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 covered
Run

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

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

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.

Erroneous code example:

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

let x = Some("s".to_string());

match x {
    op_string @ Some(s) => {}, // error: use of moved value
    None => {},
}
Run

See also the error E0303.

E0009

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

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:

#![feature(move_ref_pattern)]

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.

Erroneous code example:

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

In this example, Y cannot refer to X here. 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

E0014

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

Constants can only be initialized by a constant value or, in a future version of Rust, a call to a const function. This error indicates the use of a path (like a::b, or x) denoting something other than one of these allowed items.

Erroneous code example:

const FOO: i32 = { let x = 0; x }; // 'x' isn't a constant nor a function!
Run

To avoid it, you have to replace the non-constant value:

const FOO: i32 = { const X : i32 = 0; X };
// or even:
const FOO2: i32 = { 0 }; // but brackets are useless here
Run

E0015

A constant item was initialized with something that is not a constant expression.

Erroneous code example:

This example deliberately fails to compile
fn create_some() -> Option<u8> {
    Some(1)
}

const FOO: Option<u8> = create_some(); // error!
Run

The only functions that can be called in static or constant expressions are const functions, and struct/enum constructors.

To fix this error, you can declare create_some as a constant function:

const fn create_some() -> Option<u8> { // declared as a const function
    Some(1)
}

const FOO: Option<u8> = create_some(); // ok!

// These are also working:
struct Bar {
    x: u8,
}

const OTHER_FOO: Option<u8> = Some(1);
const BAR: Bar = Bar {x: 1};
Run

E0023

A pattern attempted to extract an incorrect number of fields from a variant.

Erroneous code example:

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

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

match x {
    Fruit::Apple(a) => {}, // error!
    _ => {}
}
Run

A pattern used to match against an enum variant must provide a sub-pattern for each field of the enum variant.

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

A struct pattern attempted to extract a non-existent field from a struct.

Erroneous code example:

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

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

match thing {
    Thing { x, z } => {} // error: `Thing::z` field doesn't exist
}
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. Struct fields are identified by the name used before the colon : so struct patterns should resemble the declaration of the struct type being matched.

struct Thing {
    x: u32,
    y: u32,
}

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

match thing {
    Thing { x, y: z } => {} // we renamed `y` to `z`
}
Run

E0027

A pattern for a struct fails to specify a sub-pattern for every one of the struct's fields.

Erroneous code 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

To fix this error, ensure that each field from the struct's definition is mentioned in the pattern, or use .. to ignore unwanted fields. Example:

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

Something other than numbers and characters has been used for a range.

Erroneous code example:

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

// The ordering relation for strings cannot 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

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.

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.

Erroneous code 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

A trait type has been dereferenced.

Erroneous code example:

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

A pointer to a trait type cannot be implicitly dereferenced by a pattern. Every trait defines a type, but because the size of trait implementers 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.

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 parameters 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 parameterized).

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 implementations
Run

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: ?Sized> {}

trait Trait: Super<Self> {
}

struct Foo;

impl Super<Foo> for Foo{}

impl Trait for Foo {}

fn main() {
    let x: Box<dyn Trait>;
}
Run

Here, the supertrait might have methods as follows:

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

If the trait Trait 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.

Erroneous code example:

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

It is unnecessary to do this since drop is called automatically whenever a value goes out of scope. However, if you really need to drop a value by hand, you can use the std::mem::drop function:

struct Foo {
    x: i32,
}
impl Drop for Foo {
    fn drop(&mut self) {
        println!("kaboom");
    }
}
fn main() {
    let mut x = Foo { x: -7 };
    drop(x); // ok!
}
Run

E0044

You cannot use type or const 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 generic parameter with the specializations that you need:

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

E0045

Variadic parameters have been used on a non-C ABI function.

Erroneous code example:

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

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

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. 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

An attempted implementation of a trait method has the wrong number of type or const parameters.

Erroneous code example:

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

For example, the Foo trait has a method foo with a type parameter T, but the implementation of foo for the type Bar is missing this parameter. To fix this error, they must have the same type parameters:

trait Foo {
    fn foo<T: Default>(x: T) -> Self;
}

struct Bar;

impl Foo for Bar {
    fn foo<T: Default>(x: T) -> Self { // ok!
        Bar
    }
}
Run

E0050

An attempted implementation of a trait method has the wrong number of function parameters.

