" and "POST" instead. This behavior is likely not what you want, so rustc warns when that happens.

Qualified names are good practice, and most code works well with them. But if you prefer them unqualified, you can import the variants into scope:

use Method::*;
enum Method { GET, POST }Run

If you want others to be able to import variants from your module directly, use pub use:

pub use Method::*;
pub enum Method { GET, POST }Run

E0178

In types, the + type operator has low precedence, so it is often necessary to use parentheses.

For example:

This example deliberately fails to compile
trait Foo {}

struct Bar<'a> {
    w: &'a Foo + Copy,   // error, use &'a (Foo + Copy)
    x: &'a Foo + 'a,     // error, use &'a (Foo + 'a)
    y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
    z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
}Run

More details can be found in RFC 438.

E0183

No description.

E0184

Explicitly implementing both Drop and Copy for a type is currently disallowed. This feature can make some sense in theory, but the current implementation is incorrect and can lead to memory unsafety (see issue #20126), so it has been disabled for now.

E0185

An associated function for a trait was defined to be static, but an implementation of the trait declared the same function to be a method (i.e. to take a self parameter).

Here's an example of this error:

This example deliberately fails to compile
trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {
    // error, method `foo` has a `&self` declaration in the impl, but not in
    // the trait
    fn foo(&self) {}
}Run

E0186

An associated function for a trait was defined to be a method (i.e. to take a self parameter), but an implementation of the trait declared the same function to be static.

Here's an example of this error:

This example deliberately fails to compile
trait Foo {
    fn foo(&self);
}

struct Bar;

impl Foo for Bar {
    // error, method `foo` has a `&self` declaration in the trait, but not in
    // the impl
    fn foo() {}
}Run

E0191

Trait objects need to have all associated types specified. Erroneous code example:

This example deliberately fails to compile
trait Trait {
    type Bar;
}

type Foo = Trait; // error: the value of the associated type `Bar` (from
                  //        the trait `Trait`) must be specifiedRun

Please verify you specified all associated types of the trait and that you used the right trait. Example:

trait Trait {
    type Bar;
}

type Foo = Trait<Bar=i32>; // ok!Run

E0192

Negative impls are only allowed for auto traits. For more information see the opt-in builtin traits RFC.

E0193

Note: this error code is no longer emitted by the compiler.

where clauses must use generic type parameters: it does not make sense to use them otherwise. An example causing this error:

trait Foo {
    fn bar(&self);
}

#[derive(Copy,Clone)]
struct Wrapper<T> {
    Wrapped: T
}

impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
    fn bar(&self) { }
}Run

This use of a where clause is strange - a more common usage would look something like the following:

trait Foo {
    fn bar(&self);
}

#[derive(Copy,Clone)]
struct Wrapper<T> {
    Wrapped: T
}
impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
    fn bar(&self) { }
}Run

Here, we're saying that the implementation exists on Wrapper only when the wrapped type T implements Clone. The where clause is important because some types will not implement Clone, and thus will not get this method.

In our erroneous example, however, we're referencing a single concrete type. Since we know for certain that Wrapper<u32> implements Clone, there's no reason to also specify it in a where clause.

E0194

A type parameter was declared which shadows an existing one. An example of this error:

This example deliberately fails to compile
trait Foo<T> {
    fn do_something(&self) -> T;
    fn do_something_else<T: Clone>(&self, bar: T);
}Run

In this example, the trait Foo and the trait method do_something_else both define a type parameter T. This is not allowed: if the method wishes to define a type parameter, it must use a different name for it.

E0195

Your method's lifetime parameters do not match the trait declaration. Erroneous code example:

This example deliberately fails to compile
trait Trait {
    fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
}

struct Foo;

impl Trait for Foo {
    fn bar<'a,'b>(x: &'a str, y: &'b str) {
    // error: lifetime parameters or bounds on method `bar`
    // do not match the trait declaration
    }
}Run

The lifetime constraint 'b for bar() implementation does not match the trait declaration. Ensure lifetime declarations match exactly in both trait declaration and implementation. Example:

trait Trait {
    fn t<'a,'b:'a>(x: &'a str, y: &'b str);
}

struct Foo;

impl Trait for Foo {
    fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
    }
}Run

E0197

Inherent implementations (one that do not implement a trait but provide methods associated with a type) are always safe because they are not implementing an unsafe trait. Removing the unsafe keyword from the inherent implementation will resolve this error.

This example deliberately fails to compile
struct Foo;

// this will cause this error
unsafe impl Foo { }
// converting it to this will fix it
impl Foo { }Run

E0198

A negative implementation is one that excludes a type from implementing a particular trait. Not being able to use a trait is always a safe operation, so negative implementations are always safe and never need to be marked as unsafe.

This example deliberately fails to compile
#![feature(optin_builtin_traits)]

struct Foo;

// unsafe is unnecessary
unsafe impl !Clone for Foo { }Run

This will compile:

This example is not tested
#![feature(optin_builtin_traits)]

struct Foo;

auto trait Enterprise {}

impl !Enterprise for Foo { }Run

Please note that negative impls are only allowed for auto traits.

E0199

Safe traits should not have unsafe implementations, therefore marking an implementation for a safe trait unsafe will cause a compiler error. Removing the unsafe marker on the trait noted in the error will resolve this problem.

This example deliberately fails to compile
struct Foo;

trait Bar { }

// this won't compile because Bar is safe
unsafe impl Bar for Foo { }
// this will compile
impl Bar for Foo { }Run

E0200

Unsafe traits must have unsafe implementations. This error occurs when an implementation for an unsafe trait isn't marked as unsafe. This may be resolved by marking the unsafe implementation as unsafe.

This example deliberately fails to compile
struct Foo;

unsafe trait Bar { }

// this won't compile because Bar is unsafe and impl isn't unsafe
impl Bar for Foo { }
// this will compile
unsafe impl Bar for Foo { }Run

E0201

It is an error to define two associated items (like methods, associated types, associated functions, etc.) with the same identifier.

For example:

This example deliberately fails to compile
struct Foo(u8);

impl Foo {
    fn bar(&self) -> bool { self.0 > 5 }
    fn bar() {} // error: duplicate associated function
}

trait Baz {
    type Quux;
    fn baz(&self) -> bool;
}

impl Baz for Foo {
    type Quux = u32;

    fn baz(&self) -> bool { true }

    // error: duplicate method
    fn baz(&self) -> bool { self.0 > 5 }

    // error: duplicate associated type
    type Quux = u32;
}Run

Note, however, that items with the same name are allowed for inherent impl blocks that don't overlap:

struct Foo<T>(T);

impl Foo<u8> {
    fn bar(&self) -> bool { self.0 > 5 }
}

impl Foo<bool> {
    fn bar(&self) -> bool { self.0 }
}Run

E0202

Inherent associated types were part of RFC 195 but are not yet implemented. See the tracking issue for the status of this implementation.

E0203

No description.

E0204

An attempt to implement the Copy trait for a struct failed because one of the fields does not implement Copy. To fix this, you must implement Copy for the mentioned field. Note that this may not be possible, as in the example of

This example deliberately fails to compile
struct Foo {
    foo : Vec<u32>,
}

impl Copy for Foo { }Run

This fails because Vec<T> does not implement Copy for any T.

Here's another example that will fail:

This example deliberately fails to compile
#[derive(Copy)]
struct Foo<'a> {
    ty: &'a mut bool,
}Run

This fails because &mut T is not Copy, even when T is Copy (this differs from the behavior for &T, which is always Copy).

E0206

You can only implement Copy for a struct or enum. Both of the following examples will fail, because neither [u8; 256] nor &'static mut Bar (mutable reference to Bar) is a struct or enum:

This example deliberately fails to compile
type Foo = [u8; 256];
impl Copy for Foo { } // error

#[derive(Copy, Clone)]
struct Bar;
impl Copy for &'static mut Bar { } // errorRun

E0207

Any type parameter or lifetime parameter of an impl must meet at least one of the following criteria:

Error example 1

Suppose we have a struct Foo and we would like to define some methods for it. The following definition leads to a compiler error:

This example deliberately fails to compile
struct Foo;

impl<T: Default> Foo {
// error: the type parameter `T` is not constrained by the impl trait, self
// type, or predicates [E0207]
    fn get(&self) -> T {
        <T as Default>::default()
    }
}Run

The problem is that the parameter T does not appear in the self type (Foo) of the impl. In this case, we can fix the error by moving the type parameter from the impl to the method get:

struct Foo;

// Move the type parameter from the impl to the method
impl Foo {
    fn get<T: Default>(&self) -> T {
        <T as Default>::default()
    }
}Run

Error example 2

As another example, suppose we have a Maker trait and want to establish a type FooMaker that makes Foos:

This example deliberately fails to compile
trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker for FooMaker {
// error: the type parameter `T` is not constrained by the impl trait, self
// type, or predicates [E0207]
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}Run

This fails to compile because T does not appear in the trait or in the implementing type.

One way to work around this is to introduce a phantom type parameter into FooMaker, like so:

use std::marker::PhantomData;

trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

// Add a type parameter to `FooMaker`
struct FooMaker<T> {
    phantom: PhantomData<T>,
}

impl<T: Default> Maker for FooMaker<T> {
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo {
            foo: <T as Default>::default(),
        }
    }
}Run

Another way is to do away with the associated type in Maker and use an input type parameter instead:

// Use a type parameter instead of an associated type here
trait Maker<Item> {
    fn make(&mut self) -> Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker<Foo<T>> for FooMaker {
    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}Run

Additional information

For more information, please see RFC 447.

E0208

No description.

E0210

This error indicates a violation of one of Rust's orphan rules for trait implementations. The rule concerns the use of type parameters in an implementation of a foreign trait (a trait defined in another crate), and states that type parameters must be "covered" by a local type. To understand what this means, it is perhaps easiest to consider a few examples.

If ForeignTrait is a trait defined in some external crate foo, then the following trait impl is an error:

This example deliberately fails to compile
extern crate foo;
use foo::ForeignTrait;

impl<T> ForeignTrait for T { } // errorRun

To work around this, it can be covered with a local type, MyType:

struct MyType<T>(T);
impl<T> ForeignTrait for MyType<T> { } // OkRun

Please note that a type alias is not sufficient.

For another example of an error, suppose there's another trait defined in foo named ForeignTrait2 that takes two type parameters. Then this impl results in the same rule violation:

This example is not tested
struct MyType2;
impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // errorRun

The reason for this is that there are two appearances of type parameter T in the impl header, both as parameters for ForeignTrait2. The first appearance is uncovered, and so runs afoul of the orphan rule.

Consider one more example:

This example is not tested
impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // OkRun

This only differs from the previous impl in that the parameters T and MyType<T> for ForeignTrait2 have been swapped. This example does not violate the orphan rule; it is permitted.

To see why that last example was allowed, you need to understand the general rule. Unfortunately this rule is a bit tricky to state. Consider an impl:

This example is not tested
impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }Run

where P1, ..., Pm are the type parameters of the impl and T0, ..., Tn are types. One of the types T0, ..., Tn must be a local type (this is another orphan rule, see the explanation for E0117). Let i be the smallest integer such that Ti is a local type. Then no type parameter can appear in any of the Tj for j < i.

For information on the design of the orphan rules, see RFC 1023.

E0212

No description.

E0214

A generic type was described using parentheses rather than angle brackets. For example:

This example deliberately fails to compile
fn main() {
    let v: Vec(&str) = vec!["foo"];
}Run

This is not currently supported: v should be defined as Vec<&str>. Parentheses are currently only used with generic types when defining parameters for Fn-family traits.

E0220

You used an associated type which isn't defined in the trait. Erroneous code example:

This example deliberately fails to compile
trait T1 {
    type Bar;
}

type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`

// or:

trait T2 {
    type Bar;

    // error: Baz is used but not declared
    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
}Run

Make sure that you have defined the associated type in the trait body. Also, verify that you used the right trait or you didn't misspell the associated type name. Example:

trait T1 {
    type Bar;
}

type Foo = T1<Bar=i32>; // ok!

// or:

trait T2 {
    type Bar;
    type Baz; // we declare `Baz` in our trait.

    // and now we can use it here:
    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
}Run

E0221

An attempt was made to retrieve an associated type, but the type was ambiguous. For example:

This example deliberately fails to compile
trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: Self::A;
    }
}Run

In this example, Foo defines an associated type A. Bar inherits that type from Foo, and defines another associated type of the same name. As a result, when we attempt to use Self::A, it's ambiguous whether we mean the A defined by Foo or the one defined by Bar.

There are two options to work around this issue. The first is simply to rename one of the types. Alternatively, one can specify the intended type using the following syntax:

trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: <Self as Bar>::A;
    }
}Run

E0223

An attempt was made to retrieve an associated type, but the type was ambiguous. For example:

This example deliberately fails to compile
trait MyTrait {type X; }

fn main() {
    let foo: MyTrait::X;
}Run

The problem here is that we're attempting to take the type of X from MyTrait. Unfortunately, the type of X is not defined, because it's only made concrete in implementations of the trait. A working version of this code might look like:

trait MyTrait {type X; }
struct MyStruct;

impl MyTrait for MyStruct {
    type X = u32;
}

fn main() {
    let foo: <MyStruct as MyTrait>::X;
}Run

This syntax specifies that we want the X type from MyTrait, as made concrete in MyStruct. The reason that we cannot simply use MyStruct::X is that MyStruct might implement two different traits with identically-named associated types. This syntax allows disambiguation between the two.

E0224

No description.

E0225

You attempted to use multiple types as bounds for a closure or trait object. Rust does not currently support this. A simple example that causes this error:

This example deliberately fails to compile
fn main() {
    let _: Box<dyn std::io::Read + std::io::Write>;
}Run

Auto traits such as Send and Sync are an exception to this rule: It's possible to have bounds of one non-builtin trait, plus any number of auto traits. For example, the following compiles correctly:

fn main() {
    let _: Box<dyn std::io::Read + Send + Sync>;
}Run

E0226

No description.

E0227

No description.

E0228

No description.

E0229

An associated type binding was done outside of the type parameter declaration and where clause. Erroneous code example:

This example deliberately fails to compile
pub trait Foo {
    type A;
    fn boo(&self) -> <Self as Foo>::A;
}

struct Bar;

impl Foo for isize {
    type A = usize;
    fn boo(&self) -> usize { 42 }
}

fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
// error: associated type bindings are not allowed hereRun

To solve this error, please move the type bindings in the type parameter declaration:

fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!Run

Or in the where clause:

fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}Run

E0230

The #[rustc_on_unimplemented] attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

This example deliberately fails to compile
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`Run

There will be an error about bool not implementing Index<u8>, followed by a note saying "the type bool cannot be indexed by u8".

As you can see, you can specify type parameters in curly braces for substitution with the actual types (using the regular format string syntax) in a given situation. Furthermore, {Self} will substitute to the type (in this case, bool) that we tried to use.

This error appears when the curly braces contain an identifier which doesn't match with any of the type parameters or the string Self. This might happen if you misspelled a type parameter, or if you intended to use literal curly braces. If it is the latter, escape the curly braces with a second curly brace of the same type; e.g. a literal { is {{.

E0231

The #[rustc_on_unimplemented] attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

This example deliberately fails to compile
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`Run

there will be an error about bool not implementing Index<u8>, followed by a note saying "the type bool cannot be indexed by u8".

As you can see, you can specify type parameters in curly braces for substitution with the actual types (using the regular format string syntax) in a given situation. Furthermore, {Self} will substitute to the type (in this case, bool) that we tried to use.

This error appears when the curly braces do not contain an identifier. Please add one of the same name as a type parameter. If you intended to use literal braces, use {{ and }} to escape them.

E0232

The #[rustc_on_unimplemented] attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

This example deliberately fails to compile
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`Run

there will be an error about bool not implementing Index<u8>, followed by a note saying "the type bool cannot be indexed by u8".

For this to work, some note must be specified. An empty attribute will not do anything, please remove the attribute or add some helpful note for users of the trait.

E0243

This error indicates that not enough type parameters were found in a type or trait.

For example, the Foo struct below is defined to be generic in T, but the type parameter is missing in the definition of Bar:

This example deliberately fails to compile
struct Foo<T> { x: T }

struct Bar { x: Foo }Run

E0244

This error indicates that too many type parameters were found in a type or trait.

For example, the Foo struct below has no type parameters, but is supplied with two in the definition of Bar:

This example deliberately fails to compile
struct Foo { x: bool }

struct Bar<S, T> { x: Foo<S, T> }Run

E0251

Note: this error code is no longer emitted by the compiler.

Two items of the same name cannot be imported without rebinding one of the items under a new local name.

An example of this error:

use foo::baz;
use bar::*; // error, do `use foo::baz as quux` instead on the previous line

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}Run

E0252

Two items of the same name cannot be imported without rebinding one of the items under a new local name.

Erroneous code example:

This example deliberately fails to compile
use foo::baz;
use bar::baz; // error, do `use bar::baz as quux` instead

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}Run

You can use aliases in order to fix this error. Example:

use foo::baz as foo_baz;
use bar::baz; // ok!

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}Run

Or you can reference the item with its parent:

use bar::baz;

fn main() {
    let x = foo::baz; // ok!
}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}Run

E0253

Attempt was made to import an unimportable value. This can happen when trying to import a method from a trait.

Erroneous code example:

This example deliberately fails to compile
mod foo {
    pub trait MyTrait {
        fn do_something();
    }
}

use foo::MyTrait::do_something;
// error: `do_something` is not directly importable

fn main() {}Run

It's invalid to directly import methods belonging to a trait or concrete type.

E0254

Attempt was made to import an item whereas an extern crate with this name has already been imported.

Erroneous code example:

This example deliberately fails to compile
extern crate core;

mod foo {
    pub trait core {
        fn do_something();
    }
}

use foo::core;  // error: an extern crate named `core` has already
                //        been imported in this module

fn main() {}Run

To fix issue issue, you have to rename at least one of the two imports. Example:

extern crate core as libcore; // ok!

mod foo {
    pub trait core {
        fn do_something();
    }
}

use foo::core;

fn main() {}Run

E0255

You can't import a value whose name is the same as another value defined in the module.

Erroneous code example:

This example deliberately fails to compile
use bar::foo; // error: an item named `foo` is already in scope

fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {}Run

You can use aliases in order to fix this error. Example:

use bar::foo as bar_foo; // ok!

fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {}Run

Or you can reference the item with its parent:

fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {
    bar::foo(); // we get the item by referring to its parent
}Run

E0256

Note: this error code is no longer emitted by the compiler.

You can't import a type or module when the name of the item being imported is the same as another type or submodule defined in the module.

An example of this error:

This example deliberately fails to compile
use foo::Bar; // error

type Bar = u32;

mod foo {
    pub mod Bar { }
}

fn main() {}Run

E0259

The name chosen for an external crate conflicts with another external crate that has been imported into the current module.

Erroneous code example:

This example deliberately fails to compile
extern crate core;
extern crate libc as core;

fn main() {}Run

The solution is to choose a different name that doesn't conflict with any external crate imported into the current module.