Erroneous code example:

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

For example, the Foo trait has a method foo with two function parameters (&self and u8), but the implementation of foo for the type Bar omits the u8 parameter. To fix this error, they must have the same parameters:

trait Foo {
    fn foo(&self, x: u8) -> bool;
}

struct Bar;

impl Foo for Bar {
    fn foo(&self, x: u8) -> bool { // ok!
        true
    }
}
Run

E0053

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

Erroneous code example:

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.

Erroneous code example:

This example deliberately fails to compile
let x = 5;

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

If you are trying to cast a numeric type to a bool, you can compare it with zero instead:

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="4"]

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

An invalid number of arguments was given when calling a closure.

Erroneous code example:

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 parameters
Run

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.

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:

This example deliberately fails to compile
use std::os::raw::{c_char, c_int};

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

unsafe { printf(); } // error!
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

An invalid number of arguments was passed when calling a function.

Erroneous code example:

This example deliberately fails to compile
fn f(u: i32) {}

f(); // error!
Run

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

A struct's or struct-like enum variant's field 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

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. Each field should be specified exactly one time. Example:

struct Foo {
    x: i32,
}

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

E0063

A struct's or struct-like enum variant's field 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

An invalid left-hand side expression was used on an assignment operation.

Erroneous code example:

This example deliberately fails to compile
12 += 1; // error!
Run

You need to have a place expression to be able to assign it something. For example:

let mut x: i8 = 12;
x += 1; // ok!
Run

E0069

The compiler found a function whose body contains a return; statement but whose return type is not ().

Erroneous code example:

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

An assignment operator was used on a non-place expression.

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 cannot assign value to a function!
    SomeStruct::x = 12; // error: SomeStruct a structure name but it is used
                        //        like a variable!
}
Run

The left-hand side of an assignment operator must be a place expression. A 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.

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

A structure-literal syntax was used 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

A recursive type has infinite size because it doesn't have an indirection.

Erroneous code example:

This example deliberately fails to compile
struct ListNode {
    head: u8,
    tail: Option<ListNode>, // error: no indirection here so impossible to
                            //        compute the type's size
}
Run

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 Rc). 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.

In the example, the 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> } // error
Run

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

A #[simd] attribute was applied to an empty tuple struct.

Erroneous code example:

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

#[repr(simd)]
struct Bad; // error!
Run

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.

Fixed example:

#![feature(repr_simd)]

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

E0076

All types in a tuple struct aren't the same when using the #[simd] attribute.

Erroneous code example:

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

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

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.

Fixed example:

#![feature(repr_simd)]

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

E0077

A tuple struct's element isn't a machine type when using the #[simd] attribute.

Erroneous code example:

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

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

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.

Fixed example:

#![feature(repr_simd)]

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

E0080

A constant value failed to get evaluated.

Erroneous code example:

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

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

Ensure that the expressions given can be evaluated as the desired integer type.

See the Custom Discriminants section of the Reference for more information about setting custom integer types on fieldless enums using the repr attribute.

E0081

A discriminant value is present more than once.

Erroneous code example:

This example deliberately fails to compile
enum Enum {
    P = 3,
    X = 3, // error!
    Y = 5,
}
Run

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 it impossible to distinguish them.

enum Enum {
    P,
    X = 3, // ok!
    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, // error!
}
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 enum
Run

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 representation of your enum:

enum NightsWatch {}
Run

E0087

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

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

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

fn main() {
    foo::<f64, bool>(); // error: wrong number of type arguments:
                        //        expected 1, found 2
}
Run

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

E0088

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

You gave too many lifetime arguments. Erroneous code example:

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

fn main() {
    f::<'static>() // error: wrong number of lifetime arguments:
                   //        expected 0, found 1
}
Run

Please check you give the right number of lifetime arguments. 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

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

Too few type arguments were supplied for a function. For example:

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

fn main() {
    foo::<f64>(); // error: wrong number of type arguments: expected 2, found 1
}
Run

Note that if a function takes multiple type arguments 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: wrong number of type arguments:
                      //        expected 2, found 1
    foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
}
Run

E0090

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

You gave too few lifetime arguments. Example:

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

fn main() {
    foo::<'static>(); // error: wrong number of lifetime arguments:
                      //        expected 2, found 1
}
Run

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

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

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

E0091

An unnecessary type or const parameter was given 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 unused
Run

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

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

E0092

An undefined atomic operation function was declared.