Correct example:

extern crate core;
extern crate libc as other_name;

fn main() {}Run

E0260

The name for an item declaration conflicts with an external crate's name.

Erroneous code example:

This example deliberately fails to compile
extern crate core;

struct core;

fn main() {}Run

There are two possible solutions:

Solution #1: Rename the item.

extern crate core;

struct xyz;Run

Solution #2: Import the crate with a different name.

extern crate core as xyz;

struct abc;Run

See the Declaration Statements section of the reference for more information about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

E0261

When using a lifetime like 'a in a type, it must be declared before being used.

These two examples illustrate the problem:

This example deliberately fails to compile
// error, use of undeclared lifetime name `'a`
fn foo(x: &'a str) { }

struct Foo {
    // error, use of undeclared lifetime name `'a`
    x: &'a str,
}Run

These can be fixed by declaring lifetime parameters:

fn foo<'a>(x: &'a str) {}

struct Foo<'a> {
    x: &'a str,
}Run

E0262

Declaring certain lifetime names in parameters is disallowed. For example, because the 'static lifetime is a special built-in lifetime name denoting the lifetime of the entire program, this is an error:

This example deliberately fails to compile
// error, invalid lifetime parameter name `'static`
fn foo<'static>(x: &'static str) { }Run

E0263

A lifetime name cannot be declared more than once in the same scope. For example:

This example deliberately fails to compile
// error, lifetime name `'a` declared twice in the same scope
fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }Run

E0264

An unknown external lang item was used. Erroneous code example:

This example deliberately fails to compile
#![feature(lang_items)]

extern "C" {
    #[lang = "cake"] // error: unknown external lang item: `cake`
    fn cake();
}Run

A list of available external lang items is available in src/librustc/middle/weak_lang_items.rs. Example:

#![feature(lang_items)]

extern "C" {
    #[lang = "panic_impl"] // ok!
    fn cake();
}Run

E0267

This error indicates the use of a loop keyword (break or continue) inside a closure but outside of any loop. Erroneous code example:

This example deliberately fails to compile
let w = || { break; }; // error: `break` inside of a closureRun

break and continue keywords can be used as normal inside closures as long as they are also contained within a loop. To halt the execution of a closure you should instead use a return statement. Example:

let w = || {
    for _ in 0..10 {
        break;
    }
};

w();Run

E0268

This error indicates the use of a loop keyword (break or continue) outside of a loop. Without a loop to break out of or continue in, no sensible action can be taken. Erroneous code example:

This example deliberately fails to compile
fn some_func() {
    break; // error: `break` outside of loop
}Run

Please verify that you are using break and continue only in loops. Example:

fn some_func() {
    for _ in 0..10 {
        break; // ok!
    }
}Run

E0271

This is because of a type mismatch between the associated type of some trait (e.g. T::Bar, where T implements trait Quux { type Bar; }) and another type U that is required to be equal to T::Bar, but is not. Examples follow.

Here is a basic example:

This example deliberately fails to compile
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }

foo(3_i8);Run

Here is that same example again, with some explanatory comments:

This example deliberately fails to compile
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
//                    ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
//                        |            |
//         This says `foo` can         |
//           only be used with         |
//              some type that         |
//         implements `Trait`.         |
//                                     |
//                             This says not only must
//                             `T` be an impl of `Trait`
//                             but also that the impl
//                             must assign the type `u32`
//                             to the associated type.
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }
//~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//      |                             |
// `i8` does have                     |
// implementation                     |
// of `Trait`...                      |
//                     ... but it is an implementation
//                     that assigns `&'static str` to
//                     the associated type.

foo(3_i8);
// Here, we invoke `foo` with an `i8`, which does not satisfy
// the constraint `<i8 as Trait>::AssociatedType=u32`, and
// therefore the type-checker complains with this error code.Run

To avoid those issues, you have to make the types match correctly. So we can fix the previous examples like this:

// Basic Example:
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }

foo(3_i8);

// For-Loop Example:
let vs = vec![1, 2, 3, 4];
for v in &vs {
    match v {
        &1 => {}
        _ => {}
    }
}Run

E0275

This error occurs when there was a recursive trait requirement that overflowed before it could be evaluated. Often this means that there is unbounded recursion in resolving some type bounds.

For example, in the following code:

This example deliberately fails to compile
trait Foo {}

struct Bar<T>(T);

impl<T> Foo for T where Bar<T>: Foo {}Run

To determine if a T is Foo, we need to check if Bar<T> is Foo. However, to do this check, we need to determine that Bar<Bar<T>> is Foo. To determine this, we check if Bar<Bar<Bar<T>>> is Foo, and so on. This is clearly a recursive requirement that can't be resolved directly.

Consider changing your trait bounds so that they're less self-referential.

E0276

This error occurs when a bound in an implementation of a trait does not match the bounds specified in the original trait. For example:

This example deliberately fails to compile
trait Foo {
    fn foo<T>(x: T);
}

impl Foo for bool {
    fn foo<T>(x: T) where T: Copy {}
}Run

Here, all types implementing Foo must have a method foo<T>(x: T) which can take any type T. However, in the impl for bool, we have added an extra bound that T is Copy, which isn't compatible with the original trait.

Consider removing the bound from the method or adding the bound to the original method definition in the trait.

E0277

You tried to use a type which doesn't implement some trait in a place which expected that trait. Erroneous code example:

This example deliberately fails to compile
// here we declare the Foo trait with a bar method
trait Foo {
    fn bar(&self);
}

// we now declare a function which takes an object implementing the Foo trait
fn some_func<T: Foo>(foo: T) {
    foo.bar();
}

fn main() {
    // we now call the method with the i32 type, which doesn't implement
    // the Foo trait
    some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
}Run

In order to fix this error, verify that the type you're using does implement the trait. Example:

trait Foo {
    fn bar(&self);
}

fn some_func<T: Foo>(foo: T) {
    foo.bar(); // we can now use this method since i32 implements the
               // Foo trait
}

// we implement the trait on the i32 type
impl Foo for i32 {
    fn bar(&self) {}
}

fn main() {
    some_func(5i32); // ok!
}Run

Or in a generic context, an erroneous code example would look like:

This example deliberately fails to compile
fn some_func<T>(foo: T) {
    println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
                           //        implemented for the type `T`
}

fn main() {
    // We now call the method with the i32 type,
    // which *does* implement the Debug trait.
    some_func(5i32);
}Run

Note that the error here is in the definition of the generic function: Although we only call it with a parameter that does implement Debug, the compiler still rejects the function: It must work with all possible input types. In order to make this example compile, we need to restrict the generic type we're accepting:

use std::fmt;

// Restrict the input type to types that implement Debug.
fn some_func<T: fmt::Debug>(foo: T) {
    println!("{:?}", foo);
}

fn main() {
    // Calling the method is still fine, as i32 implements Debug.
    some_func(5i32);

    // This would fail to compile now:
    // struct WithoutDebug;
    // some_func(WithoutDebug);
}Run

Rust only looks at the signature of the called function, as such it must already specify all requirements that will be used for every type parameter.

E0278

No description.

E0279

No description.

E0280

No description.

E0281

Note: this error code is no longer emitted by the compiler.

You tried to supply a type which doesn't implement some trait in a location which expected that trait. This error typically occurs when working with Fn-based types. Erroneous code example:

fn foo<F: Fn(usize)>(x: F) { }

fn main() {
    // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
    // but the trait `core::ops::Fn<(usize,)>` is required
    // [E0281]
    foo(|y: String| { });
}

The issue in this case is that foo is defined as accepting a Fn with one argument of type String, but the closure we attempted to pass to it requires one arguments of type usize.

E0282

This error indicates that type inference did not result in one unique possible type, and extra information is required. In most cases this can be provided by adding a type annotation. Sometimes you need to specify a generic type parameter manually.

A common example is the collect method on Iterator. It has a generic type parameter with a FromIterator bound, which for a char iterator is implemented by Vec and String among others. Consider the following snippet that reverses the characters of a string:

This example deliberately fails to compile
let x = "hello".chars().rev().collect();Run

In this case, the compiler cannot infer what the type of x should be: Vec<char> and String are both suitable candidates. To specify which type to use, you can use a type annotation on x:

let x: Vec<char> = "hello".chars().rev().collect();Run

It is not necessary to annotate the full type. Once the ambiguity is resolved, the compiler can infer the rest:

let x: Vec<_> = "hello".chars().rev().collect();Run

Another way to provide the compiler with enough information, is to specify the generic type parameter:

let x = "hello".chars().rev().collect::<Vec<char>>();Run

Again, you need not specify the full type if the compiler can infer it:

let x = "hello".chars().rev().collect::<Vec<_>>();Run

Apart from a method or function with a generic type parameter, this error can occur when a type parameter of a struct or trait cannot be inferred. In that case it is not always possible to use a type annotation, because all candidates have the same return type. For instance:

This example deliberately fails to compile
struct Foo<T> {
    num: T,
}

impl<T> Foo<T> {
    fn bar() -> i32 {
        0
    }

    fn baz() {
        let number = Foo::bar();
    }
}Run

This will fail because the compiler does not know which instance of Foo to call bar on. Change Foo::bar() to Foo::<T>::bar() to resolve the error.

E0283

This error occurs when the compiler doesn't have enough information to unambiguously choose an implementation.

For example:

This example deliberately fails to compile
trait Generator {
    fn create() -> u32;
}

struct Impl;

impl Generator for Impl {
    fn create() -> u32 { 1 }
}

struct AnotherImpl;

impl Generator for AnotherImpl {
    fn create() -> u32 { 2 }
}

fn main() {
    let cont: u32 = Generator::create();
    // error, impossible to choose one of Generator trait implementation
    // Impl or AnotherImpl? Maybe anything else?
}Run

To resolve this error use the concrete type:

trait Generator {
    fn create() -> u32;
}

struct AnotherImpl;

impl Generator for AnotherImpl {
    fn create() -> u32 { 2 }
}

fn main() {
    let gen1 = AnotherImpl::create();

    // if there are multiple methods with same name (different traits)
    let gen2 = <AnotherImpl as Generator>::create();
}Run

E0284

No description.

E0296

This error indicates that the given recursion limit could not be parsed. Ensure that the value provided is a positive integer between quotes.

Erroneous code example:

This example deliberately fails to compile
#![recursion_limit]

fn main() {}Run

And a working example:

#![recursion_limit="1000"]

fn main() {}Run

E0297

Note: this error code is no longer emitted by the compiler.

Patterns used to bind names must be irrefutable. That is, they must guarantee that a name will be extracted in all cases. Instead of pattern matching the loop variable, consider using a match or if let inside the loop body. For instance:

This example deliberately fails to compile
let xs : Vec<Option<i32>> = vec![Some(1), None];

// This fails because `None` is not covered.
for Some(x) in xs {
    // ...
}Run

Match inside the loop instead:

let xs : Vec<Option<i32>> = vec![Some(1), None];

for item in xs {
    match item {
        Some(x) => {},
        None => {},
    }
}Run

Or use if let:

let xs : Vec<Option<i32>> = vec![Some(1), None];

for item in xs {
    if let Some(x) = item {
        // ...
    }
}Run

E0301

Mutable borrows are not allowed in pattern guards, because matching cannot have side effects. Side effects could alter the matched object or the environment on which the match depends in such a way, that the match would not be exhaustive. For instance, the following would not match any arm if mutable borrows were allowed:

This example deliberately fails to compile
match Some(()) {
    None => { },
    option if option.take().is_none() => {
        /* impossible, option is `Some` */
    },
    Some(_) => { } // When the previous match failed, the option became `None`.
}Run

E0302

Assignments are not allowed in pattern guards, because matching cannot have side effects. Side effects could alter the matched object or the environment on which the match depends in such a way, that the match would not be exhaustive. For instance, the following would not match any arm if assignments were allowed:

This example deliberately fails to compile
match Some(()) {
    None => { },
    option if { option = None; false } => { },
    Some(_) => { } // When the previous match failed, the option became `None`.
}Run

E0303

In certain cases it is possible for sub-bindings to violate memory safety. Updates to the borrow checker in a future version of Rust may remove this restriction, but for now patterns must be rewritten without sub-bindings.

Before:

This example deliberately fails to compile
match Some("hi".to_string()) {
    ref op_string_ref @ Some(s) => {},
    None => {},
}Run

After:

match Some("hi".to_string()) {
    Some(ref s) => {
        let op_string_ref = &Some(s);
        // ...
    },
    None => {},
}Run

The op_string_ref binding has type &Option<&String> in both cases.

See also https://github.com/rust-lang/rust/issues/14587

E0307

No description.

E0308

This error occurs when the compiler was unable to infer the concrete type of a variable. It can occur for several cases, the most common of which is a mismatch in the expected type that the compiler inferred for a variable's initializing expression, and the actual type explicitly assigned to the variable.

For example:

This example deliberately fails to compile
let x: i32 = "I am not a number!";
//     ~~~   ~~~~~~~~~~~~~~~~~~~~
//      |             |
//      |    initializing expression;
//      |    compiler infers type `&str`
//      |
//    type `i32` assigned to variable `x`Run

E0309

Types in type definitions have lifetimes associated with them that represent how long the data stored within them is guaranteed to be live. This lifetime must be as long as the data needs to be alive, and missing the constraint that denotes this will cause this error.

This example deliberately fails to compile
// This won't compile because T is not constrained, meaning the data
// stored in it is not guaranteed to last as long as the reference
struct Foo<'a, T> {
    foo: &'a T
}Run

This will compile, because it has the constraint on the type parameter:

struct Foo<'a, T: 'a> {
    foo: &'a T
}Run

To see why this is important, consider the case where T is itself a reference (e.g., T = &str). If we don't include the restriction that T: 'a, the following code would be perfectly legal:

This example deliberately fails to compile
struct Foo<'a, T> {
    foo: &'a T
}

fn main() {
    let v = "42".to_string();
    let f = Foo{foo: &v};
    drop(v);
    println!("{}", f.foo); // but we've already dropped v!
}Run

E0310

Types in type definitions have lifetimes associated with them that represent how long the data stored within them is guaranteed to be live. This lifetime must be as long as the data needs to be alive, and missing the constraint that denotes this will cause this error.

This example deliberately fails to compile
// This won't compile because T is not constrained to the static lifetime
// the reference needs
struct Foo<T> {
    foo: &'static T
}Run

This will compile, because it has the constraint on the type parameter:

struct Foo<T: 'static> {
    foo: &'static T
}Run

E0311

No description.

E0312

No description.

E0313

No description.

E0314

No description.

E0315

No description.

E0316

No description.

E0317

This error occurs when an if expression without an else block is used in a context where a type other than () is expected, for example a let expression:

This example deliberately fails to compile
fn main() {
    let x = 5;
    let a = if x == 5 { 1 };
}Run

An if expression without an else block has the type (), so this is a type error. To resolve it, add an else block having the same type as the if block.

E0320

No description.

E0321

A cross-crate opt-out trait was implemented on something which wasn't a struct or enum type. Erroneous code example:

This example deliberately fails to compile
#![feature(optin_builtin_traits)]

struct Foo;

impl !Sync for Foo {}

unsafe impl Send for &'static Foo {}
// error: cross-crate traits with a default impl, like `core::marker::Send`,
//        can only be implemented for a struct/enum type, not
//        `&'static Foo`Run

Only structs and enums are permitted to impl Send, Sync, and other opt-out trait, and the struct or enum must be local to the current crate. So, for example, unsafe impl Send for Rc<Foo> is not allowed.

E0322

The Sized trait is a special trait built-in to the compiler for types with a constant size known at compile-time. This trait is automatically implemented for types as needed by the compiler, and it is currently disallowed to explicitly implement it for a type.

E0323

An associated const was implemented when another trait item was expected. Erroneous code example:

This example deliberately fails to compile
trait Foo {
    type N;
}

struct Bar;

impl Foo for Bar {
    const N : u32 = 0;
    // error: item `N` is an associated const, which doesn't match its
    //        trait `<Bar as Foo>`
}Run

Please verify that the associated const wasn't misspelled and the correct trait was implemented. Example:

struct Bar;

trait Foo {
    type N;
}

impl Foo for Bar {
    type N = u32; // ok!
}Run

Or:

struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    const N : u32 = 0; // ok!
}Run

E0324

A method was implemented when another trait item was expected. Erroneous code example:

This example deliberately fails to compile
struct Bar;

trait Foo {
    const N : u32;

    fn M();
}

impl Foo for Bar {
    fn N() {}
    // error: item `N` is an associated method, which doesn't match its
    //        trait `<Bar as Foo>`
}Run

To fix this error, please verify that the method name wasn't misspelled and verify that you are indeed implementing the correct trait items. Example:

struct Bar;

trait Foo {
    const N : u32;

    fn M();
}

impl Foo for Bar {
    const N : u32 = 0;

    fn M() {} // ok!
}Run

E0325

An associated type was implemented when another trait item was expected. Erroneous code example:

This example deliberately fails to compile
struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    type N = u32;
    // error: item `N` is an associated type, which doesn't match its
    //        trait `<Bar as Foo>`
}Run

Please verify that the associated type name wasn't misspelled and your implementation corresponds to the trait definition. Example:

struct Bar;

trait Foo {
    type N;
}

impl Foo for Bar {
    type N = u32; // ok!
}Run

Or:

struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    const N : u32 = 0; // ok!
}Run

E0326

The types of any associated constants in a trait implementation must match the types in the trait definition. This error indicates that there was a mismatch.

Here's an example of this error:

This example deliberately fails to compile
trait Foo {
    const BAR: bool;
}

struct Bar;

impl Foo for Bar {
    const BAR: u32 = 5; // error, expected bool, found u32
}Run

E0328

The Unsize trait should not be implemented directly. All implementations of Unsize are provided automatically by the compiler.

Erroneous code example:

This example deliberately fails to compile
#![feature(unsize)]

use std::marker::Unsize;

pub struct MyType;

impl<T> Unsize<T> for MyType {}Run

If you are defining your own smart pointer type and would like to enable conversion from a sized to an unsized type with the DST coercion system, use CoerceUnsized instead.

#![feature(coerce_unsized)]

use std::ops::CoerceUnsized;

pub struct MyType<T: ?Sized> {
    field_with_unsized_type: T,
}

impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
    where T: CoerceUnsized<U> {}Run

E0364

Private items cannot be publicly re-exported. This error indicates that you attempted to pub use a type or value that was not itself public.

Erroneous code example:

This example deliberately fails to compile
mod foo {
    const X: u32 = 1;
}

pub use foo::X;

fn main() {}Run

The solution to this problem is to ensure that the items that you are re-exporting are themselves marked with pub:

mod foo {
    pub const X: u32 = 1;
}

pub use foo::X;

fn main() {}Run

See the 'Use Declarations' section of the reference for more information on this topic:

https://doc.rust-lang.org/reference.html#use-declarations

E0365

Private modules cannot be publicly re-exported. This error indicates that you attempted to pub use a module that was not itself public.

Erroneous code example:

This example deliberately fails to compile
mod foo {
    pub const X: u32 = 1;
}

pub use foo as foo2;

fn main() {}Run

The solution to this problem is to ensure that the module that you are re-exporting is itself marked with pub:

pub mod foo {
    pub const X: u32 = 1;
}

pub use foo as foo2;

fn main() {}Run

See the 'Use Declarations' section of the reference for more information on this topic:

https://doc.rust-lang.org/reference.html#use-declarations

E0366

An attempt was made to implement Drop on a concrete specialization of a generic type. An example is shown below:

This example deliberately fails to compile
struct Foo<T> {
    t: T
}

impl Drop for Foo<u32> {
    fn drop(&mut self) {}
}Run

This code is not legal: it is not possible to specialize Drop to a subset of implementations of a generic type. One workaround for this is to wrap the generic type, as shown below:

struct Foo<T> {
    t: T
}

struct Bar {
    t: Foo<u32>
}

impl Drop for Bar {
    fn drop(&mut self) {}
}Run

E0367

An attempt was made to implement Drop on a specialization of a generic type. An example is shown below:

This example deliberately fails to compile
trait Foo{}

struct MyStruct<T> {
    t: T
}

impl<T: Foo> Drop for MyStruct<T> {
    fn drop(&mut self) {}
}Run

This code is not legal: it is not possible to specialize Drop to a subset of implementations of a generic type. In order for this code to work, MyStruct must also require that T implements Foo. Alternatively, another option is to wrap the generic type in another that specializes appropriately:

trait Foo{}

struct MyStruct<T> {
    t: T
}

struct MyStructWrapper<T: Foo> {
    t: MyStruct<T>
}

impl <T: Foo> Drop for MyStructWrapper<T> {
    fn drop(&mut self) {}
}Run

E0368

This error indicates that a binary assignment operator like += or ^= was applied to a type that doesn't support it. For example:

This example deliberately fails to compile
let mut x = 12f32; // error: binary operation `<<` cannot be applied to
                   //        type `f32`

x <<= 2;Run

To fix this error, please check that this type implements this binary operation. Example:

let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait

x <<= 2; // ok!Run

It is also possible to overload most operators for your own type by implementing the [OP]Assign traits from std::ops.

Another problem you might be facing is this: suppose you've overloaded the + operator for some type Foo by implementing the std::ops::Add trait for Foo, but you find that using += does not work, as in this example:

This example deliberately fails to compile
use std::ops::Add;

struct Foo(u32);

impl Add for Foo {
    type Output = Foo;

    fn add(self, rhs: Foo) -> Foo {
        Foo(self.0 + rhs.0)
    }
}

fn main() {
    let mut x: Foo = Foo(5);
    x += Foo(7); // error, `+= cannot be applied to the type `Foo`
}Run

This is because AddAssign is not automatically implemented, so you need to manually implement it for your type.

E0369

A binary operation was attempted on a type which doesn't support it. Erroneous code example:

This example deliberately fails to compile
let x = 12f32; // error: binary operation `<<` cannot be applied to
               //        type `f32`

x << 2;Run

To fix this error, please check that this type implements this binary operation. Example:

let x = 12u32; // the `u32` type does implement it:
               // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html

x << 2; // ok!Run

It is also possible to overload most operators for your own type by implementing traits from std::ops.

String concatenation appends the string on the right to the string on the left and may require reallocation. This requires ownership of the string on the left. If something should be added to a string literal, move the literal to the heap by allocating it with to_owned() like in "Your text".to_owned().

E0370

The maximum value of an enum was reached, so it cannot be automatically set in the next enum value. Erroneous code example:

This example deliberately fails to compile
#[deny(overflowing_literals)]
enum Foo {
    X = 0x7fffffffffffffff,
    Y, // error: enum discriminant overflowed on value after
       //        9223372036854775807: i64; set explicitly via
       //        Y = -9223372036854775808 if that is desired outcome
}Run

To fix this, please set manually the next enum value or put the enum variant with the maximum value at the end of the enum. Examples:

enum Foo {
    X = 0x7fffffffffffffff,
    Y = 0, // ok!
}Run

Or:

enum Foo {
    Y = 0, // ok!
    X = 0x7fffffffffffffff,
}Run

E0371

When Trait2 is a subtrait of Trait1 (for example, when Trait2 has a definition like trait Trait2: Trait1 { ... }), it is not allowed to implement Trait1 for Trait2. This is because Trait2 already implements Trait1 by definition, so it is not useful to do this.

Example:

This example deliberately fails to compile
trait Foo { fn foo(&self) { } }
trait Bar: Foo { }
trait Baz: Bar { }

impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
impl Baz for Bar { } // Note: This is OKRun

E0373

This error occurs when an attempt is made to use data captured by a closure, when that data may no longer exist. It's most commonly seen when attempting to return a closure:

This example deliberately fails to compile
fn foo() -> Box<Fn(u32) -> u32> {
    let x = 0u32;
    Box::new(|y| x + y)
}Run

Notice that x is stack-allocated by foo(). By default, Rust captures closed-over data by reference. This means that once foo() returns, x no longer exists. An attempt to access x within the closure would thus be unsafe.

Another situation where this might be encountered is when spawning threads:

This example deliberately fails to compile
fn foo() {
    let x = 0u32;
    let y = 1u32;

    let thr = std::thread::spawn(|| {
        x + y
    });
}Run

Since our new thread runs in parallel, the stack frame containing x and y may well have disappeared by the time we try to use them. Even if we call thr.join() within foo (which blocks until thr has completed, ensuring the stack frame won't disappear), we will not succeed: the compiler cannot prove that this behaviour is safe, and so won't let us do it.

The solution to this problem is usually to switch to using a move closure. This approach moves (or copies, where possible) data into the closure, rather than taking references to it. For example:

fn foo() -> Box<Fn(u32) -> u32> {
    let x = 0u32;
    Box::new(move |y| x + y)
}Run

Now that the closure has its own copy of the data, there's no need to worry about safety.

E0374

A struct without a field containing an unsized type cannot implement CoerceUnsized. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

This example deliberately fails to compile
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: i32,
}

// error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
    where T: CoerceUnsized<U> {}Run

CoerceUnsized is used to coerce one struct containing an unsized type into another struct containing a different unsized type. If the struct doesn't have any fields of unsized types then you don't need explicit coercion to get the types you want. To fix this you can either not try to implement CoerceUnsized or you can add a field that is unsized to the struct.

Example:

#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

// We don't need to impl `CoerceUnsized` here.
struct Foo {
    a: i32,
}

// We add the unsized type field to the struct.
struct Bar<T: ?Sized> {
    a: i32,
    b: T,
}

// The struct has an unsized field so we can implement
// `CoerceUnsized` for it.
impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
    where T: CoerceUnsized<U> {}Run

Note that CoerceUnsized is mainly used by smart pointers like Box, Rc and Arc to be able to mark that they can coerce unsized types that they are pointing at.

E0375

A struct with more than one field containing an unsized type cannot implement CoerceUnsized. This only occurs when you are trying to coerce one of the types in your struct to another type in the struct. In this case we try to impl CoerceUnsized from T to U which are both types that the struct takes. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

This example deliberately fails to compile
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized, U: ?Sized> {
    a: i32,
    b: T,
    c: U,
}

// error: Struct `Foo` has more than one unsized field.
impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}Run

CoerceUnsized only allows for coercion from a structure with a single unsized type field to another struct with a single unsized type field. In fact Rust only allows for a struct to have one unsized type in a struct and that unsized type must be the last field in the struct. So having two unsized types in a single struct is not allowed by the compiler. To fix this use only one field containing an unsized type in the struct and then use multiple structs to manage each unsized type field you need.

Example:

#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: i32,
    b: T,
}

impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
    where T: CoerceUnsized<U> {}

fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
    Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
}Run

E0376

The type you are trying to impl CoerceUnsized for is not a struct. CoerceUnsized can only be implemented for a struct. Unsized types are already able to be coerced without an implementation of CoerceUnsized whereas a struct containing an unsized type needs to know the unsized type field it's containing is able to be coerced. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

This example deliberately fails to compile
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

// error: The type `U` is not a struct
impl<T, U> CoerceUnsized<U> for Foo<T> {}Run

The CoerceUnsized trait takes a struct type. Make sure the type you are providing to CoerceUnsized is a struct with only the last field containing an unsized type.

Example:

#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T> {
    a: T,
}

// The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}Run

Note that in Rust, structs can only contain an unsized type if the field containing the unsized type is the last and only unsized type field in the struct.

E0377

No description.

E0379

Trait methods cannot be declared const by design. For more information, see RFC 911.

E0380

Auto traits cannot have methods or associated items. For more information see the opt-in builtin traits RFC.

E0381

It is not allowed to use or capture an uninitialized variable. For example:

This example deliberately fails to compile
fn main() {
    let x: i32;
    let y = x; // error, use of possibly uninitialized variable
}Run

To fix this, ensure that any declared variables are initialized before being used. Example:

fn main() {
    let x: i32 = 0;
    let y = x; // ok!
}Run

E0382

This error occurs when an attempt is made to use a variable after its contents have been moved elsewhere. For example:

This example deliberately fails to compile
struct MyStruct { s: u32 }

fn main() {
    let mut x = MyStruct{ s: 5u32 };
    let y = x;
    x.s = 6;
    println!("{}", x.s);
}Run

Since MyStruct is a type that is not marked Copy, the data gets moved out of x when we set y. This is fundamental to Rust's ownership system: outside of workarounds like Rc, a value cannot be owned by more than one variable.

Sometimes we don't need to move the value. Using a reference, we can let another function borrow the value without changing its ownership. In the example below, we don't actually have to move our string to calculate_length, we can give it a reference to it with & instead.

fn main() {
    let s1 = String::from("hello");

    let len = calculate_length(&s1);

    println!("The length of '{}' is {}.", s1, len);
}

fn calculate_length(s: &String) -> usize {
    s.len()
}Run

A mutable reference can be created with &mut.

Sometimes we don't want a reference, but a duplicate. All types marked Clone can be duplicated by calling .clone(). Subsequent changes to a clone do not affect the original variable.

Most types in the standard library are marked Clone. The example below demonstrates using clone() on a string. s1 is first set to "many", and then copied to s2. Then the first character of s1 is removed, without affecting s2. "any many" is printed to the console.

fn main() {
    let mut s1 = String::from("many");
    let s2 = s1.clone();
    s1.remove(0);
    println!("{} {}", s1, s2);
}Run

If we control the definition of a type, we can implement Clone on it ourselves with #[derive(Clone)].

Some types have no ownership semantics at all and are trivial to duplicate. An example is i32 and the other number types. We don't have to call .clone() to clone them, because they are marked Copy in addition to Clone. Implicit cloning is more convenient in this case. We can mark our own types Copy if all their members also are marked Copy.

In the example below, we implement a Point type. Because it only stores two integers, we opt-out of ownership semantics with Copy. Then we can let p2 = p1 without p1 being moved.

#[derive(Copy, Clone)]
struct Point { x: i32, y: i32 }

fn main() {
    let mut p1 = Point{ x: -1, y: 2 };
    let p2 = p1;
    p1.x = 1;
    println!("p1: {}, {}", p1.x, p1.y);
    println!("p2: {}, {}", p2.x, p2.y);
}Run

Alternatively, if we don't control the struct's definition, or mutable shared ownership is truly required, we can use Rc and RefCell:

use std::cell::RefCell;
use std::rc::Rc;

struct MyStruct { s: u32 }

fn main() {
    let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 }));
    let y = x.clone();
    x.borrow_mut().s = 6;
    println!("{}", x.borrow().s);
}Run

With this approach, x and y share ownership of the data via the Rc (reference count type). RefCell essentially performs runtime borrow checking: ensuring that at most one writer or multiple readers can access the data at any one time.

If you wish to learn more about ownership in Rust, start with the chapter in the Book:

https://doc.rust-lang.org/book/first-edition/ownership.html

E0383

This error occurs when an attempt is made to partially reinitialize a structure that is currently uninitialized.

For example, this can happen when a drop has taken place:

This example deliberately fails to compile
struct Foo {
    a: u32,
}
impl Drop for Foo {
    fn drop(&mut self) { /* ... */ }
}

let mut x = Foo { a: 1 };
drop(x); // `x` is now uninitialized
x.a = 2; // error, partial reinitialization of uninitialized structure `t`Run

This error can be fixed by fully reinitializing the structure in question:

struct Foo {
    a: u32,
}
impl Drop for Foo {
    fn drop(&mut self) { /* ... */ }
}

let mut x = Foo { a: 1 };
drop(x);
x = Foo { a: 2 };Run

E0384

This error occurs when an attempt is made to reassign an immutable variable. For example:

This example deliberately fails to compile
fn main() {
    let x = 3;
    x = 5; // error, reassignment of immutable variable
}Run

By default, variables in Rust are immutable. To fix this error, add the keyword mut after the keyword let when declaring the variable. For example:

fn main() {
    let mut x = 3;
    x = 5;
}Run

E0387

This error occurs when an attempt is made to mutate or mutably reference data that a closure has captured immutably. Examples of this error are shown below:

This example deliberately fails to compile
// Accepts a function or a closure that captures its environment immutably.
// Closures passed to foo will not be able to mutate their closed-over state.
fn foo<F: Fn()>(f: F) { }

// Attempts to mutate closed-over data. Error message reads:
// `cannot assign to data in a captured outer variable...`
fn mutable() {
    let mut x = 0u32;
    foo(|| x = 2);
}

// Attempts to take a mutable reference to closed-over data.  Error message
// reads: `cannot borrow data mutably in a captured outer variable...`
fn mut_addr() {
    let mut x = 0u32;
    foo(|| { let y = &mut x; });
}Run

The problem here is that foo is defined as accepting a parameter of type Fn. Closures passed into foo will thus be inferred to be of type Fn, meaning that they capture their context immutably.

If the definition of foo is under your control, the simplest solution is to capture the data mutably. This can be done by defining foo to take FnMut rather than Fn:

fn foo<F: FnMut()>(f: F) { }Run

Alternatively, we can consider using the Cell and RefCell types to achieve interior mutability through a shared reference. Our example's mutable function could be redefined as below:

use std::cell::Cell;

fn foo<F: Fn()>(f: F) { }

fn mutable() {
    let x = Cell::new(0u32);
    foo(|| x.set(2));
}Run

You can read more about cell types in the API documentation:

https://doc.rust-lang.org/std/cell/

E0388

E0388 was removed and is no longer issued.

E0389

An attempt was made to mutate data using a non-mutable reference. This commonly occurs when attempting to assign to a non-mutable reference of a mutable reference (&(&mut T)).

Example of erroneous code:

This example deliberately fails to compile
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };
    let fancy_ref = &(&mut fancy);
    fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
    println!("{}", fancy_ref.num);
}Run

Here, &mut fancy is mutable, but &(&mut fancy) is not. Creating an immutable reference to a value borrows it immutably. There can be multiple references of type &(&mut T) that point to the same value, so they must be immutable to prevent multiple mutable references to the same value.

To fix this, either remove the outer reference:

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };

    let fancy_ref = &mut fancy;
    // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)

    fancy_ref.num = 6; // No error!

    println!("{}", fancy_ref.num);
}Run

Or make the outer reference mutable:

struct FancyNum {
    num: u8
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };

    let fancy_ref = &mut (&mut fancy);
    // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)

    fancy_ref.num = 6; // No error!

    println!("{}", fancy_ref.num);
}Run

E0390

You tried to implement methods for a primitive type. Erroneous code example:

This example deliberately fails to compile
struct Foo {
    x: i32
}

impl *mut Foo {}
// error: only a single inherent implementation marked with
//        `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitiveRun

This isn't allowed, but using a trait to implement a method is a good solution. Example:

struct Foo {
    x: i32
}

trait Bar {
    fn bar();
}

impl Bar for *mut Foo {
    fn bar() {} // ok!
}Run

E0391

This error indicates that some types or traits depend on each other and therefore cannot be constructed.

The following example contains a circular dependency between two traits:

This example deliberately fails to compile
trait FirstTrait : SecondTrait {

}

trait SecondTrait : FirstTrait {

}Run

E0392

This error indicates that a type or lifetime parameter has been declared but not actually used. Here is an example that demonstrates the error:

This example deliberately fails to compile
enum Foo<T> {
    Bar,
}Run

If the type parameter was included by mistake, this error can be fixed by simply removing the type parameter, as shown below:

enum Foo {
    Bar,
}Run

Alternatively, if the type parameter was intentionally inserted, it must be used. A simple fix is shown below:

enum Foo<T> {
    Bar(T),
}Run

This error may also commonly be found when working with unsafe code. For example, when using raw pointers one may wish to specify the lifetime for which the pointed-at data is valid. An initial attempt (below) causes this error:

This example deliberately fails to compile
struct Foo<'a, T> {
    x: *const T,
}Run

We want to express the constraint that Foo should not outlive 'a, because the data pointed to by T is only valid for that lifetime. The problem is that there are no actual uses of 'a. It's possible to work around this by adding a PhantomData type to the struct, using it to tell the compiler to act as if the struct contained a borrowed reference &'a T:

use std::marker::PhantomData;

struct Foo<'a, T: 'a> {
    x: *const T,
    phantom: PhantomData<&'a T>
}Run

PhantomData can also be used to express information about unused type parameters.

E0393

A type parameter which references Self in its default value was not specified. Example of erroneous code:

This example deliberately fails to compile
trait A<T=Self> {}

fn together_we_will_rule_the_galaxy(son: &A) {}
// error: the type parameter `T` must be explicitly specified in an
//        object type because its default value `Self` references the
//        type `Self`Run

A trait object is defined over a single, fully-defined trait. With a regular default parameter, this parameter can just be substituted in. However, if the default parameter is Self, the trait changes for each concrete type; i.e. i32 will be expected to implement A<i32>, bool will be expected to implement A<bool>, etc... These types will not share an implementation of a fully-defined trait; instead they share implementations of a trait with different parameters substituted in for each implementation. This is irreconcilable with what we need to make a trait object work, and is thus disallowed. Making the trait concrete by explicitly specifying the value of the defaulted parameter will fix this issue. Fixed example:

trait A<T=Self> {}

fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!Run

E0395

The value assigned to a constant scalar must be known at compile time, which is not the case when comparing raw pointers.

Erroneous code example:

This example deliberately fails to compile
static FOO: i32 = 42;
static BAR: i32 = 42;

static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
// error: raw pointers cannot be compared in statics!Run

The address assigned by the linker to FOO and BAR may or may not be identical, so the value of BAZ can't be determined.

If you want to do the comparison, please do it at run-time.

For example:

static FOO: i32 = 42;
static BAR: i32 = 42;

let baz: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
// baz isn't a constant expression so it's okRun

E0396

The value behind a raw pointer can't be determined at compile-time (or even link-time), which means it can't be used in a constant expression. Erroneous code example:

This example deliberately fails to compile
const REG_ADDR: *const u8 = 0x5f3759df as *const u8;

const VALUE: u8 = unsafe { *REG_ADDR };
// error: raw pointers cannot be dereferenced in constantsRun

A possible fix is to dereference your pointer at some point in run-time.