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 compiler/rustc_codegen_llvm/src/intrinsic.rs and in library/core/src/intrinsics.rs in the Rust source code. Example:

#![feature(intrinsics)]

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

E0093

An unknown intrinsic function was declared.

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 compiler/rustc_codegen_llvm/src/intrinsic.rs and in library/core/src/intrinsics.rs in the Rust source code. Example:

#![feature(intrinsics)]

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

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

E0094

An invalid number of type parameters was given 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).

Erroneous code examples:

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; // correct
Run

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

E0107

An incorrect number of generic arguments were provided.

Erroneous code example:

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

struct Bar { x: Foo }             // error: wrong number of type arguments:
                                  //        expected 1, found 0
struct Baz<S, T> { x: Foo<S, T> } // error: wrong number of type arguments:
                                  //        expected 1, found 2

fn foo<T, U>(x: T, y: U) {}
fn f() {}

fn main() {
    let x: bool = true;
    foo::<bool>(x);                 // error: wrong number of type arguments:
                                    //        expected 2, found 1
    foo::<bool, i32, i32>(x, 2, 4); // error: wrong number of type arguments:
                                    //        expected 2, found 3
    f::<'static>();                 // error: wrong number of lifetime arguments
                                    //        expected 0, found 1
}
Run

When using/declaring an item with generic arguments, you must provide the exact same number:

struct Foo<T> { x: T }

struct Bar<T> { x: Foo<T> }               // ok!
struct Baz<S, T> { x: Foo<S>, y: Foo<T> } // ok!

fn foo<T, U>(x: T, y: U) {}
fn f() {}

fn main() {
    let x: bool = true;
    foo::<bool, u32>(x, 12);              // ok!
    f();                                  // ok!
}
Run

E0109

You tried to provide a generic argument to a type which doesn't need it.

Erroneous code example:

This example deliberately fails to compile
type X = u32<i32>; // error: type arguments are not allowed for this type
type Y = bool<'static>; // error: lifetime parameters are not allowed on
                        //        this type
Run

Check that you used the correct argument and that the definition is correct.

Example:

type X = u32; // ok!
type Y = bool; // ok!
Run

Note that generic arguments for enum variant constructors go after the variant, not after the enum. For example, you would write Option::None::<u32>, rather than Option::<u32>::None.

E0110

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

You tried to provide a lifetime to a type which doesn't need it. See E0109 for more details.

E0116

An inherent implementation was defined for a type outside the current crate.

Erroneous code example:

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

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 above is not allowed since Vec is defined in the standard library.

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 above
Run

E0117

Only traits defined in the current crate can be implemented for arbitrary types.

Erroneous code example:

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

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

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

An inherent implementation was defined for something which isn't a struct, enum, union, or trait object.

Erroneous code example:

This example deliberately fails to compile
impl (u8, u8) { // error: no nominal 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

Instead of defining an inherent implementation on a reference, you could also move the reference inside the implementation:

This example deliberately fails to compile
struct Foo;

impl &Foo { // error: no nominal type found for inherent implementation
    fn bar(self, other: Self) {}
}
Run

becomes

struct Foo;

impl Foo {
    fn bar(&self, other: &Self) {}
}
Run

E0119

There are conflicting trait implementations for the same type.

Erroneous code example:

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

Drop was implemented on a trait, which is not allowed: only structs and enums can implement Drop.

Erroneous code example:

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:

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:

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

The type placeholder _ was used within a type on an item's signature.

Erroneous code example:

This example deliberately fails to compile
fn foo() -> _ { 5 } // error

static BAR: _ = "test"; // error
Run

In those cases, you need to provide the type explicitly:

fn foo() -> i32 { 5 } // ok!

static BAR: &str = "test"; // ok!
Run

The type placeholder _ can be used outside item's signature as follows:

let x = "a4a".split('4')
    .collect::<Vec<_>>(); // No need to precise the Vec's generic type.
Run

E0124

A struct was declared with two fields having 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

A type parameter with default value is using forward declared identifier.

Erroneous code example:

This example deliberately fails to compile
struct Foo<T = U, U = ()> {
    field1: T,
    field2: U,
}
// error: type parameters with a default cannot use forward declared
//        identifiers
Run

Type parameter defaults can only use parameters that occur before them. Since type parameters are evaluated in-order, this issue could be fixed by doing:

struct Foo<U = (), T = U> {
    field1: T,
    field2: U,
}
Run

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

E0130

A pattern was declared 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

To fix this error, 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

The main function was defined with generic parameters.

Erroneous code example:

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

It is not possible to define the main function with generic parameters. It must not take any arguments.

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, view the unstable book):

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 the unsafe section of the Book for more details.

E0136

More than one main function was found.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    // ...
}

// ...

fn main() { // error!
    // ...
}
Run

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

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]` attribute
Run

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' functions
Run

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 = "owned_box"]
struct Foo; // error: duplicate lang item found: `owned_box`
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 the unstable book.

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.

E0158

An associated const has been referenced in a pattern.

Erroneous code example:

This example deliberately fails to compile
enum EFoo { A, B, C, D }

trait Foo {
    const X: EFoo;
}

fn test<A: Foo>(arg: EFoo) {
    match arg {
        A::X => { // error!
            println!("A::X");
        }
    }
}
Run

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 whose size was not known at compile time.

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

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

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:

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

Something which is neither a tuple struct nor a tuple variant was used as a pattern.

Erroneous code example:

This example deliberately fails to compile
enum A {
    B,
    C,
}

impl A {
    fn new() {}
}

fn bar(foo: A) {
    match foo {
        A::new() => (), // error!
        _ => {}
    }
}
Run

This error means that an attempt was made to match something which is neither a tuple struct nor a tuple variant. Only these two elements are allowed as a pattern:

enum A {
    B,
    C,
}

impl A {
    fn new() {}
}

fn bar(foo: A) {
    match foo {
        A::B => (), // ok!
        _ => {}
    }
}
Run

E0165

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

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:

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

A pattern binding is using the same name as one of the variants of a type.

Erroneous code example:

This example deliberately fails to compile
enum Method {
    GET,
    POST,
}

fn is_empty(s: Method) -> bool {
    match s {
        GET => true,
        _ => false
    }
}

fn main() {}
Run

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

The + type operator was used in an ambiguous context.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

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

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

trait Foo {}

struct Bar<'a> {
    x: &'a (Foo + 'a),     // ok!
    y: &'a mut (Foo + 'a), // ok!
    z: fn() -> (Foo + 'a), // ok!
}
Run

More details can be found in RFC 438.

E0183

No description.

E0184

The Copy trait was implemented on a type with a Drop implementation.

Erroneous code example:

This example deliberately fails to compile
#[derive(Copy)]
struct Foo; // error!

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

Explicitly implementing both Drop and Copy trait on 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).

Erroneous code example:

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

When a type implements a trait's associated function, it has to use the same signature. So in this case, since Foo::foo does not take any argument and does not return anything, its implementation on Bar should be the same:

trait Foo {
    fn foo();
}

struct Bar;

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

Erroneous code example:

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

When a type implements a trait's associated function, it has to use the same signature. So in this case, since Foo::foo takes self as argument and does not return anything, its implementation on Bar should be the same:

trait Foo {
    fn foo(&self);
}

struct Bar;

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

E0191

An associated type wasn't specified for a trait object.

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 specified
Run

Trait objects need to have all associated types specified. Please verify that all associated types of the trait were specified and the correct trait was used. Example:

trait Trait {
    type Bar;
}

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

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.

E0195

The lifetime parameters of the method 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

An inherent implementation was marked unsafe.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

unsafe impl Foo { } // error!
Run

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.

struct Foo;

impl Foo { } // ok!
Run

E0198

A negative implementation was marked as unsafe.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

unsafe impl !Clone for Foo { } // error!
Run

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 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

A trait implementation was marked as unsafe while the trait is safe.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

trait Bar { }

unsafe impl Bar for Foo { } // error!
Run

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:

struct Foo;

trait Bar { }

impl Bar for Foo { } // ok!
Run

E0200

An unsafe trait was implemented without an unsafe implementation.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

unsafe trait Bar { }

impl Bar for Foo { } // error!
Run

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.

struct Foo;

unsafe trait Bar { }

unsafe impl Bar for Foo { } // ok!
Run

E0201

Two associated items (like methods, associated types, associated functions, etc.) were defined with the same identifier.

Erroneous code 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.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

impl Foo {
    type Bar = isize; // error!
}
Run

E0203

Having multiple relaxed default bounds is unsupported.