For example:

const REG_ADDR: *const u8 = 0x5f3759df as *const u8;

let reg_value = unsafe { *REG_ADDR };Run

E0398

Note: this error code is no longer emitted by the compiler.

In Rust 1.3, the default object lifetime bounds are expected to change, as described in RFC 1156. You are getting a warning because the compiler thinks it is possible that this change will cause a compilation error in your code. It is possible, though unlikely, that this is a false alarm.

The heart of the change is that where &'a Box<SomeTrait> used to default to &'a Box<SomeTrait+'a>, it now defaults to &'a Box<SomeTrait+'static> (here, SomeTrait is the name of some trait type). Note that the only types which are affected are references to boxes, like &Box<SomeTrait> or &[Box<SomeTrait>]. More common types like &SomeTrait or Box<SomeTrait> are unaffected.

To silence this warning, edit your code to use an explicit bound. Most of the time, this means that you will want to change the signature of a function that you are calling. For example, if the error is reported on a call like foo(x), and foo is defined as follows:

fn foo(arg: &Box<SomeTrait>) { /* ... */ }Run

You might change it to:

fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }Run

This explicitly states that you expect the trait object SomeTrait to contain references (with a maximum lifetime of 'a).

E0399

You implemented a trait, overriding one or more of its associated types but did not reimplement its default methods.

Example of erroneous code:

This example deliberately fails to compile
#![feature(associated_type_defaults)]

pub trait Foo {
    type Assoc = u8;
    fn bar(&self) {}
}

impl Foo for i32 {
    // error - the following trait items need to be reimplemented as
    //         `Assoc` was overridden: `bar`
    type Assoc = i32;
}Run

To fix this, add an implementation for each default method from the trait:

#![feature(associated_type_defaults)]

pub trait Foo {
    type Assoc = u8;
    fn bar(&self) {}
}

impl Foo for i32 {
    type Assoc = i32;
    fn bar(&self) {} // ok!
}Run

E0401

Inner items do not inherit type parameters from the functions they are embedded in.

Erroneous code example:

This example deliberately fails to compile
fn foo<T>(x: T) {
    fn bar(y: T) { // T is defined in the "outer" function
        // ..
    }
    bar(x);
}Run

Nor will this:

This example deliberately fails to compile
fn foo<T>(x: T) {
    type MaybeT = Option<T>;
    // ...
}Run

Or this:

This example deliberately fails to compile
fn foo<T>(x: T) {
    struct Foo {
        x: T,
    }
    // ...
}Run

Items inside functions are basically just like top-level items, except that they can only be used from the function they are in.

There are a couple of solutions for this.

If the item is a function, you may use a closure:

fn foo<T>(x: T) {
    let bar = |y: T| { // explicit type annotation may not be necessary
        // ..
    };
    bar(x);
}Run

For a generic item, you can copy over the parameters:

fn foo<T>(x: T) {
    fn bar<T>(y: T) {
        // ..
    }
    bar(x);
}Run
fn foo<T>(x: T) {
    type MaybeT<T> = Option<T>;
}Run

Be sure to copy over any bounds as well:

fn foo<T: Copy>(x: T) {
    fn bar<T: Copy>(y: T) {
        // ..
    }
    bar(x);
}Run
fn foo<T: Copy>(x: T) {
    struct Foo<T: Copy> {
        x: T,
    }
}Run

This may require additional type hints in the function body.

In case the item is a function inside an impl, defining a private helper function might be easier:

impl<T> Foo<T> {
    pub fn foo(&self, x: T) {
        self.bar(x);
    }

    fn bar(&self, y: T) {
        // ..
    }
}Run

For default impls in traits, the private helper solution won't work, however closures or copying the parameters should still work.

E0403

Some type parameters have the same name.

Erroneous code example:

This example deliberately fails to compile
fn foo<T, T>(s: T, u: T) {} // error: the name `T` is already used for a type
                            //        parameter in this type parameter listRun

Please verify that none of the type parameters are misspelled, and rename any clashing parameters. Example:

fn foo<T, Y>(s: T, u: Y) {} // ok!Run

E0404

You tried to use something which is not a trait in a trait position, such as a bound or impl.

Erroneous code example:

This example deliberately fails to compile
struct Foo;
struct Bar;

impl Foo for Bar {} // error: `Foo` is not a traitRun

Another erroneous code example:

This example deliberately fails to compile
struct Foo;

fn bar<T: Foo>(t: T) {} // error: `Foo` is not a traitRun

Please verify that you didn't misspell the trait's name or otherwise use the wrong identifier. Example:

trait Foo {
    // some functions
}
struct Bar;

impl Foo for Bar { // ok!
    // functions implementation
}Run

or

trait Foo {
    // some functions
}

fn bar<T: Foo>(t: T) {} // ok!Run

E0405

The code refers to a trait that is not in scope.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

impl SomeTrait for Foo {} // error: trait `SomeTrait` is not in scopeRun

Please verify that the name of the trait wasn't misspelled and ensure that it was imported. Example:

// solution 1:
use some_file::SomeTrait;

// solution 2:
trait SomeTrait {
    // some functions
}

struct Foo;

impl SomeTrait for Foo { // ok!
    // implements functions
}Run

E0407

A definition of a method not in the implemented trait was given in a trait implementation.

Erroneous code example:

This example deliberately fails to compile
trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // error: method `b` is not a member of trait `Foo`
}Run

Please verify you didn't misspell the method name and you used the correct trait. First example:

trait Foo {
    fn a();
    fn b();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // ok!
}Run

Second example:

trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
}

impl Bar {
    fn b() {}
}Run

E0408

An "or" pattern was used where the variable bindings are not consistently bound across patterns.

Erroneous code example:

This example deliberately fails to compile
match x {
    Some(y) | None => { /* use y */ } // error: variable `y` from pattern #1 is
                                      //        not bound in pattern #2
    _ => ()
}Run

Here, y is bound to the contents of the Some and can be used within the block corresponding to the match arm. However, in case x is None, we have not specified what y is, and the block will use a nonexistent variable.

To fix this error, either split into multiple match arms:

let x = Some(1);
match x {
    Some(y) => { /* use y */ }
    None => { /* ... */ }
}Run

or, bind the variable to a field of the same type in all sub-patterns of the or pattern:

let x = (0, 2);
match x {
    (0, y) | (y, 0) => { /* use y */}
    _ => {}
}Run

In this example, if x matches the pattern (0, _), the second field is set to y. If it matches (_, 0), the first field is set to y; so in all cases y is set to some value.

E0409

An "or" pattern was used where the variable bindings are not consistently bound across patterns.

Erroneous code example:

This example deliberately fails to compile
let x = (0, 2);
match x {
    (0, ref y) | (y, 0) => { /* use y */} // error: variable `y` is bound with
                                          //        different mode in pattern #2
                                          //        than in pattern #1
    _ => ()
}Run

Here, y is bound by-value in one case and by-reference in the other.

To fix this error, just use the same mode in both cases. Generally using ref or ref mut where not already used will fix this:

let x = (0, 2);
match x {
    (0, ref y) | (ref y, 0) => { /* use y */}
    _ => ()
}Run

Alternatively, split the pattern:

let x = (0, 2);
match x {
    (y, 0) => { /* use y */ }
    (0, ref y) => { /* use y */}
    _ => ()
}Run

E0411

The Self keyword was used outside an impl or a trait.

Erroneous code example:

This example deliberately fails to compile
<Self>::foo; // error: use of `Self` outside of an impl or traitRun

The Self keyword represents the current type, which explains why it can only be used inside an impl or a trait. It gives access to the associated items of a type:

trait Foo {
    type Bar;
}

trait Baz : Foo {
    fn bar() -> Self::Bar; // like this
}Run

However, be careful when two types have a common associated type:

This example deliberately fails to compile
trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> Self::Bar;
    // error: ambiguous associated type `Bar` in bounds of `Self`
}Run

This problem can be solved by specifying from which trait we want to use the Bar type:

trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> <Self as Foo>::Bar; // ok!
}Run

E0412

The type name used is not in scope.

Erroneous code examples:

This example deliberately fails to compile
impl Something {} // error: type name `Something` is not in scope

// or:

trait Foo {
    fn bar(N); // error: type name `N` is not in scope
}

// or:

fn foo(x: T) {} // type name `T` is not in scopeRun

To fix this error, please verify you didn't misspell the type name, you did declare it or imported it into the scope. Examples:

struct Something;

impl Something {} // ok!

// or:

trait Foo {
    type N;

    fn bar(_: Self::N); // ok!
}

// or:

fn foo<T>(x: T) {} // ok!Run

Another case that causes this error is when a type is imported into a parent module. To fix this, you can follow the suggestion and use File directly or use super::File; which will import the types from the parent namespace. An example that causes this error is below:

This example deliberately fails to compile
use std::fs::File;

mod foo {
    fn some_function(f: File) {}
}Run
use std::fs::File;

mod foo {
    // either
    use super::File;
    // or
    // use std::fs::File;
    fn foo(f: File) {}
}Run

E0415

More than one function parameter have the same name.

Erroneous code example:

This example deliberately fails to compile
fn foo(f: i32, f: i32) {} // error: identifier `f` is bound more than
                          //        once in this parameter listRun

Please verify you didn't misspell parameters' name. Example:

fn foo(f: i32, g: i32) {} // ok!Run

E0416

An identifier is bound more than once in a pattern.

Erroneous code example:

This example deliberately fails to compile
match (1, 2) {
    (x, x) => {} // error: identifier `x` is bound more than once in the
                 //        same pattern
}Run

Please verify you didn't misspell identifiers' name. Example:

match (1, 2) {
    (x, y) => {} // ok!
}Run

Or maybe did you mean to unify? Consider using a guard:

match (A, B, C) {
    (x, x2, see) if x == x2 => { /* A and B are equal, do one thing */ }
    (y, z, see) => { /* A and B unequal; do another thing */ }
}Run

E0422

You are trying to use an identifier that is either undefined or not a struct. Erroneous code example:

This example deliberately fails to compile
fn main () {
    let x = Foo { x: 1, y: 2 };
}Run

In this case, Foo is undefined, so it inherently isn't anything, and definitely not a struct.

This example deliberately fails to compile
fn main () {
    let foo = 1;
    let x = foo { x: 1, y: 2 };
}Run

In this case, foo is defined, but is not a struct, so Rust can't use it as one.

E0423

An identifier was used like a function name or a value was expected and the identifier exists but it belongs to a different namespace.

For (an erroneous) example, here a struct variant name were used as a function:

This example deliberately fails to compile
struct Foo { a: bool };

let f = Foo();
// error: expected function, found `Foo`
// `Foo` is a struct name, but this expression uses it like a function nameRun

Please verify you didn't misspell the name of what you actually wanted to use here. Example:

fn Foo() -> u32 { 0 }

let f = Foo(); // ok!Run

It is common to forget the trailing ! on macro invocations, which would also yield this error:

This example deliberately fails to compile
println("");
// error: expected function, found macro `println`
// did you mean `println!(...)`? (notice the trailing `!`)Run

Another case where this error is emitted is when a value is expected, but something else is found:

This example deliberately fails to compile
pub mod a {
    pub const I: i32 = 1;
}

fn h1() -> i32 {
    a.I
    //~^ ERROR expected value, found module `a`
    // did you mean `a::I`?
}Run

E0424

The self keyword was used in a static method.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

impl Foo {
    fn bar(self) {}

    fn foo() {
        self.bar(); // error: `self` is not available in a static method.
    }
}Run

Please check if the method's argument list should have contained self, &self, or &mut self (in case you didn't want to create a static method), and add it if so. Example:

struct Foo;

impl Foo {
    fn bar(self) {}

    fn foo(self) {
        self.bar(); // ok!
    }
}Run

E0425

An unresolved name was used.

Erroneous code examples:

This example deliberately fails to compile
something_that_doesnt_exist::foo;
// error: unresolved name `something_that_doesnt_exist::foo`

// or:

trait Foo {
    fn bar() {
        Self; // error: unresolved name `Self`
    }
}

// or:

let x = unknown_variable;  // error: unresolved name `unknown_variable`Run

Please verify that the name wasn't misspelled and ensure that the identifier being referred to is valid for the given situation. Example:

enum something_that_does_exist {
    Foo,
}Run

Or:

mod something_that_does_exist {
    pub static foo : i32 = 0i32;
}

something_that_does_exist::foo; // ok!Run

Or:

let unknown_variable = 12u32;
let x = unknown_variable; // ok!Run

If the item is not defined in the current module, it must be imported using a use statement, like so:

use foo::bar;
bar();Run

If the item you are importing is not defined in some super-module of the current module, then it must also be declared as public (e.g., pub fn).

E0426

An undeclared label was used.

Erroneous code example:

This example deliberately fails to compile
loop {
    break 'a; // error: use of undeclared label `'a`
}Run

Please verify you spelt or declare the label correctly. Example:

'a: loop {
    break 'a; // ok!
}Run

E0428

A type or module has been defined more than once.

Erroneous code example:

This example deliberately fails to compile
struct Bar;
struct Bar; // error: duplicate definition of value `Bar`Run

Please verify you didn't misspell the type/module's name or remove/rename the duplicated one. Example:

struct Bar;
struct Bar2; // ok!Run

E0429

The self keyword cannot appear alone as the last segment in a use declaration.

Erroneous code example:

This example deliberately fails to compile
use std::fmt::self; // error: `self` imports are only allowed within a { } listRun

To use a namespace itself in addition to some of its members, self may appear as part of a brace-enclosed list of imports:

use std::fmt::{self, Debug};Run

If you only want to import the namespace, do so directly:

use std::fmt;Run

E0430

The self import appears more than once in the list.

Erroneous code example:

This example deliberately fails to compile
use something::{self, self}; // error: `self` import can only appear once in
                             //        the listRun

Please verify you didn't misspell the import name or remove the duplicated self import. Example:

use something::{self}; // ok!Run

E0431

An invalid self import was made.

Erroneous code example:

This example deliberately fails to compile
use {self}; // error: `self` import can only appear in an import list with a
            //        non-empty prefixRun

You cannot import the current module into itself, please remove this import or verify you didn't misspell it.

E0432

An import was unresolved.

Erroneous code example:

This example deliberately fails to compile
use something::Foo; // error: unresolved import `something::Foo`.Run

Paths in use statements are relative to the crate root. To import items relative to the current and parent modules, use the self:: and super:: prefixes, respectively. Also verify that you didn't misspell the import name and that the import exists in the module from where you tried to import it. Example:

use self::something::Foo; // ok!

mod something {
    pub struct Foo;
}Run

Or, if you tried to use a module from an external crate, you may have missed the extern crate declaration (which is usually placed in the crate root):

extern crate core; // Required to use the `core` crate

use core::any;Run

E0433

An undeclared type or module was used.

Erroneous code example:

This example deliberately fails to compile
let map = HashMap::new();
// error: failed to resolve. Use of undeclared type or module `HashMap`Run

Please verify you didn't misspell the type/module's name or that you didn't forget to import it:

use std::collections::HashMap; // HashMap has been imported.
let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!Run

E0434

This error indicates that a variable usage inside an inner function is invalid because the variable comes from a dynamic environment. Inner functions do not have access to their containing environment.

Erroneous code example:

This example deliberately fails to compile
fn foo() {
    let y = 5;
    fn bar() -> u32 {
        y // error: can't capture dynamic environment in a fn item; use the
          //        || { ... } closure form instead.
    }
}Run

Functions do not capture local variables. To fix this error, you can replace the function with a closure:

fn foo() {
    let y = 5;
    let bar = || {
        y
    };
}Run

or replace the captured variable with a constant or a static item:

fn foo() {
    static mut X: u32 = 4;
    const Y: u32 = 5;
    fn bar() -> u32 {
        unsafe {
            X = 3;
        }
        Y
    }
}Run

E0435

A non-constant value was used in a constant expression.

Erroneous code example:

This example deliberately fails to compile
let foo = 42;
let a: [u8; foo]; // error: attempt to use a non-constant value in a constantRun

To fix this error, please replace the value with a constant. Example:

let a: [u8; 42]; // ok!Run

Or:

const FOO: usize = 42;
let a: [u8; FOO]; // ok!Run

E0436

The functional record update syntax is only allowed for structs. (Struct-like enum variants don't qualify, for example.)

Erroneous code example:

This example deliberately fails to compile
enum PublicationFrequency {
    Weekly,
    SemiMonthly { days: (u8, u8), annual_special: bool },
}

fn one_up_competitor(competitor_frequency: PublicationFrequency)
                     -> PublicationFrequency {
    match competitor_frequency {
        PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
            days: (1, 15), annual_special: false
        },
        c @ PublicationFrequency::SemiMonthly{ .. } =>
            PublicationFrequency::SemiMonthly {
                annual_special: true, ..c // error: functional record update
                                          //        syntax requires a struct
        }
    }
}Run

Rewrite the expression without functional record update syntax:

enum PublicationFrequency {
    Weekly,
    SemiMonthly { days: (u8, u8), annual_special: bool },
}

fn one_up_competitor(competitor_frequency: PublicationFrequency)
                     -> PublicationFrequency {
    match competitor_frequency {
        PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
            days: (1, 15), annual_special: false
        },
        PublicationFrequency::SemiMonthly{ days, .. } =>
            PublicationFrequency::SemiMonthly {
                days, annual_special: true // ok!
        }
    }
}Run

E0437

Trait implementations can only implement associated types that are members of the trait in question. This error indicates that you attempted to implement an associated type whose name does not match the name of any associated type in the trait.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

impl Foo for i32 {
    type Bar = bool;
}Run

The solution to this problem is to remove the extraneous associated type:

trait Foo {}

impl Foo for i32 {}Run

E0438

Trait implementations can only implement associated constants that are members of the trait in question. This error indicates that you attempted to implement an associated constant whose name does not match the name of any associated constant in the trait.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

impl Foo for i32 {
    const BAR: bool = true;
}Run

The solution to this problem is to remove the extraneous associated constant:

trait Foo {}

impl Foo for i32 {}Run

E0439

The length of the platform-intrinsic function simd_shuffle wasn't specified. Erroneous code example:

This example deliberately fails to compile
#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
    // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
}Run

The simd_shuffle function needs the length of the array passed as last parameter in its name. Example:

#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
}Run

E0440

A platform-specific intrinsic function has the wrong number of type parameters. Erroneous code example:

This example deliberately fails to compile
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
    // error: platform-specific intrinsic has wrong number of type
    //        parameters
}Run

Please refer to the function declaration to see if it corresponds with yours. Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2) -> i32;
}Run

E0441

An unknown platform-specific intrinsic function was used. Erroneous code example:

This example deliberately fails to compile
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
    // error: unrecognized platform-specific intrinsic function
}Run

Please verify that the function name wasn't misspelled, and ensure that it is declared in the rust source code (in the file src/librustc_platform_intrinsics/x86.rs). Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}Run

E0442

Intrinsic argument(s) and/or return value have the wrong type. Erroneous code example:

This example deliberately fails to compile
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
             i8, i8, i8, i8, i8, i8, i8, i8);
#[repr(simd)]
struct i32x4(i32, i32, i32, i32);
#[repr(simd)]
struct i64x2(i64, i64);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
    // error: intrinsic arguments/return value have wrong type
}Run

To fix this error, please refer to the function declaration to give it the awaited types. Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}Run

E0443

Intrinsic argument(s) and/or return value have the wrong type. Erroneous code example:

This example deliberately fails to compile
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
#[repr(simd)]
struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
    // error: intrinsic argument/return value has wrong type
}Run

To fix this error, please refer to the function declaration to give it the awaited types. Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}Run

E0444

A platform-specific intrinsic function has wrong number of arguments. Erroneous code example:

This example deliberately fails to compile
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
    // error: platform-specific intrinsic has invalid number of arguments
}Run

Please refer to the function declaration to see if it corresponds with yours. Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
}Run

E0445

A private trait was used on a public type parameter bound. Erroneous code examples:

This example deliberately fails to compile
#![deny(private_in_public)]

trait Foo {
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // error: private trait in public interface
pub struct Bar2<T: Foo>(pub T); // same error
pub fn foo<T: Foo> (t: T) {} // same errorRun

To solve this error, please ensure that the trait is also public. The trait can be made inaccessible if necessary by placing it into a private inner module, but it still has to be marked with pub. Example:

pub trait Foo { // we set the Foo trait public
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // ok!
pub struct Bar2<T: Foo>(pub T); // ok!
pub fn foo<T: Foo> (t: T) {} // ok!Run

E0446

A private type was used in a public type signature. Erroneous code example:

This example deliberately fails to compile
#![deny(private_in_public)]

mod Foo {
    struct Bar(u32);

    pub fn bar() -> Bar { // error: private type in public interface
        Bar(0)
    }
}Run

To solve this error, please ensure that the type is also public. The type can be made inaccessible if necessary by placing it into a private inner module, but it still has to be marked with pub. Example:

mod Foo {
    pub struct Bar(u32); // we set the Bar type public

    pub fn bar() -> Bar { // ok!
        Bar(0)
    }
}Run

E0447

Note: this error code is no longer emitted by the compiler.