Erroneous code example:

This example deliberately fails to compile
struct Bad<T: ?Sized + ?Send>{
    inner: T
}
Run

Here the type T cannot have a relaxed bound for multiple default traits (Sized and Send). This can be fixed by only using one relaxed bound.

struct Good<T: ?Sized>{
    inner: T
}
Run

E0204

The Copy trait was implemented on a type which contains a field that doesn't implement the Copy trait.

Erroneous code example:

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

impl Copy for Foo { } // error!
Run

The Copy trait is implemented by default only on primitive types. If your type only contains primitive types, you'll be able to implement Copy on it. Otherwise, it won't be possible.

Here's another example that will fail:

This example deliberately fails to compile
#[derive(Copy)] // error!
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).

E0205

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

An attempt to implement the Copy trait for an enum failed because one of the variants does not implement Copy. To fix this, you must implement Copy for the mentioned variant. Note that this may not be possible, as in the example of

This example deliberately fails to compile
enum Foo {
    Bar(Vec<u32>),
    Baz,
}

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)]
enum Foo<'a> {
    Bar(&'a mut bool),
    Baz,
}
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

The Copy trait was implemented on a type which is neither a struct nor an enum.

Erroneous code example:

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 { } // error!
Run

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

E0207

A type parameter that is specified for impl is not constrained.

Erroneous code example:

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

Any type parameter 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 previous code example has a definition which leads to a compiler error:

The problem is that the parameter T does not appear in the implementing 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.

When implementing a foreign trait for a foreign type, the trait must have one or more type parameters. A type local to your crate must appear before any use of any type parameters.

To understand what this means, it is perhaps easier 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 { } // error
Run

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

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

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 { } // error
Run

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 { } // Ok
Run

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).

Both of the following must be true:

  1. At least one of the types T0..=Tn must be a local type. Let Ti be the first such type.
  2. No uncovered type parameters P1..=Pm may appear in T0..Ti (excluding Ti).

For information on the design of the orphan rules, see RFC 2451 and RFC 1023.

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

E0211

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

You used a function or type which doesn't fit the requirements for where it was used. Erroneous code examples:

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

extern "rust-intrinsic" {
    fn size_of<T>(); // error: intrinsic has wrong type
}

// or:

fn main() -> i32 { 0 }
// error: main function expects type: `fn() {main}`: expected (), found i32

// or:

let x = 1u8;
match x {
    0u8..=3i8 => (),
    // error: mismatched types in range: expected u8, found i8
    _ => ()
}

// or:

use std::rc::Rc;
struct Foo;

impl Foo {
    fn x(self: Rc<Foo>) {}
    // error: mismatched self type: expected `Foo`: expected struct
    //        `Foo`, found struct `alloc::rc::Rc`
}
Run

For the first code example, please check the function definition. Example:

#![feature(intrinsics)]

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

The second case example is a bit particular: the main function must always have this definition:

This example deliberately fails to compile
fn main();
Run

They never take parameters and never return types.

For the third example, when you match, all patterns must have the same type as the type you're matching on. Example:

let x = 1u8;

match x {
    0u8..=3u8 => (), // ok!
    _ => ()
}
Run

And finally, for the last example, only Box<Self>, &Self, Self, or &mut Self work as explicit self parameters. Example:

struct Foo;

impl Foo {
    fn x(self: Box<Foo>) {} // ok!
}
Run

E0212

No description.

E0214

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

Erroneous code example:

This example deliberately fails to compile
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.

The previous code example fixed:

let v: Vec<&str> = vec!["foo"];
Run

E0220

The associated type used was not 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.

Erroneous code 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

E0222

An attempt was made to constrain an associated type.

Erroneous code example:

This example deliberately fails to compile
pub trait Vehicle {
    type Color;
}

pub trait Box {
    type Color;
}

pub trait BoxCar : Box + Vehicle {}

fn dent_object<COLOR>(c: dyn BoxCar<Color=COLOR>) {} // Invalid constraint
Run

In this example, BoxCar has two super-traits: Vehicle and Box. Both of these traits define an associated type Color. BoxCar inherits two types with that name from both super-traits. Because of this, we need to use the fully qualified path syntax to refer to the appropriate Color associated type, either <BoxCar as Vehicle>::Color or <BoxCar as Box>::Color, but this syntax is not allowed to be used in a function signature.