The pub keyword was used inside a function. Erroneous code example:

fn foo() {
    pub struct Bar; // error: visibility has no effect inside functions
}Run

Since we cannot access items defined inside a function, the visibility of its items does not impact outer code. So using the pub keyword in this context is invalid.

E0448

The pub keyword was used inside a public enum. Erroneous code example:

This example deliberately fails to compile
pub enum Foo {
    pub Bar, // error: unnecessary `pub` visibility
}Run

Since the enum is already public, adding pub on one its elements is unnecessary. Example:

This example deliberately fails to compile
enum Foo {
    pub Bar, // not ok!
}Run

This is the correct syntax:

pub enum Foo {
    Bar, // ok!
}Run

E0449

A visibility qualifier was used when it was unnecessary. Erroneous code examples:

This example deliberately fails to compile
struct Bar;

trait Foo {
    fn foo();
}

pub impl Bar {} // error: unnecessary visibility qualifier

pub impl Foo for Bar { // error: unnecessary visibility qualifier
    pub fn foo() {} // error: unnecessary visibility qualifier
}Run

To fix this error, please remove the visibility qualifier when it is not required. Example:

struct Bar;

trait Foo {
    fn foo();
}

// Directly implemented methods share the visibility of the type itself,
// so `pub` is unnecessary here
impl Bar {}

// Trait methods share the visibility of the trait, so `pub` is
// unnecessary in either case
impl Foo for Bar {
    fn foo() {}
}Run

E0451

A struct constructor with private fields was invoked. Erroneous code example:

This example deliberately fails to compile
mod Bar {
    pub struct Foo {
        pub a: isize,
        b: isize,
    }
}

let f = Bar::Foo{ a: 0, b: 0 }; // error: field `b` of struct `Bar::Foo`
                                //        is privateRun

To fix this error, please ensure that all the fields of the struct are public, or implement a function for easy instantiation. Examples:

mod Bar {
    pub struct Foo {
        pub a: isize,
        pub b: isize, // we set `b` field public
    }
}

let f = Bar::Foo{ a: 0, b: 0 }; // ok!Run

Or:

mod Bar {
    pub struct Foo {
        pub a: isize,
        b: isize, // still private
    }

    impl Foo {
        pub fn new() -> Foo { // we create a method to instantiate `Foo`
            Foo { a: 0, b: 0 }
        }
    }
}

let f = Bar::Foo::new(); // ok!Run

E0452

An invalid lint attribute has been given. Erroneous code example:

This example deliberately fails to compile
#![allow(foo = "")] // error: malformed lint attributeRun

Lint attributes only accept a list of identifiers (where each identifier is a lint name). Ensure the attribute is of this form:

#![allow(foo)] // ok!
// or:
#![allow(foo, foo2)] // ok!Run

E0453

A lint check attribute was overruled by a forbid directive set as an attribute on an enclosing scope, or on the command line with the -F option.

Example of erroneous code:

This example deliberately fails to compile
#![forbid(non_snake_case)]

#[allow(non_snake_case)]
fn main() {
    let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
                      //        forbid(non_snake_case)
}Run

The forbid lint setting, like deny, turns the corresponding compiler warning into a hard error. Unlike deny, forbid prevents itself from being overridden by inner attributes.

If you're sure you want to override the lint check, you can change forbid to deny (or use -D instead of -F if the forbid setting was given as a command-line option) to allow the inner lint check attribute:

#![deny(non_snake_case)]

#[allow(non_snake_case)]
fn main() {
    let MyNumber = 2; // ok!
}Run

Otherwise, edit the code to pass the lint check, and remove the overruled attribute:

#![forbid(non_snake_case)]

fn main() {
    let my_number = 2;
}Run

E0454

A link name was given with an empty name. Erroneous code example:

This example is not tested
#[link(name = "")] extern {} // error: #[link(name = "")] given with empty nameRun

The rust compiler cannot link to an external library if you don't give it its name. Example:

#[link(name = "some_lib")] extern {} // ok!Run

E0455

Linking with kind=framework is only supported when targeting macOS, as frameworks are specific to that operating system.

Erroneous code example:

This example is not tested
#[link(name = "FooCoreServices", kind = "framework")] extern {}
// OS used to compile is Linux for exampleRun

To solve this error you can use conditional compilation:

#[cfg_attr(target="macos", link(name = "FooCoreServices", kind = "framework"))]
extern {}Run

See more: https://doc.rust-lang.org/book/first-edition/conditional-compilation.html

E0456

No description.

E0457

No description.

E0458

An unknown "kind" was specified for a link attribute. Erroneous code example:

This example is not tested
#[link(kind = "wonderful_unicorn")] extern {}
// error: unknown kind: `wonderful_unicorn`Run

Please specify a valid "kind" value, from one of the following:

E0459

A link was used without a name parameter. Erroneous code example:

This example is not tested
#[link(kind = "dylib")] extern {}
// error: #[link(...)] specified without `name = "foo"`Run

Please add the name parameter to allow the rust compiler to find the library you want. Example:

#[link(kind = "dylib", name = "some_lib")] extern {} // ok!Run

E0460

No description.

E0461

No description.

E0462

No description.

E0463

A plugin/crate was declared but cannot be found. Erroneous code example:

This example deliberately fails to compile
#![feature(plugin)]
#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`Run

You need to link your code to the relevant crate in order to be able to use it (through Cargo or the -L option of rustc example). Plugins are crates as well, and you link to them the same way.

E0464

No description.

E0465

No description.

E0466

Macro import declarations were malformed.

Erroneous code examples:

This example deliberately fails to compile
#[macro_use(a_macro(another_macro))] // error: invalid import declaration
extern crate core as some_crate;

#[macro_use(i_want = "some_macros")] // error: invalid import declaration
extern crate core as another_crate;Run

This is a syntax error at the level of attribute declarations. The proper syntax for macro imports is the following:

This example is not tested
// In some_crate:
#[macro_export]
macro_rules! get_tacos {
    ...
}

#[macro_export]
macro_rules! get_pimientos {
    ...
}

// In your crate:
#[macro_use(get_tacos, get_pimientos)] // It imports `get_tacos` and
extern crate some_crate;               // `get_pimientos` macros from some_crateRun

If you would like to import all exported macros, write macro_use with no arguments.

E0468

A non-root module attempts to import macros from another crate.

Example of erroneous code:

This example deliberately fails to compile
mod foo {
    #[macro_use(debug_assert)]  // error: must be at crate root to import
    extern crate core;          //        macros from another crate
    fn run_macro() { debug_assert!(true); }
}Run

Only extern crate imports at the crate root level are allowed to import macros.

Either move the macro import to crate root or do without the foreign macros. This will work:

#[macro_use(debug_assert)]
extern crate core;

mod foo {
    fn run_macro() { debug_assert!(true); }
}Run

E0469

A macro listed for import was not found.

Erroneous code example:

This example deliberately fails to compile
#[macro_use(drink, be_merry)] // error: imported macro not found
extern crate alloc;

fn main() {
    // ...
}Run

Either the listed macro is not contained in the imported crate, or it is not exported from the given crate.

This could be caused by a typo. Did you misspell the macro's name?

Double-check the names of the macros listed for import, and that the crate in question exports them.

A working version would be:

This example is not tested
// In some_crate crate:
#[macro_export]
macro_rules! eat {
    ...
}

#[macro_export]
macro_rules! drink {
    ...
}

// In your crate:
#[macro_use(eat, drink)]
extern crate some_crate; //ok!Run

E0472

No description.

E0473

No description.

E0474

No description.

E0475

No description.

E0476

No description.

E0477

No description.

E0478

A lifetime bound was not satisfied.

Erroneous code example:

This example deliberately fails to compile
// Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
// outlive all the superbounds from the trait (`'kiss`, in this example).

trait Wedding<'t>: 't { }

struct Prince<'kiss, 'SnowWhite> {
    child: Box<Wedding<'kiss> + 'SnowWhite>,
    // error: lifetime bound not satisfied
}Run

In this example, the 'SnowWhite lifetime is supposed to outlive the 'kiss lifetime but the declaration of the Prince struct doesn't enforce it. To fix this issue, you need to specify it:

trait Wedding<'t>: 't { }

struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
                                          // longer than 'SnowWhite.
    child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
}Run

E0479

No description.

E0480

No description.

E0481

No description.

E0482

No description.

E0483

No description.

E0484

No description.

E0485

No description.

E0486

No description.

E0487

No description.

E0488

No description.

E0489

No description.

E0490

No description.

E0491

A reference has a longer lifetime than the data it references.

Erroneous code example:

This example deliberately fails to compile
// struct containing a reference requires a lifetime parameter,
// because the data the reference points to must outlive the struct (see E0106)
struct Struct<'a> {
    ref_i32: &'a i32,
}

// However, a nested struct like this, the signature itself does not tell
// whether 'a outlives 'b or the other way around.
// So it could be possible that 'b of reference outlives 'a of the data.
struct Nested<'a, 'b> {
    ref_struct: &'b Struct<'a>, // compile error E0491
}Run

To fix this issue, you can specify a bound to the lifetime like below:

struct Struct<'a> {
    ref_i32: &'a i32,
}

// 'a: 'b means 'a outlives 'b
struct Nested<'a: 'b, 'b> {
    ref_struct: &'b Struct<'a>,
}Run

E0492

A borrow of a constant containing interior mutability was attempted. Erroneous code example:

This example deliberately fails to compile
use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};

const A: AtomicUsize = ATOMIC_USIZE_INIT;
static B: &'static AtomicUsize = &A;
// error: cannot borrow a constant which may contain interior mutability,
//        create a static insteadRun

A const represents a constant value that should never change. If one takes a & reference to the constant, then one is taking a pointer to some memory location containing the value. Normally this is perfectly fine: most values can't be changed via a shared & pointer, but interior mutability would allow it. That is, a constant value could be mutated. On the other hand, a static is explicitly a single memory location, which can be mutated at will.

So, in order to solve this error, either use statics which are Sync:

use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};

static A: AtomicUsize = ATOMIC_USIZE_INIT;
static B: &'static AtomicUsize = &A; // ok!Run

You can also have this error while using a cell type:

This example deliberately fails to compile
use std::cell::Cell;

const A: Cell<usize> = Cell::new(1);
const B: &'static Cell<usize> = &A;
// error: cannot borrow a constant which may contain interior mutability,
//        create a static instead

// or:
struct C { a: Cell<usize> }

const D: C = C { a: Cell::new(1) };
const E: &'static Cell<usize> = &D.a; // error

// or:
const F: &'static C = &D; // errorRun

This is because cell types do operations that are not thread-safe. Due to this, they don't implement Sync and thus can't be placed in statics. In this case, StaticMutex would work just fine, but it isn't stable yet: https://doc.rust-lang.org/nightly/std/sync/struct.StaticMutex.html

However, if you still wish to use these types, you can achieve this by an unsafe wrapper:

use std::cell::Cell;
use std::marker::Sync;

struct NotThreadSafe<T> {
    value: Cell<T>,
}

unsafe impl<T> Sync for NotThreadSafe<T> {}

static A: NotThreadSafe<usize> = NotThreadSafe { value : Cell::new(1) };
static B: &'static NotThreadSafe<usize> = &A; // ok!Run

Remember this solution is unsafe! You will have to ensure that accesses to the cell are synchronized.

E0493

No description.

E0495

No description.

E0496

A lifetime name is shadowing another lifetime name. Erroneous code example:

This example deliberately fails to compile
struct Foo<'a> {
    a: &'a i32,
}

impl<'a> Foo<'a> {
    fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
                           //        name that is already in scope
    }
}Run

Please change the name of one of the lifetimes to remove this error. Example:

struct Foo<'a> {
    a: &'a i32,
}

impl<'a> Foo<'a> {
    fn f<'b>(x: &'b i32) { // ok!
    }
}

fn main() {
}Run

E0497

A stability attribute was used outside of the standard library. Erroneous code example:

This example deliberately fails to compile
#[stable] // error: stability attributes may not be used outside of the
          //        standard library
fn foo() {}Run

It is not possible to use stability attributes outside of the standard library. Also, for now, it is not possible to write deprecation messages either.

E0498

No description.

E0499

A variable was borrowed as mutable more than once. Erroneous code example:

This example deliberately fails to compile
let mut i = 0;
let mut x = &mut i;
let mut a = &mut i;
// error: cannot borrow `i` as mutable more than once at a timeRun

Please note that in rust, you can either have many immutable references, or one mutable reference. Take a look at https://doc.rust-lang.org/stable/book/references-and-borrowing.html for more information. Example:

let mut i = 0;
let mut x = &mut i; // ok!

// or:
let mut i = 0;
let a = &i; // ok!
let b = &i; // still ok!
let c = &i; // still ok!Run

E0500

A borrowed variable was used in another closure. Example of erroneous code:

This example deliberately fails to compile
fn you_know_nothing(jon_snow: &mut i32) {
    let nights_watch = || {
        *jon_snow = 2;
    };
    let starks = || {
        *jon_snow = 3; // error: closure requires unique access to `jon_snow`
                       //        but it is already borrowed
    };
}Run

In here, jon_snow is already borrowed by the nights_watch closure, so it cannot be borrowed by the starks closure at the same time. To fix this issue, you can put the closure in its own scope:

fn you_know_nothing(jon_snow: &mut i32) {
    {
        let nights_watch = || {
            *jon_snow = 2;
        };
    } // At this point, `jon_snow` is free.
    let starks = || {
        *jon_snow = 3;
    };
}Run

Or, if the type implements the Clone trait, you can clone it between closures:

fn you_know_nothing(jon_snow: &mut i32) {
    let mut jon_copy = jon_snow.clone();
    let nights_watch = || {
        jon_copy = 2;
    };
    let starks = || {
        *jon_snow = 3;
    };
}Run

E0501

This error indicates that a mutable variable is being used while it is still captured by a closure. Because the closure has borrowed the variable, it is not available for use until the closure goes out of scope.

Note that a capture will either move or borrow a variable, but in this situation, the closure is borrowing the variable. Take a look at http://rustbyexample.com/fn/closures/capture.html for more information about capturing.

Example of erroneous code:

This example deliberately fails to compile
fn inside_closure(x: &mut i32) {
    // Actions which require unique access
}

fn outside_closure(x: &mut i32) {
    // Actions which require unique access
}

fn foo(a: &mut i32) {
    let bar = || {
        inside_closure(a)
    };
    outside_closure(a); // error: cannot borrow `*a` as mutable because previous
                        //        closure requires unique access.
}Run

To fix this error, you can place the closure in its own scope:

fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    {
        let bar = || {
            inside_closure(a)
        };
    } // borrow on `a` ends.
    outside_closure(a); // ok!
}Run

Or you can pass the variable as a parameter to the closure:

fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    let bar = |s: &mut i32| {
        inside_closure(s)
    };
    outside_closure(a);
    bar(a);
}Run

It may be possible to define the closure later:

fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    outside_closure(a);
    let bar = || {
        inside_closure(a)
    };
}Run

E0502

This error indicates that you are trying to borrow a variable as mutable when it has already been borrowed as immutable.

Example of erroneous code:

This example deliberately fails to compile
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    let ref y = a; // a is borrowed as immutable.
    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
            //        as immutable
}Run

To fix this error, ensure that you don't have any other references to the variable before trying to access it mutably:

fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    bar(a);
    let ref y = a; // ok!
}Run

For more information on the rust ownership system, take a look at https://doc.rust-lang.org/stable/book/references-and-borrowing.html.

E0503

A value was used after it was mutably borrowed.

Example of erroneous code:

This example deliberately fails to compile
fn main() {
    let mut value = 3;
    // Create a mutable borrow of `value`. This borrow
    // lives until the end of this function.
    let _borrow = &mut value;
    let _sum = value + 1; // error: cannot use `value` because
                          //        it was mutably borrowed
}Run

In this example, value is mutably borrowed by borrow and cannot be used to calculate sum. This is not possible because this would violate Rust's mutability rules.

You can fix this error by limiting the scope of the borrow:

fn main() {
    let mut value = 3;
    // By creating a new block, you can limit the scope
    // of the reference.
    {
        let _borrow = &mut value; // Use `_borrow` inside this block.
    }
    // The block has ended and with it the borrow.
    // You can now use `value` again.
    let _sum = value + 1;
}Run

Or by cloning value before borrowing it:

fn main() {
    let mut value = 3;
    // We clone `value`, creating a copy.
    let value_cloned = value.clone();
    // The mutable borrow is a reference to `value` and
    // not to `value_cloned`...
    let _borrow = &mut value;
    // ... which means we can still use `value_cloned`,
    let _sum = value_cloned + 1;
    // even though the borrow only ends here.
}Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/stable/book/references-and-borrowing.html

E0504

This error occurs when an attempt is made to move a borrowed variable into a closure.