In order to encode this kind of constraint, a where clause and a new type parameter are needed:

pub trait Vehicle {
    type Color;
}

pub trait Box {
    type Color;
}

pub trait BoxCar : Box + Vehicle {}

// Introduce a new `CAR` type parameter
fn foo<CAR, COLOR>(
    c: CAR,
) where
    // Bind the type parameter `CAR` to the trait `BoxCar`
    CAR: BoxCar,
    // Further restrict `<BoxCar as Vehicle>::Color` to be the same as the
    // type parameter `COLOR`
    CAR: Vehicle<Color = COLOR>,
    // We can also simultaneously restrict the other trait's associated type
    CAR: Box<Color = COLOR>
{}
Run

E0223

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

Erroneous code 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

A trait object was declared with no traits.

Erroneous code example:

This example deliberately fails to compile
type Foo = dyn 'static +;
Run

Rust does not currently support this.

To solve, ensure that the trait object has at least one trait:

type Foo = dyn 'static + Copy;
Run

E0225

Multiple types were used as bounds for a closure or trait object.

Erroneous code example:

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

Rust does not currently support this.

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

More than one explicit lifetime bound was used on a trait object.

Example of erroneous code:

This example deliberately fails to compile
trait Foo {}

type T<'a, 'b> = dyn Foo + 'a + 'b; // error: Trait object `arg` has two
                                    //        lifetime bound, 'a and 'b.
Run

Here T is a trait object with two explicit lifetime bounds, 'a and 'b.

Only a single explicit lifetime bound is permitted on trait objects. To fix this error, consider removing one of the lifetime bounds:

trait Foo {}

type T<'a> = dyn Foo + 'a;
Run

E0227

No description.

E0228

The lifetime bound for this object type cannot be deduced from context and must be specified.

Erroneous code example:

This example deliberately fails to compile
trait Trait { }

struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
    x: &'a i32,
    y: &'b i32,
    z: T,
}

type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>;
Run

When a trait object is used as a type argument of a generic type, Rust will try to infer its lifetime if unspecified. However, this isn't possible when the containing type has more than one lifetime bound.

The above example can be resolved by either reducing the number of lifetime bounds to one or by making the trait object lifetime explicit, like so:

trait Trait { }

struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
    x: &'a i32,
    y: &'b i32,
    z: T,
}

type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + 'b>;
Run

For more information, see RFC 599 and its amendment RFC 1156.

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 here
Run

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(rustc_attrs)]

#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // error
trait BadAnnotation<A> {}
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(rustc_attrs)]

#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error!
trait BadAnnotation<A> {}
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(rustc_attrs)]

#[rustc_on_unimplemented(lorem="")] // error!
trait BadAnnotation {}
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

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

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

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

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 this 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 std 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 std 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.

E0261

An undeclared lifetime was used.

Erroneous code example:

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:

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

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

Impl blocks declare lifetime parameters separately. You need to add lifetime parameters to an impl block if you're implementing a type that has a lifetime parameter of its own. For example:

This example deliberately fails to compile
struct Foo<'a> {
    x: &'a str,
}

// error,  use of undeclared lifetime name `'a`
impl Foo<'a> {
    fn foo<'a>(x: &'a str) {}
}
Run

This is fixed by declaring the impl block like this:

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

// correct
impl<'a> Foo<'a> {
    fn foo(x: &'a str) {}
}
Run

E0262

An invalid name was used for a lifetime parameter.

Erroneous code example:

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

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:

E0263

A lifetime was declared more than once in the same scope.

Erroneous code example:

This example deliberately fails to compile
fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str, z: &'a str) { // error!
}
Run

Two lifetimes cannot have the same name. To fix this example, change the second 'a lifetime into something else ('c for example):

fn foo<'a, 'b, 'c>(x: &'a str, y: &'b str, z: &'c str) { // ok!
}
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/middle/weak_lang_items.rs. Example:

#![feature(lang_items)]

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

E0267

A loop keyword (break or continue) was used 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 closure
Run

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

A loop keyword (break or continue) was used outside of a loop.

Erroneous code example:

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

Without a loop to break out of or continue in, no sensible action can be taken. Please verify that you are using break and continue only in loops. Example:

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

E0271

A type mismatched an associated type of a trait.

Erroneous code example:

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

The issue can be resolved by changing the associated type:

  1. in the foo implementation:
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);
Run
  1. in the Trait implementation for i8:
trait Trait { type AssociatedType; }

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

impl Trait for i8 { type AssociatedType = u32; }

foo(3_i8);
Run

E0275

An evaluation of a trait requirement overflowed.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

struct Bar<T>(T);

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

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

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

A trait implementation has stricter requirements than the trait definition.

Erroneous code 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.

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:

This example deliberately fails to compile
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| { });
}
Run

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

The compiler could not infer a type and asked for a type annotation.

Erroneous code example:

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

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:

In the first code example, 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

An implementation cannot be chosen unambiguously because of lack of information.

Erroneous code 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
    // Should it be Impl or AnotherImpl, maybe something else?
}
Run

This error can be solved by adding type annotations that provide the missing information to the compiler. In this case, the solution is to use a 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

This error occurs when the compiler is unable to unambiguously infer the return type of a function or method which is generic on return type, such as the collect method for Iterators.

For example:

This example deliberately fails to compile
fn main() {
    let n: u32 = 1;
    let mut d: u64 = 2;
    d = d + n.into();
}
Run

Here we have an addition of d and n.into(). Hence, n.into() can return any type T where u64: Add<T>. On the other hand, the into method can return any type where u32: Into<T>.

The author of this code probably wants into() to return a u64, but the compiler can't be sure that there isn't another type T where both u32: Into<T> and u64: Add<T>.

To resolve this error, use a concrete type for the intermediate expression:

fn main() {
    let n: u32 = 1;
    let mut d: u64 = 2;
    let m: u64 = n.into();
    d = d + m;
}
Run

Note that the type of v can now be inferred from the type of temp.

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

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

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

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

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

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

Sub-bindings, e.g. ref x @ Some(ref y) are now allowed under #![feature(bindings_after_at)] and checked to make sure that memory safety is upheld.


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 Issue 14587.

E0307

The self parameter in a method has an invalid "receiver type".

Erroneous code example:

This example deliberately fails to compile
struct Foo;
struct Bar;

trait Trait {
    fn foo(&self);
}

impl Trait for Foo {
    fn foo(self: &Bar) {}
}
Run

Methods take a special first parameter, of which there are three variants: self, &self, and &mut self. These are syntactic sugar for self: Self, self: &Self, and self: &mut Self respectively.

trait Trait {
    fn foo(&self);
//         ^^^^^ `self` here is a reference to the receiver object
}

impl Trait for Foo {
    fn foo(&self) {}
//         ^^^^^ the receiver type is `&Foo`
}
Run

The type Self acts as an alias to the type of the current trait implementer, or "receiver type". Besides the already mentioned Self, &Self and &mut Self valid receiver types, the following are also valid: self: Box<Self>, self: Rc<Self>, self: Arc<Self>, and self: Pin<P> (where P is one of the previous types except Self). Note that Self can also be the underlying implementing type, like Foo in the following example:

impl Trait for Foo {
    fn foo(self: &Foo) {}
}
Run

This error will be emitted by the compiler when using an invalid receiver type, like in the following example:

This example deliberately fails to compile
impl Trait for Foo {
    fn foo(self: &Bar) {}
}
Run

The nightly feature Arbitrary self types extends the accepted set of receiver types to also include any type that can dereference to Self:

#![feature(arbitrary_self_types)]

struct Foo;
struct Bar;

// Because you can dereference `Bar` into `Foo`...
impl std::ops::Deref for Bar {
    type Target = Foo;

    fn deref(&self) -> &Foo {
        &Foo
    }
}

impl Foo {
    fn foo(self: Bar) {}
//         ^^^^^^^^^ ...it can be used as the receiver type
}
Run

E0308

Expected type did not match the received type.

Erroneous code 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

This error occurs when the compiler is unable to infer the concrete type of a variable. It can occur in several cases, the most common being a mismatch between two types: the type the author explicitly assigned, and the type the compiler inferred.

E0309

A parameter type is missing an explicit lifetime bound and may not live long enough.