Example of erroneous code:

This example deliberately fails to compile
struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let x = move || {
        println!("child function: {}", fancy_num.num);
        // error: cannot move `fancy_num` into closure because it is borrowed
    };

    x();
    println!("main function: {}", fancy_ref.num);
}Run

Here, fancy_num is borrowed by fancy_ref and so cannot be moved into the closure x. There is no way to move a value into a closure while it is borrowed, as that would invalidate the borrow.

If the closure can't outlive the value being moved, try using a reference rather than moving:

struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let x = move || {
        // fancy_ref is usable here because it doesn't move `fancy_num`
        println!("child function: {}", fancy_ref.num);
    };

    x();

    println!("main function: {}", fancy_num.num);
}Run

If the value has to be borrowed and then moved, try limiting the lifetime of the borrow using a scoped block:

struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };

    {
        let fancy_ref = &fancy_num;
        println!("main function: {}", fancy_ref.num);
        // `fancy_ref` goes out of scope here
    }

    let x = move || {
        // `fancy_num` can be moved now (no more references exist)
        println!("child function: {}", fancy_num.num);
    };

    x();
}Run

If the lifetime of a reference isn't enough, such as in the case of threading, consider using an Arc to create a reference-counted value:

use std::sync::Arc;
use std::thread;

struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_ref1 = Arc::new(FancyNum { num: 5 });
    let fancy_ref2 = fancy_ref1.clone();

    let x = thread::spawn(move || {
        // `fancy_ref1` can be moved and has a `'static` lifetime
        println!("child thread: {}", fancy_ref1.num);
    });

    x.join().expect("child thread should finish");
    println!("main thread: {}", fancy_ref2.num);
}Run

E0505

A value was moved out while it was still borrowed.

Erroneous code example:

This example deliberately fails to compile
struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(x);
    }
}Run

Here, the function eat takes the ownership of x. However, x cannot be moved because it was borrowed to _ref_to_val. To fix that you can do few different things:

Examples:

struct Value {}

fn eat(val: &Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(&x); // pass by reference, if it's possible
    }
}Run

Or:

struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
    }
    eat(x); // release borrow and then move it.
}Run

Or:

#[derive(Clone, Copy)] // implement Copy trait
struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(x); // it will be copied here.
    }
}Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/stable/book/references-and-borrowing.html

E0506

This error occurs when an attempt is made to assign to a borrowed value.

Example of erroneous code:

This example deliberately fails to compile
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;
    fancy_num = FancyNum { num: 6 };
    // error: cannot assign to `fancy_num` because it is borrowed

    println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
}Run

Because fancy_ref still holds a reference to fancy_num, fancy_num can't be assigned to a new value as it would invalidate the reference.

Alternatively, we can move out of fancy_num into a second fancy_num:

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    let moved_num = fancy_num;
    fancy_num = FancyNum { num: 6 };

    println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
}Run

If the value has to be borrowed, try limiting the lifetime of the borrow using a scoped block:

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };

    {
        let fancy_ref = &fancy_num;
        println!("Ref: {}", fancy_ref.num);
    }

    // Works because `fancy_ref` is no longer in scope
    fancy_num = FancyNum { num: 6 };
    println!("Num: {}", fancy_num.num);
}Run

Or by moving the reference into a function:

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };

    print_fancy_ref(&fancy_num);

    // Works because function borrow has ended
    fancy_num = FancyNum { num: 6 };
    println!("Num: {}", fancy_num.num);
}

fn print_fancy_ref(fancy_ref: &FancyNum){
    println!("Ref: {}", fancy_ref.num);
}Run

E0507

You tried to move out of a value which was borrowed. Erroneous code example:

This example deliberately fails to compile
use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // error: cannot move out of borrowed content
}Run

Here, the nothing_is_true method takes the ownership of self. However, self cannot be moved because .borrow() only provides an &TheDarkKnight, which is a borrow of the content owned by the RefCell. To fix this error, you have three choices:

Examples:

use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(&self) {} // First case, we don't take ownership
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // ok!
}Run

Or:

use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);
    let x = x.into_inner(); // we get back ownership

    x.nothing_is_true(); // ok!
}Run

Or:

use std::cell::RefCell;

#[derive(Clone, Copy)] // we implement the Copy trait
struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // ok!
}Run

Moving a member out of a mutably borrowed struct will also cause E0507 error:

This example deliberately fails to compile
struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

struct Batcave {
    knight: TheDarkKnight
}

fn main() {
    let mut cave = Batcave {
        knight: TheDarkKnight
    };
    let borrowed = &mut cave;

    borrowed.knight.nothing_is_true(); // E0507
}Run

It is fine only if you put something back. mem::replace can be used for that:

use std::mem;

let mut cave = Batcave {
    knight: TheDarkKnight
};
let borrowed = &mut cave;

mem::replace(&mut borrowed.knight, TheDarkKnight).nothing_is_true(); // ok!Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/book/first-edition/references-and-borrowing.html

E0508

A value was moved out of a non-copy fixed-size array.

Example of erroneous code:

This example deliberately fails to compile
struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`,
                           //        a non-copy fixed-size array
}Run

The first element was moved out of the array, but this is not possible because NonCopy does not implement the Copy trait.

Consider borrowing the element instead of moving it:

struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    let _value = &array[0]; // Borrowing is allowed, unlike moving.
}Run

Alternatively, if your type implements Clone and you need to own the value, consider borrowing and then cloning:

#[derive(Clone)]
struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    // Now you can clone the array element.
    let _value = array[0].clone();
}Run

E0509

This error occurs when an attempt is made to move out of a value whose type implements the Drop trait.

Example of erroneous code:

This example deliberately fails to compile
struct FancyNum {
    num: usize
}

struct DropStruct {
    fancy: FancyNum
}

impl Drop for DropStruct {
    fn drop(&mut self) {
        // Destruct DropStruct, possibly using FancyNum
    }
}

fn main() {
    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
    let fancy_field = drop_struct.fancy; // Error E0509
    println!("Fancy: {}", fancy_field.num);
    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
}Run

Here, we tried to move a field out of a struct of type DropStruct which implements the Drop trait. However, a struct cannot be dropped if one or more of its fields have been moved.

Structs implementing the Drop trait have an implicit destructor that gets called when they go out of scope. This destructor may use the fields of the struct, so moving out of the struct could make it impossible to run the destructor. Therefore, we must think of all values whose type implements the Drop trait as single units whose fields cannot be moved.

This error can be fixed by creating a reference to the fields of a struct, enum, or tuple using the ref keyword:

struct FancyNum {
    num: usize
}

struct DropStruct {
    fancy: FancyNum
}

impl Drop for DropStruct {
    fn drop(&mut self) {
        // Destruct DropStruct, possibly using FancyNum
    }
}

fn main() {
    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
    let ref fancy_field = drop_struct.fancy; // No more errors!
    println!("Fancy: {}", fancy_field.num);
    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
}Run

Note that this technique can also be used in the arms of a match expression:

struct FancyNum {
    num: usize
}

enum DropEnum {
    Fancy(FancyNum)
}

impl Drop for DropEnum {
    fn drop(&mut self) {
        // Destruct DropEnum, possibly using FancyNum
    }
}

fn main() {
    // Creates and enum of type `DropEnum`, which implements `Drop`
    let drop_enum = DropEnum::Fancy(FancyNum{num: 10});
    match drop_enum {
        // Creates a reference to the inside of `DropEnum::Fancy`
        DropEnum::Fancy(ref fancy_field) => // No error!
            println!("It was fancy-- {}!", fancy_field.num),
    }
    // implicit call to `drop_enum.drop()` as drop_enum goes out of scope
}Run

E0511

Invalid monomorphization of an intrinsic function was used. Erroneous code example:

This example is not tested
#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

fn main() {
    unsafe { simd_add(0, 1); }
    // error: invalid monomorphization of `simd_add` intrinsic
}Run

The generic type has to be a SIMD type. Example:

#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
#[derive(Copy, Clone)]
struct i32x2(i32, i32);

extern "platform-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

unsafe { simd_add(i32x2(0, 0), i32x2(1, 2)); } // ok!Run

E0512

Transmute with two differently sized types was attempted. Erroneous code example:

This example deliberately fails to compile
fn takes_u8(_: u8) {}

fn main() {
    unsafe { takes_u8(::std::mem::transmute(0u16)); }
    // error: transmute called with types of different sizes
}Run

Please use types with same size or use the expected type directly. Example:

fn takes_u8(_: u8) {}

fn main() {
    unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
    // or:
    unsafe { takes_u8(0u8); } // ok!
}Run

E0514

No description.

E0516

The typeof keyword is currently reserved but unimplemented. Erroneous code example:

This example deliberately fails to compile
fn main() {
    let x: typeof(92) = 92;
}Run

Try using type inference instead. Example:

fn main() {
    let x = 92;
}Run

E0517

This error indicates that a #[repr(..)] attribute was placed on an unsupported item.

Examples of erroneous code:

This example deliberately fails to compile
#[repr(C)]
type Foo = u8;

#[repr(packed)]
enum Foo {Bar, Baz}

#[repr(u8)]
struct Foo {bar: bool, baz: bool}

#[repr(C)]
impl Foo {
    // ...
}Run

These attributes do not work on typedefs, since typedefs are just aliases.

Representations like #[repr(u8)], #[repr(i64)] are for selecting the discriminant size for enums with no data fields on any of the variants, e.g. enum Color {Red, Blue, Green}, effectively setting the size of the enum to the size of the provided type. Such an enum can be cast to a value of the same type as well. In short, #[repr(u8)] makes the enum behave like an integer with a constrained set of allowed values.

Only field-less enums can be cast to numerical primitives, so this attribute will not apply to structs.

#[repr(packed)] reduces padding to make the struct size smaller. The representation of enums isn't strictly defined in Rust, and this attribute won't work on enums.

#[repr(simd)] will give a struct consisting of a homogeneous series of machine types (i.e. u8, i32, etc) a representation that permits vectorization via SIMD. This doesn't make much sense for enums since they don't consist of a single list of data.

E0518

This error indicates that an #[inline(..)] attribute was incorrectly placed on something other than a function or method.

Examples of erroneous code:

This example deliberately fails to compile
#[inline(always)]
struct Foo;

#[inline(never)]
impl Foo {
    // ...
}Run

#[inline] hints the compiler whether or not to attempt to inline a method or function. By default, the compiler does a pretty good job of figuring this out itself, but if you feel the need for annotations, #[inline(always)] and #[inline(never)] can override or force the compiler's decision.

If you wish to apply this attribute to all methods in an impl, manually annotate each method; it is not possible to annotate the entire impl with an #[inline] attribute.

E0519

No description.

E0520

A non-default implementation was already made on this type so it cannot be specialized further. Erroneous code example:

This example deliberately fails to compile
#![feature(specialization)]

trait SpaceLlama {
    fn fly(&self);
}

// applies to all T
impl<T> SpaceLlama for T {
    default fn fly(&self) {}
}

// non-default impl
// applies to all `Clone` T and overrides the previous impl
impl<T: Clone> SpaceLlama for T {
    fn fly(&self) {}
}

// since `i32` is clone, this conflicts with the previous implementation
impl SpaceLlama for i32 {
    default fn fly(&self) {}
    // error: item `fly` is provided by an `impl` that specializes
    //        another, but the item in the parent `impl` is not marked
    //        `default` and so it cannot be specialized.
}Run

Specialization only allows you to override default functions in implementations.

To fix this error, you need to mark all the parent implementations as default. Example:

#![feature(specialization)]

trait SpaceLlama {
    fn fly(&self);
}

// applies to all T
impl<T> SpaceLlama for T {
    default fn fly(&self) {} // This is a parent implementation.
}

// applies to all `Clone` T; overrides the previous impl
impl<T: Clone> SpaceLlama for T {
    default fn fly(&self) {} // This is a parent implementation but was
                             // previously not a default one, causing the error
}

// applies to i32, overrides the previous two impls
impl SpaceLlama for i32 {
    fn fly(&self) {} // And now that's ok!
}Run

E0522

The lang attribute is intended for marking special items that are built-in to Rust itself. This includes special traits (like Copy and Sized) that affect how the compiler behaves, as well as special functions that may be automatically invoked (such as the handler for out-of-bounds accesses when indexing a slice). Erroneous code example:

This example deliberately fails to compile
#![feature(lang_items)]

#[lang = "cookie"]
fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
    loop {}
}Run

E0523

No description.

E0524

No description.

E0525

A closure was used but didn't implement the expected trait.

Erroneous code example:

This example deliberately fails to compile
struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x); // error: expected a closure that implements
                              //        the `Fn` trait, but this closure only
                              //        implements `FnOnce`
    bar(closure);
}Run

In the example above, closure is an FnOnce closure whereas the bar function expected an Fn closure. In this case, it's simple to fix the issue, you just have to implement Copy and Clone traits on struct X and it'll be ok:

#[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x);
    bar(closure); // ok!
}Run

To understand better how closures work in Rust, read: https://doc.rust-lang.org/book/first-edition/closures.html

E0526

No description.

E0527

The number of elements in an array or slice pattern differed from the number of elements in the array being matched.

Example of erroneous code:

This example deliberately fails to compile
let r = &[1, 2, 3, 4];
match r {
    &[a, b] => { // error: pattern requires 2 elements but array
                 //        has 4
        println!("a={}, b={}", a, b);
    }
}Run

Ensure that the pattern is consistent with the size of the matched array. Additional elements can be matched with ..:

#![feature(slice_patterns)]

let r = &[1, 2, 3, 4];
match r {
    &[a, b, ..] => { // ok!
        println!("a={}, b={}", a, b);
    }
}Run

E0528

An array or slice pattern required more elements than were present in the matched array.

Example of erroneous code:

This example deliberately fails to compile
#![feature(slice_patterns)]

let r = &[1, 2];
match r {
    &[a, b, c, rest..] => { // error: pattern requires at least 3
                            //        elements but array has 2
        println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
    }
}Run

Ensure that the matched array has at least as many elements as the pattern requires. You can match an arbitrary number of remaining elements with ..:

#![feature(slice_patterns)]

let r = &[1, 2, 3, 4, 5];
match r {
    &[a, b, c, rest..] => { // ok!
        // prints `a=1, b=2, c=3 rest=[4, 5]`
        println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
    }
}Run

E0529

An array or slice pattern was matched against some other type.

Example of erroneous code:

This example deliberately fails to compile
let r: f32 = 1.0;
match r {
    [a, b] => { // error: expected an array or slice, found `f32`
        println!("a={}, b={}", a, b);
    }
}Run

Ensure that the pattern and the expression being matched on are of consistent types:

let r = [1.0, 2.0];
match r {
    [a, b] => { // ok!
        println!("a={}, b={}", a, b);
    }
}Run

E0530

A binding shadowed something it shouldn't.

Erroneous code example:

This example deliberately fails to compile
static TEST: i32 = 0;

let r: (i32, i32) = (0, 0);
match r {
    TEST => {} // error: match bindings cannot shadow statics
}Run

To fix this error, just change the binding's name in order to avoid shadowing one of the following:

Fixed example:

static TEST: i32 = 0;

let r: (i32, i32) = (0, 0);
match r {
    something => {} // ok!
}Run

E0531

No description.

E0532

Pattern arm did not match expected kind.

Erroneous code example:

This example deliberately fails to compile
enum State {
    Succeeded,
    Failed(String),
}

fn print_on_failure(state: &State) {
    match *state {
        // error: expected unit struct/variant or constant, found tuple
        //        variant `State::Failed`
        State::Failed => println!("Failed"),
        _ => ()
    }
}Run

To fix this error, ensure the match arm kind is the same as the expression matched.

Fixed example:

enum State {
    Succeeded,
    Failed(String),
}

fn print_on_failure(state: &State) {
    match *state {
        State::Failed(ref msg) => println!("Failed with {}", msg),
        _ => ()
    }
}Run

E0533

No description.

E0534

The inline attribute was malformed.

Erroneous code example:

This example is not tested
#[inline()] // error: expected one argument
pub fn something() {}

fn main() {}Run

The parenthesized inline attribute requires the parameter to be specified:

#[inline(always)]
fn something() {}Run

or:

#[inline(never)]
fn something() {}Run

Alternatively, a paren-less version of the attribute may be used to hint the compiler about inlining opportunity:

#[inline]
fn something() {}Run

For more information about the inline attribute, read: https://doc.rust-lang.org/reference.html#inline-attributes

E0535

An unknown argument was given to the inline attribute.

Erroneous code example:

This example is not tested
#[inline(unknown)] // error: invalid argument
pub fn something() {}

fn main() {}Run

The inline attribute only supports two arguments:

All other arguments given to the inline attribute will return this error. Example:

#[inline(never)] // ok!
pub fn something() {}

fn main() {}Run

For more information about the inline attribute, https: read://doc.rust-lang.org/reference.html#inline-attributes

E0536

The not cfg-predicate was malformed.

Erroneous code example:

This example deliberately fails to compile
#[cfg(not())] // error: expected 1 cfg-pattern
pub fn something() {}

pub fn main() {}Run

The not predicate expects one cfg-pattern. Example:

#[cfg(not(target_os = "linux"))] // ok!
pub fn something() {}

pub fn main() {}Run

For more information about the cfg attribute, read: https://doc.rust-lang.org/reference.html#conditional-compilation

E0537

An unknown predicate was used inside the cfg attribute.

Erroneous code example:

This example deliberately fails to compile
#[cfg(unknown())] // error: invalid predicate `unknown`
pub fn something() {}

pub fn main() {}Run

The cfg attribute supports only three kinds of predicates:

Example:

#[cfg(not(target_os = "linux"))] // ok!
pub fn something() {}

pub fn main() {}Run

For more information about the cfg attribute, read: https://doc.rust-lang.org/reference.html#conditional-compilation

E0538

Attribute contains same meta item more than once.

Erroneous code example:

This example deliberately fails to compile
#[deprecated(
    since="1.0.0",
    note="First deprecation note.",
    note="Second deprecation note." // error: multiple same meta item
)]
fn deprecated_function() {}Run

Meta items are the key-value pairs inside of an attribute. Each key may only be used once in each attribute.

To fix the problem, remove all but one of the meta items with the same key.

Example:

#[deprecated(
    since="1.0.0",
    note="First deprecation note."
)]
fn deprecated_function() {}Run

E0539

No description.

E0540

No description.

E0541

An unknown meta item was used.

Erroneous code example:

This example deliberately fails to compile
#[deprecated(
    since="1.0.0",
    // error: unknown meta item
    reason="Example invalid meta item. Should be 'note'")
]
fn deprecated_function() {}Run

Meta items are the key-value pairs inside of an attribute. The keys provided must be one of the valid keys for the specified attribute.

To fix the problem, either remove the unknown meta item, or rename it if you provided the wrong name.

In the erroneous code example above, the wrong name was provided, so changing to a correct one it will fix the error. Example:

#[deprecated(
    since="1.0.0",
    note="This is a valid meta item for the deprecated attribute."
)]
fn deprecated_function() {}Run

E0542

No description.

E0543

No description.

E0544

No description.

E0545

No description.

E0546

No description.

E0547

No description.

E0548

No description.

E0549

No description.

E0550

No description.

E0551

No description.

E0552

A unrecognized representation attribute was used.

Erroneous code example:

This example deliberately fails to compile
#[repr(D)] // error: unrecognized representation hint
struct MyStruct {
    my_field: usize
}Run

You can use a repr attribute to tell the compiler how you want a struct or enum to be laid out in memory.

Make sure you're using one of the supported options:

#[repr(C)] // ok!
struct MyStruct {
    my_field: usize
}Run

For more information about specifying representations, see the "Alternative Representations" section of the Rustonomicon.

E0553

No description.

E0554

Feature attributes are only allowed on the nightly release channel. Stable or beta compilers will not comply.

Example of erroneous code (on a stable compiler):

This example is not tested
#![feature(non_ascii_idents)] // error: #![feature] may not be used on the
                              //        stable release channelRun

If you need the feature, make sure to use a nightly release of the compiler (but be warned that the feature may be removed or altered in the future).

E0555

No description.

E0556

No description.

E0557

A feature attribute named a feature that has been removed.

Erroneous code example:

This example deliberately fails to compile
#![feature(managed_boxes)] // error: feature has been removedRun

Delete the offending feature attribute.

E0558

The export_name attribute was malformed.

Erroneous code example:

This example is not tested
#[export_name] // error: `export_name` attribute has invalid format
pub fn something() {}

fn main() {}Run

The export_name attribute expects a string in order to determine the name of the exported symbol. Example:

#[export_name = "some_function"] // ok!
pub fn something() {}

fn main() {}Run

E0559

An unknown field was specified into an enum's structure variant.

Erroneous code example:

This example deliberately fails to compile
enum Field {
    Fool { x: u32 },
}

let s = Field::Fool { joke: 0 };
// error: struct variant `Field::Fool` has no field named `joke`Run

Verify you didn't misspell the field's name or that the field exists. Example:

enum Field {
    Fool { joke: u32 },
}

let s = Field::Fool { joke: 0 }; // ok!Run

E0560

An unknown field was specified into a structure.

Erroneous code example:

This example deliberately fails to compile
struct Simba {
    mother: u32,
}

let s = Simba { mother: 1, father: 0 };
// error: structure `Simba` has no field named `father`Run

Verify you didn't misspell the field's name or that the field exists. Example:

struct Simba {
    mother: u32,
    father: u32,
}

let s = Simba { mother: 1, father: 0 }; // ok!Run

E0561

No description.

E0562

Abstract return types (written impl Trait for some trait Trait) are only allowed as function return types.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    let count_to_ten: impl Iterator<Item=usize> = 0..10;
    // error: `impl Trait` not allowed outside of function and inherent method
    //        return types
    for i in count_to_ten {
        println!("{}", i);
    }
}Run

Make sure impl Trait only appears in return-type position.

fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
    0..n
}

fn main() {
    for i in count_to_n(10) {  // ok!
        println!("{}", i);
    }
}Run

See RFC 1522 for more details.

E0564

No description.

E0565

A literal was used in an attribute that doesn't support literals.

Erroneous code example:

This example is not tested
#![feature(attr_literals)]

#[inline("always")] // error: unsupported literal
pub fn something() {}Run

Literals in attributes are new and largely unsupported. Work to support literals where appropriate is ongoing. Try using an unquoted name instead:

#[inline(always)]
pub fn something() {}Run

E0566

No description.

E0567

No description.

E0568

No description.

E0569

If an impl has a generic parameter with the #[may_dangle] attribute, then that impl must be declared as an `unsafe impl.

Erroneous code example:

This example deliberately fails to compile
#![feature(dropck_eyepatch)]

struct Foo<X>(X);
impl<#[may_dangle] X> Drop for Foo<X> {
    fn drop(&mut self) { }
}Run

In this example, we are asserting that the destructor for Foo will not access any data of type X, and require this assertion to be true for overall safety in our program. The compiler does not currently attempt to verify this assertion; therefore we must tag this impl as unsafe.

E0570

The requested ABI is unsupported by the current target.

The rust compiler maintains for each target a blacklist of ABIs unsupported on that target. If an ABI is present in such a list this usually means that the target / ABI combination is currently unsupported by llvm.

If necessary, you can circumvent this check using custom target specifications.

E0571

A break statement with an argument appeared in a non-loop loop.

Example of erroneous code:

This example deliberately fails to compile
let result = while true {
    if satisfied(i) {
        break 2*i; // error: `break` with value from a `while` loop
    }
    i += 1;
};Run

The break statement can take an argument (which will be the value of the loop expression if the break statement is executed) in loop loops, but not for, while, or while let loops.

Make sure break value; statements only occur in loop loops:

let result = loop { // ok!
    if satisfied(i) {
        break 2*i;
    }
    i += 1;
};Run

E0572

A return statement was found outside of a function body.

Erroneous code example:

This example deliberately fails to compile
const FOO: u32 = return 0; // error: return statement outside of function body

fn main() {}Run

To fix this issue, just remove the return keyword or move the expression into a function. Example:

const FOO: u32 = 0;

fn some_fn() -> u32 {
    return FOO;
}

fn main() {
    some_fn();
}Run

E0573

No description.

E0574

No description.

E0575

No description.

E0576

No description.

E0577

No description.

E0578

No description.

E0579

When matching against an exclusive range, the compiler verifies that the range is non-empty. Exclusive range patterns include the start point but not the end point, so this is equivalent to requiring the start of the range to be less than the end of the range.

For example:

This example deliberately fails to compile
match 5u32 {
    // This range is ok, albeit pointless.
    1 .. 2 => {}
    // This range is empty, and the compiler can tell.
    5 .. 5 => {}
}Run

E0580

The main function was incorrectly declared.

Erroneous code example:

This example deliberately fails to compile
fn main(x: i32) { // error: main function has wrong type
    println!("{}", x);
}Run

The main function prototype should never take arguments. Example:

fn main() {
    // your code
}Run

If you want to get command-line arguments, use std::env::args. To exit with a specified exit code, use std::process::exit.

E0581

In a fn type, a lifetime appears only in the return type, and not in the arguments types.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    // Here, `'a` appears only in the return type:
    let x: for<'a> fn() -> &'a i32;
}Run

To fix this issue, either use the lifetime in the arguments, or use 'static. Example:

fn main() {
    // Here, `'a` appears only in the return type:
    let x: for<'a> fn(&'a i32) -> &'a i32;
    let y: fn() -> &'static i32;
}Run

Note: The examples above used to be (erroneously) accepted by the compiler, but this was since corrected. See issue #33685 for more details.

E0582

A lifetime appears only in an associated-type binding, and not in the input types to the trait.

Erroneous code example:

This example deliberately fails to compile
fn bar<F>(t: F)
    // No type can satisfy this requirement, since `'a` does not
    // appear in any of the input types (here, `i32`):
    where F: for<'a> Fn(i32) -> Option<&'a i32>
{
}

fn main() { }Run

To fix this issue, either use the lifetime in the inputs, or use 'static. Example:

fn bar<F, G>(t: F, u: G)
    where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
          G: Fn(i32) -> Option<&'static i32>,
{
}

fn main() { }Run

Note: The examples above used to be (erroneously) accepted by the compiler, but this was since corrected. See issue #33685 for more details.

E0583

A file wasn't found for an out-of-line module.

Erroneous code example:

This example is not tested
mod file_that_doesnt_exist; // error: file not found for module

fn main() {}Run

Please be sure that a file corresponding to the module exists. If you want to use a module named file_that_doesnt_exist, you need to have a file named file_that_doesnt_exist.rs or file_that_doesnt_exist/mod.rs in the same directory.

E0584

No description.

E0585

A documentation comment that doesn't document anything was found.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    // The following doc comment will fail:
    /// This is a useless doc comment!
}Run

Documentation comments need to be followed by items, including functions, types, modules, etc. Examples:

/// I'm documenting the following struct:
struct Foo;

/// I'm documenting the following function:
fn foo() {}Run

E0586

An inclusive range was used with no end.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
    let x = &tmp[1..=]; // error: inclusive range was used with no end
}Run

An inclusive range needs an end in order to include it. If you just need a start and no end, use a non-inclusive range (with ..):

fn main() {
    let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
    let x = &tmp[1..]; // ok!
}Run

Or put an end to your inclusive range:

fn main() {
    let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
    let x = &tmp[1..=3]; // ok!
}Run

E0587

No description.

E0588

No description.

E0589

The value of N that was specified for repr(align(N)) was not a power of two, or was greater than 2^29.

This example deliberately fails to compile
#[repr(align(15))] // error: invalid `repr(align)` attribute: not a power of two
enum Foo {
    Bar(u64),
}Run

E0590

break or continue must include a label when used in the condition of a while loop.

Example of erroneous code:

This example deliberately fails to compile
while break {}Run

To fix this, add a label specifying which loop is being broken out of:

'foo: while break 'foo {}Run

E0591

Per RFC 401, if you have a function declaration foo:

// For the purposes of this explanation, all of these
// different kinds of `fn` declarations are equivalent:
struct S;
fn foo(x: S) { /* ... */ }
extern "C" { fn foo(x: S); }
impl S { fn foo(self) { /* ... */ } }Run

the type of foo is not fn(S), as one might expect. Rather, it is a unique, zero-sized marker type written here as typeof(foo). However, typeof(foo) can be coerced to a function pointer fn(S), so you rarely notice this:

let x: fn(S) = foo; // OK, coercesRun

The reason that this matter is that the type fn(S) is not specific to any particular function: it's a function pointer. So calling x() results in a virtual call, whereas foo() is statically dispatched, because the type of foo tells us precisely what function is being called.

As noted above, coercions mean that most code doesn't have to be concerned with this distinction. However, you can tell the difference when using transmute to convert a fn item into a fn pointer.

This is sometimes done as part of an FFI:

This example deliberately fails to compile
extern "C" fn foo(userdata: Box<i32>) {
    /* ... */
}

let f: extern "C" fn(*mut i32) = transmute(foo);
callback(f);Run

Here, transmute is being used to convert the types of the fn arguments. This pattern is incorrect because, because the type of foo is a function item (typeof(foo)), which is zero-sized, and the target type (fn()) is a function pointer, which is not zero-sized. This pattern should be rewritten. There are a few possible ways to do this:

The same applies to transmutes to *mut fn(), which were observedin practice. Note though that use of this type is generally incorrect. The intention is typically to describe a function pointer, but just fn() alone suffices for that. *mut fn() is a pointer to a fn pointer. (Since these values are typically just passed to C code, however, this rarely makes a difference in practice.)

E0592

No description.

E0593

You tried to supply an Fn-based type with an incorrect number of arguments than what was expected.

Erroneous code example:

This example deliberately fails to compile
fn foo<F: Fn()>(x: F) { }

fn main() {
    // [E0593] closure takes 1 argument but 0 arguments are required
    foo(|y| { });
}Run

E0594

No description.

E0595

Closures cannot mutate immutable captured variables.

Erroneous code example:

This example deliberately fails to compile
let x = 3; // error: closure cannot assign to immutable local variable `x`
let mut c = || { x += 1 };Run

Make the variable binding mutable:

let mut x = 3; // ok!
let mut c = || { x += 1 };Run

E0596

This error occurs because you tried to mutably borrow a non-mutable variable.

Example of erroneous code:

This example deliberately fails to compile
let x = 1;
let y = &mut x; // error: cannot borrow mutablyRun

In here, x isn't mutable, so when we try to mutably borrow it in y, it fails. To fix this error, you need to make x mutable:

let mut x = 1;
let y = &mut x; // ok!Run

E0597

This error occurs because a borrow was made inside a variable which has a greater lifetime than the borrowed one.

Example of erroneous code:

This example deliberately fails to compile
struct Foo<'a> {
    x: Option<&'a u32>,
}

let mut x = Foo { x: None };
let y = 0;
x.x = Some(&y); // error: `y` does not live long enoughRun

In here, x is created before y and therefore has a greater lifetime. Always keep in mind that values in a scope are dropped in the opposite order they are created. So to fix the previous example, just make the y lifetime greater than the x's one:

struct Foo<'a> {
    x: Option<&'a u32>,
}

let y = 0;
let mut x = Foo { x: None };
x.x = Some(&y);Run

E0598

No description.

E0599

This error occurs when a method is used on a type which doesn't implement it:

Erroneous code example:

This example deliberately fails to compile
struct Mouth;

let x = Mouth;
x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
               //        in the current scopeRun

E0600

An unary operator was used on a type which doesn't implement it.

Example of erroneous code:

This example deliberately fails to compile
enum Question {
    Yes,
    No,
}

!Question::Yes; // error: cannot apply unary operator `!` to type `Question`Run

In this case, Question would need to implement the std::ops::Not trait in order to be able to use ! on it. Let's implement it:

use std::ops::Not;

enum Question {
    Yes,
    No,
}

// We implement the `Not` trait on the enum.
impl Not for Question {
    type Output = bool;

    fn not(self) -> bool {
        match self {
            Question::Yes => false, // If the `Answer` is `Yes`, then it
                                    // returns false.
            Question::No => true, // And here we do the opposite.
        }
    }
}

assert_eq!(!Question::Yes, false);
assert_eq!(!Question::No, true);Run

E0601

No main function was found in a binary crate. To fix this error, add a main function. For example:

fn main() {
    // Your program will start here.
    println!("Hello world!");
}Run

If you don't know the basics of Rust, you can go look to the Rust Book to get started: https://doc.rust-lang.org/book/

E0602

An unknown lint was used on the command line.

Erroneous example:

rustc -D bogus omse_file.rs

Maybe you just misspelled the lint name or the lint doesn't exist anymore. Either way, try to update/remove it in order to fix the error.

E0603

A private item was used outside its scope.

Erroneous code example:

This example deliberately fails to compile
mod SomeModule {
    const PRIVATE: u32 = 0x_a_bad_1dea_u32; // This const is private, so we
                                            // can't use it outside of the
                                            // `SomeModule` module.
}

println!("const value: {}", SomeModule::PRIVATE); // error: constant `PRIVATE`
                                                  //        is privateRun

In order to fix this error, you need to make the item public by using the pub keyword. Example:

mod SomeModule {
    pub const PRIVATE: u32 = 0x_a_bad_1dea_u32; // We set it public by using the
                                                // `pub` keyword.
}

println!("const value: {}", SomeModule::PRIVATE); // ok!Run

E0604

A cast to char was attempted on a type other than u8.

Erroneous code example:

This example deliberately fails to compile
0u32 as char; // error: only `u8` can be cast as `char`, not `u32`Run

As the error message indicates, only u8 can be cast into char. Example:

let c = 86u8 as char; // ok!
assert_eq!(c, 'V');Run

For more information about casts, take a look at The Book: https://doc.rust-lang.org/book/first-edition/casting-between-types.html

E0605

An invalid cast was attempted.

Erroneous code examples:

This example deliberately fails to compile
let x = 0u8;
x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`

// Another example

let v = 0 as *const u8; // So here, `v` is a `*const u8`.
v as &u8; // error: non-primitive cast: `*const u8` as `&u8`Run

Only primitive types can be cast into each other. Examples:

let x = 0u8;
x as u32; // ok!

let v = 0 as *const u8;
v as *const i8; // ok!Run

For more information about casts, take a look at The Book: https://doc.rust-lang.org/book/first-edition/casting-between-types.html

E0606

An incompatible cast was attempted.

Erroneous code example:

This example deliberately fails to compile
let x = &0u8; // Here, `x` is a `&u8`.
let y: u32 = x as u32; // error: casting `&u8` as `u32` is invalidRun

When casting, keep in mind that only primitive types can be cast into each other. Example:

let x = &0u8;
let y: u32 = *x as u32; // We dereference it first and then cast it.Run

For more information about casts, take a look at The Book: https://doc.rust-lang.org/book/first-edition/casting-between-types.html

E0607

A cast between a thin and a fat pointer was attempted.

Erroneous code example:

This example deliberately fails to compile
let v = 0 as *const u8;
v as *const [u8];Run

First: what are thin and fat pointers?

Thin pointers are "simple" pointers: they are purely a reference to a memory address.

Fat pointers are pointers referencing Dynamically Sized Types (also called DST). DST don't have a statically known size, therefore they can only exist behind some kind of pointers that contain additional information. Slices and trait objects are DSTs. In the case of slices, the additional information the fat pointer holds is their size.

To fix this error, don't try to cast directly between thin and fat pointers.

For more information about casts, take a look at The Book: https://doc.rust-lang.org/book/first-edition/casting-between-types.html

E0608

An attempt to index into a type which doesn't implement the std::ops::Index trait was performed.

Erroneous code example:

This example deliberately fails to compile
0u8[2]; // error: cannot index into a value of type `u8`Run

To be able to index into a type it needs to implement the std::ops::Index trait. Example:

let v: Vec<u8> = vec![0, 1, 2, 3];

// The `Vec` type implements the `Index` trait so you can do:
println!("{}", v[2]);Run

E0609

Attempted to access a non-existent field in a struct.

Erroneous code example:

This example deliberately fails to compile
struct StructWithFields {
    x: u32,
}

let s = StructWithFields { x: 0 };
println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`Run

To fix this error, check that you didn't misspell the field's name or that the field actually exists. Example:

struct StructWithFields {
    x: u32,
}

let s = StructWithFields { x: 0 };
println!("{}", s.x); // ok!Run

E0610

Attempted to access a field on a primitive type.

Erroneous code example:

This example deliberately fails to compile
let x: u32 = 0;
println!("{}", x.foo); // error: `{integer}` is a primitive type, therefore
                       //        doesn't have fieldsRun

Primitive types are the most basic types available in Rust and don't have fields. To access data via named fields, struct types are used. Example:

// We declare struct called `Foo` containing two fields:
struct Foo {
    x: u32,
    y: i64,
}

// We create an instance of this struct:
let variable = Foo { x: 0, y: -12 };
// And we can now access its fields:
println!("x: {}, y: {}", variable.x, variable.y);Run

For more information about primitives and structs, take a look at The Book: https://doc.rust-lang.org/book/first-edition/primitive-types.html https://doc.rust-lang.org/book/first-edition/structs.html

E0614

Attempted to dereference a variable which cannot be dereferenced.

Erroneous code example:

This example deliberately fails to compile
let y = 0u32;
*y; // error: type `u32` cannot be dereferencedRun

Only types implementing std::ops::Deref can be dereferenced (such as &T). Example:

let y = 0u32;
let x = &y;
// So here, `x` is a `&u32`, so we can dereference it:
*x; // ok!Run

E0615

Attempted to access a method like a field.

Erroneous code example:

This example deliberately fails to compile
struct Foo {
    x: u32,
}

impl Foo {
    fn method(&self) {}
}

let f = Foo { x: 0 };
f.method; // error: attempted to take value of method `method` on type `Foo`Run

If you want to use a method, add () after it:

f.method();Run

However, if you wanted to access a field of a struct check that the field name is spelled correctly. Example:

println!("{}", f.x);Run

E0616

Attempted to access a private field on a struct.

Erroneous code example:

This example deliberately fails to compile
mod some_module {
    pub struct Foo {
        x: u32, // So `x` is private in here.
    }

    impl Foo {
        pub fn new() -> Foo { Foo { x: 0 } }
    }
}

let f = some_module::Foo::new();
println!("{}", f.x); // error: field `x` of struct `some_module::Foo` is privateRun

If you want to access this field, you have two options:

  1. Set the field public:
mod some_module {
    pub struct Foo {
        pub x: u32, // `x` is now public.
    }

    impl Foo {
        pub fn new() -> Foo { Foo { x: 0 } }
    }
}

let f = some_module::Foo::new();
println!("{}", f.x); // ok!Run
  1. Add a getter function:
mod some_module {
    pub struct Foo {
        x: u32, // So `x` is still private in here.
    }

    impl Foo {
        pub fn new() -> Foo { Foo { x: 0 } }

        // We create the getter function here:
        pub fn get_x(&self) -> &u32 { &self.x }
    }
}

let f = some_module::Foo::new();
println!("{}", f.get_x()); // ok!Run

E0617

Attempted to pass an invalid type of variable into a variadic function.

Erroneous code example:

This example deliberately fails to compile
extern {
    fn printf(c: *const i8, ...);
}

unsafe {
    printf(::std::ptr::null(), 0f32);
    // error: can't pass an `f32` to variadic function, cast to `c_double`
}Run

Certain Rust types must be cast before passing them to a variadic function, because of arcane ABI rules dictated by the C standard. To fix the error, cast the value to the type specified by the error message (which you may need to import from std::os::raw).

E0618

Attempted to call something which isn't a function nor a method.

Erroneous code examples:

This example deliberately fails to compile
enum X {
    Entry,
}

X::Entry(); // error: expected function, found `X::Entry`

// Or even simpler:
let x = 0i32;
x(); // error: expected function, found `i32`Run

Only functions and methods can be called using (). Example:

// We declare a function:
fn i_am_a_function() {}

// And we call it:
i_am_a_function();Run

E0619

Note: this error code is no longer emitted by the compiler.

The type-checker needed to know the type of an expression, but that type had not yet been inferred.

Erroneous code example:

This example deliberately fails to compile
let mut x = vec![];
match x.pop() {
    Some(v) => {
        // Here, the type of `v` is not (yet) known, so we
        // cannot resolve this method call:
        v.to_uppercase(); // error: the type of this value must be known in
                          //        this context
    }
    None => {}
}Run

Type inference typically proceeds from the top of the function to the bottom, figuring out types as it goes. In some cases -- notably method calls and overloadable operators like * -- the type checker may not have enough information yet to make progress. This can be true even if the rest of the function provides enough context (because the type-checker hasn't looked that far ahead yet). In this case, type annotations can be used to help it along.

To fix this error, just specify the type of the variable. Example:

let mut x: Vec<String> = vec![]; // We precise the type of the vec elements.
match x.pop() {
    Some(v) => {
        v.to_uppercase(); // Since rustc now knows the type of the vec elements,
                          // we can use `v`'s methods.
    }
    None => {}
}Run

E0620

A cast to an unsized type was attempted.

Erroneous code example:

This example deliberately fails to compile
let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
                                  //        as `[usize]`Run

In Rust, some types don't have a known size at compile-time. For example, in a slice type like [u32], the number of elements is not known at compile-time and hence the overall size cannot be computed. As a result, such types can only be manipulated through a reference (e.g., &T or &mut T) or other pointer-type (e.g., Box or Rc). Try casting to a reference instead:

let x = &[1_usize, 2] as &[usize]; // ok!Run

E0621

This error code indicates a mismatch between the lifetimes appearing in the function signature (i.e., the parameter types and the return type) and the data-flow found in the function body.

Erroneous code example:

This example deliberately fails to compile
fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
                                             //        required in the type of
                                             //        `y`
    if x > y { x } else { y }
}Run

In the code above, the function is returning data borrowed from either x or y, but the 'a annotation indicates that it is returning data only from x. To fix the error, the signature and the body must be made to match. Typically, this is done by updating the function signature. So, in this case, we change the type of y to &'a i32, like so:

fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
    if x > y { x } else { y }
}Run

Now the signature indicates that the function data borrowed from either x or y. Alternatively, you could change the body to not return data from y:

fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
    x
}Run

E0622

An intrinsic was declared without being a function.

Erroneous code example:

This example deliberately fails to compile
#![feature(intrinsics)]
extern "rust-intrinsic" {
    pub static breakpoint : unsafe extern "rust-intrinsic" fn();
    // error: intrinsic must be a function
}

fn main() { unsafe { breakpoint(); } }Run

An intrinsic is a function available for use in a given programming language whose implementation is handled specially by the compiler. In order to fix this error, just declare a function.

E0623

No description.

E0624

A private item was used outside of its scope.

Erroneous code example:

This example deliberately fails to compile
mod inner {
    pub struct Foo;

    impl Foo {
        fn method(&self) {}
    }
}

let foo = inner::Foo;
foo.method(); // error: method `method` is privateRun

Two possibilities are available to solve this issue:

  1. Only use the item in the scope it has been defined:
mod inner {
    pub struct Foo;

    impl Foo {
        fn method(&self) {}
    }

    pub fn call_method(foo: &Foo) { // We create a public function.
        foo.method(); // Which calls the item.
    }
}

let foo = inner::Foo;
inner::call_method(&foo); // And since the function is public, we can call the
                          // method through it.Run
  1. Make the item public:
mod inner {
    pub struct Foo;

    impl Foo {
        pub fn method(&self) {} // It's now public.
    }
}

let foo = inner::Foo;
foo.method(); // Ok!Run

E0625

No description.

E0626

This error occurs because a borrow in a generator persists across a yield point.

This example deliberately fails to compile
let mut b = || {
    let a = &String::new(); // <-- This borrow...
    yield (); // ...is still in scope here, when the yield occurs.
    println!("{}", a);
};
unsafe { b.resume() };Run

At present, it is not permitted to have a yield that occurs while a borrow is still in scope. To resolve this error, the borrow must either be "contained" to a smaller scope that does not overlap the yield or else eliminated in another way. So, for example, we might resolve the previous example by removing the borrow and just storing the integer by value:

let mut b = || {
    let a = 3;
    yield ();
    println!("{}", a);
};
unsafe { b.resume() };Run

This is a very simple case, of course. In more complex cases, we may wish to have more than one reference to the value that was borrowed -- in those cases, something like the Rc or Arc types may be useful.

This error also frequently arises with iteration:

This example deliberately fails to compile
let mut b = || {
  let v = vec![1,2,3];
  for &x in &v { // <-- borrow of `v` is still in scope...
    yield x; // ...when this yield occurs.
  }
};
unsafe { b.resume() };Run

Such cases can sometimes be resolved by iterating "by value" (or using into_iter()) to avoid borrowing:

let mut b = || {
  let v = vec![1,2,3];
  for x in v { // <-- Take ownership of the values instead!
    yield x; // <-- Now yield is OK.
  }
};
unsafe { b.resume() };Run

If taking ownership is not an option, using indices can work too:

let mut b = || {
  let v = vec![1,2,3];
  let len = v.len(); // (*)
  for i in 0..len {
    let x = v[i]; // (*)
    yield x; // <-- Now yield is OK.
  }
};
unsafe { b.resume() };

// (*) -- Unfortunately, these temporaries are currently required.
// See <https://github.com/rust-lang/rust/issues/43122>.Run

E0627

No description.

E0628

No description.

E0629

No description.

E0630

No description.

E0631

No description.

E0632

No description.

E0633

The unwind attribute was malformed.

Erroneous code example:

This example is not tested
#[unwind()] // error: expected one argument
pub extern fn something() {}

fn main() {}Run

The #[unwind] attribute should be used as follows:

NB. The default behavior here is "allowed", but this is unspecified and likely to change in the future.

E0634

No description.

E0637

No description.

E0638

This error indicates that the struct or enum must be matched non-exhaustively as it has been marked as non_exhaustive.

When applied within a crate, downstream users of the crate will need to use the _ pattern when matching enums and use the .. pattern when matching structs.

For example, in the below example, since the enum is marked as non_exhaustive, it is required that downstream crates match non-exhaustively on it.

This example is not tested
use std::error::Error as StdError;

#[non_exhaustive] pub enum Error {
   Message(String),
   Other,
}

impl StdError for Error {
   fn description(&self) -> &str {
        // This will not error, despite being marked as non_exhaustive, as this
        // enum is defined within the current crate, it can be matched
        // exhaustively.
        match *self {
           Message(ref s) => s,
           Other => "other or unknown error",
        }
   }
}Run

An example of matching non-exhaustively on the above enum is provided below:

This example is not tested
use mycrate::Error;

// This will not error as the non_exhaustive Error enum has been matched with a
// wildcard.
match error {
   Message(ref s) => ...,
   Other => ...,
   _ => ...,
}Run

Similarly, for structs, match with .. to avoid this error.

E0639

This error indicates that the struct or enum cannot be instantiated from outside of the defining crate as it has been marked as non_exhaustive and as such more fields/variants may be added in future that could cause adverse side effects for this code.

It is recommended that you look for a new function or equivalent in the crate's documentation.

E0640

No description.

E0641

No description.

E0642

No description.

E0643

This error indicates that there is a mismatch between generic parameters and impl Trait parameters in a trait declaration versus its impl.

This example deliberately fails to compile
trait Foo {
    fn foo(&self, _: &impl Iterator);
}
impl Foo for () {
    fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible
                                          // signature for trait
}Run

E0644

A closure or generator was constructed that references its own type.

Erroneous example:

fn fix<F>(f: &F)
  where F: Fn(&F)
{
  f(&f);
}

fn main() {
  fix(&|y| {
    // Here, when `x` is called, the parameter `y` is equal to `x`.
  });
}

Rust does not permit a closure to directly reference its own type, either through an argument (as in the example above) or by capturing itself through its environment. This restriction helps keep closure inference tractable.

The easiest fix is to rewrite your closure into a top-level function, or into a method. In some cases, you may also be able to have your closure call itself by capturing a &Fn() object or fn() pointer that refers to itself. That is permitting, since the closure would be invoking itself via a virtual call, and hence does not directly reference its own type.

E0645

No description.

E0646

It is not possible to define main with a where clause. Erroneous code example:

This example deliberately fails to compile
fn main() where i32: Copy { // error: main function is not allowed to have
                            // a where clause
}Run

E0647

It is not possible to define start with a where clause. Erroneous code example:

This example deliberately fails to compile
#![feature(start)]

#[start]
fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
    //^ error: start function is not allowed to have a where clause
    0
}Run

E0648

export_name attributes may not contain null characters (\0).

This example deliberately fails to compile
#[export_name="\0foo"] // error: `export_name` may not contain null characters
pub fn bar() {}Run

E0657

No description.

E0658

An unstable feature was used.

Erroneous code example:

This example deliberately fails to compile
#[repr(u128)] // error: use of unstable library feature 'repr128'
enum Foo {
    Bar(u64),
}Run

If you're using a stable or a beta version of rustc, you won't be able to use any unstable features. In order to do so, please switch to a nightly version of rustc (by using rustup).

If you're using a nightly version of rustc, just add the corresponding feature to be able to use it:

#![feature(repr128)]

#[repr(u128)] // ok!
enum Foo {
    Bar(u64),
}Run

E0659

An item usage is ambiguous.

Erroneous code example:

This example deliberately fails to compile
pub mod moon {
    pub fn foo() {}
}

pub mod earth {
    pub fn foo() {}
}

mod collider {
    pub use moon::*;
    pub use earth::*;
}

fn main() {
    collider::foo(); // ERROR: `foo` is ambiguous
}Run

This error generally appears when two items with the same name are imported into a module. Here, the foo functions are imported and reexported from the collider module and therefore, when we're using collider::foo(), both functions collide.

To solve this error, the best solution is generally to keep the path before the item when using it. Example:

pub mod moon {
    pub fn foo() {}
}

pub mod earth {
    pub fn foo() {}
}

mod collider {
    pub use moon;
    pub use earth;
}

fn main() {
    collider::moon::foo(); // ok!
    collider::earth::foo(); // ok!
}Run

E0666

No description.

E0667

No description.

E0687

No description.

E0688

No description.

E0689

This error indicates that the numeric value for the method being passed exists but the type of the numeric value or binding could not be identified.

The error happens on numeric literals:

This example deliberately fails to compile
2.0.neg();Run

and on numeric bindings without an identified concrete type:

This example deliberately fails to compile
let x = 2.0;
x.neg();  // same error as aboveRun

Because of this, you must give the numeric literal or binding a type:

use std::ops::Neg;

let _ = 2.0_f32.neg();
let x: f32 = 2.0;
let _ = x.neg();
let _ = (2.0 as f32).neg();Run

E0690

A struct with the representation hint repr(transparent) had zero or more than on fields that were not guaranteed to be zero-sized.

Erroneous code example:

This example deliberately fails to compile
#[repr(transparent)]
struct LengthWithUnit<U> { // error: transparent struct needs exactly one
    value: f32,            //        non-zero-sized field, but has 2
    unit: U,
}Run

Because transparent structs are represented exactly like one of their fields at run time, said field must be uniquely determined. If there is no field, or if there are multiple fields, it is not clear how the struct should be represented. Note that fields of zero-typed types (e.g., PhantomData) can also exist alongside the field that contains the actual data, they do not count for this error. When generic types are involved (as in the above example), an error is reported because the type parameter could be non-zero-sized.

To combine repr(transparent) with type parameters, PhantomData may be useful:

use std::marker::PhantomData;

#[repr(transparent)]
struct LengthWithUnit<U> {
    value: f32,
    unit: PhantomData<U>,
}Run

E0691

A struct with the repr(transparent) representation hint contains a zero-sized field that requires non-trivial alignment.

Erroneous code example:

This example deliberately fails to compile
#![feature(repr_align, attr_literals)]

#[repr(align(32))]
struct ForceAlign32;

#[repr(transparent)]
struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
                                   //        struct has alignment larger than 1Run

A transparent struct is supposed to be represented exactly like the piece of data it contains. Zero-sized fields with different alignment requirements potentially conflict with this property. In the example above, Wrapper would have to be aligned to 32 bytes even though f32 has a smaller alignment requirement.

Consider removing the over-aligned zero-sized field:

#[repr(transparent)]
struct Wrapper(f32);Run

Alternatively, PhantomData<T> has alignment 1 for all T, so you can use it if you need to keep the field for some reason:

#![feature(repr_align, attr_literals)]

use std::marker::PhantomData;

#[repr(align(32))]
struct ForceAlign32;

#[repr(transparent)]
struct Wrapper(f32, PhantomData<ForceAlign32>);Run

Note that empty arrays [T; 0] have the same alignment requirement as the element type T. Also note that the error is conservatively reported even when the alignment of the zero-sized type is less than or equal to the data field's alignment.

E0692

A repr(transparent) type was also annotated with other, incompatible representation hints.

Erroneous code example:

This example deliberately fails to compile
#[repr(transparent, C)] // error: incompatible representation hints
struct Grams(f32);Run

A type annotated as repr(transparent) delegates all representation concerns to another type, so adding more representation hints is contradictory. Remove either the transparent hint or the other hints, like this:

#[repr(transparent)]
struct Grams(f32);Run

Alternatively, move the other attributes to the contained type:

#[repr(C)]
struct Foo {
    x: i32,
    // ...
}

#[repr(transparent)]
struct FooWrapper(Foo);Run

Note that introducing another struct just to have a place for the other attributes may have unintended side effects on the representation:

#[repr(transparent)]
struct Grams(f32);

#[repr(C)]
struct Float(f32);

#[repr(transparent)]
struct Grams2(Float); // this is not equivalent to `Grams` aboveRun

Here, Grams2 is a not equivalent to Grams -- the former transparently wraps a (non-transparent) struct containing a single float, while Grams is a transparent wrapper around a float. This can make a difference for the ABI.

E0693

No description.

E0694

No description.

E0695

A break statement without a label appeared inside a labeled block.

Example of erroneous code:

This example deliberately fails to compile
loop {
    'a: {
        break;
    }
}Run

Make sure to always label the break:

'l: loop {
    'a: {
        break 'l;
    }
}Run

Or if you want to break the labeled block:

loop {
    'a: {
        break 'a;
    }
    break;
}Run

E0696

No description.

E0697

No description.

E0698

No description.

E0699

A method was called on a raw pointer whose inner type wasn't completely known.

For example, you may have done something like:

This example deliberately fails to compile
let foo = &1;
let bar = foo as *const _;
if bar.is_null() {
    // ...
}Run

Here, the type of bar isn't known; it could be a pointer to anything. Instead, specify a type for the pointer (preferably something that makes sense for the thing you're pointing to):

let foo = &1;
let bar = foo as *const i32;
if bar.is_null() {
    // ...
}Run

Even though is_null() exists as a method on any raw pointer, Rust shows this error because Rust allows for self to have arbitrary types (behind the arbitrary_self_types feature flag).

This means that someone can specify such a function:

This example is not tested
impl Foo {
    fn is_null(self: *const Self) -> bool {
        // do something else
    }
}Run

and now when you call .is_null() on a raw pointer to Foo, there's ambiguity.

Given that we don't know what type the pointer is, and there's potential ambiguity for some types, we disallow calling methods on raw pointers when the type is unknown.

E0700

The impl Trait return type captures lifetime parameters that do not appear within the impl Trait itself.

Erroneous code example:

use std::cell::Cell;

trait Trait<'a> { }

impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }

fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
where 'x: 'y
{
    x
}

Here, the function foo returns a value of type Cell<&'x u32>, which references the lifetime 'x. However, the return type is declared as impl Trait<'y> -- this indicates that foo returns "some type that implements Trait<'y>", but it also indicates that the return type only captures data referencing the lifetime 'y. In this case, though, we are referencing data with lifetime 'x, so this function is in error.

To fix this, you must reference the lifetime 'x from the return type. For example, changing the return type to impl Trait<'y> + 'x would work:

use std::cell::Cell;

trait Trait<'a> { }

impl<'a,'b> Trait<'b> for Cell<&'a u32> { }

fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
where 'x: 'y
{
    x
}Run

E0701

This error indicates that a #[non_exhaustive] attribute was incorrectly placed on something other than a struct or enum.

Examples of erroneous code:

This example deliberately fails to compile

#[non_exhaustive]
trait Foo { }Run

E0702

This error indicates that a #[non_exhaustive] attribute had a value. The #[non_exhaustive] should be empty.

Examples of erroneous code:

This example deliberately fails to compile

#[non_exhaustive(anything)]
struct Foo;Run

E0703

No description.

E0704

No description.

E0706

No description.

E0707

No description.

E0708

No description.

E0709

No description.

E0710

No description.