Erroneous code example:

This example deliberately fails to compile
// This won't compile because the applicable impl of
// `SomeTrait` (below) requires that `T: 'a`, but the struct does
// not have a matching where-clause.
struct Foo<'a, T> {
    foo: <T as SomeTrait<'a>>::Output,
}

trait SomeTrait<'a> {
    type Output;
}

impl<'a, T> SomeTrait<'a> for T
where
    T: 'a,
{
    type Output = u32;
}
Run

The type definition contains some field whose type requires an outlives annotation. Outlives annotations (e.g., T: 'a) are used to guarantee that all the data in T is valid for at least the lifetime 'a. This scenario most commonly arises when the type contains an associated type reference like <T as SomeTrait<'a>>::Output, as shown in the previous code.

There, the where clause T: 'a that appears on the impl is not known to be satisfied on the struct. To make this example compile, you have to add a where-clause like T: 'a to the struct definition:

struct Foo<'a, T>
where
    T: 'a,
{
    foo: <T as SomeTrait<'a>>::Output
}

trait SomeTrait<'a> {
    type Output;
}

impl<'a, T> SomeTrait<'a> for T
where
    T: 'a,
{
    type Output = u32;
}
Run

E0310

A parameter type is missing a lifetime constraint or has a lifetime that does not live long enough.

Erroneous code example:

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

Type parameters in type definitions have lifetimes associated with them that represent how long the data stored within them is guaranteed to 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 will compile, because it has the constraint on the type parameter:

struct Foo<T: 'static> {
    foo: &'static T
}
Run

E0311

No description.

E0312

Reference's lifetime of borrowed content doesn't match the expected lifetime.

Erroneous code example:

This example deliberately fails to compile
pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'static str {
    if maybestr.is_none() {
        "(none)"
    } else {
        let s: &'a str = maybestr.as_ref().unwrap();
        s  // Invalid lifetime!
    }
}
Run

To fix this error, either lessen the expected lifetime or find a way to not have to use this reference outside of its current scope (by running the code directly in the same block for example?):

// In this case, we can fix the issue by switching from "static" lifetime to 'a
pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'a str {
    if maybestr.is_none() {
        "(none)"
    } else {
        let s: &'a str = maybestr.as_ref().unwrap();
        s  // Ok!
    }
}
Run

E0313

No description.

E0314

No description.

E0315

No description.

E0316

No description.

E0317

An if expression is missing an else block.

Erroneous code example:

This example deliberately fails to compile
let x = 5;
let a = if x == 5 {
    1
};
Run

This error occurs when an if expression without an else block is used in a context where a type other than () is expected. In the previous code example, the let expression was expecting a value but since there was no else, no value was returned.

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.

So to fix the previous code example:

let x = 5;
let a = if x == 5 {
    1
} else {
    2
};
Run

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 was implemented explicitly.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

impl Sized for Foo {} // error!
Run

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

An implementation of a trait doesn't match the type constraint.

Erroneous code example:

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

The types of any associated constants in a trait implementation must match the types in the trait definition.

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

E0329

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

An attempt was made to access an associated constant through either a generic type parameter or Self. This is not supported yet. An example causing this error is shown below:

trait Foo {
    const BAR: f64;
}

struct MyStruct;

impl Foo for MyStruct {
    const BAR: f64 = 0f64;
}

fn get_bar_bad<F: Foo>(t: F) -> f64 {
    F::BAR
}
Run

Currently, the value of BAR for a particular type can only be accessed through a concrete type, as shown below:

trait Foo {
    const BAR: f64;
}

struct MyStruct;

impl Foo for MyStruct {
    const BAR: f64 = 0f64;
}

fn get_bar_good() -> f64 {
    <MyStruct as Foo>::BAR
}
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 a {
    fn foo() {}

    mod a {
        pub use super::foo; // error!
    }
}
Run

The solution to this problem is to ensure that the items that you are re-exporting are themselves marked with pub:

mod a {
    pub fn foo() {} // ok!

    mod a {
        pub use super::foo;
    }
}
Run

See the Use Declarations section of the reference for more information on this topic.

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.

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.

Erroneous code example:

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

A binary assignment operator like += or ^= was applied to a type that doesn't support it.

Erroneous code 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
#[repr(i64)]
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:

#[repr(i64)]
enum Foo {
    X = 0x7fffffffffffffff,
    Y = 0, // ok!
}
Run

Or:

#[repr(i64)]
enum Foo {
    Y = 0, // ok!
    X = 0x7fffffffffffffff,
}
Run

E0371

A trait was implemented on another which already automatically implemented it.

Erroneous code examples:

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 OK
Run

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.

E0373

A captured variable in a closure may not live long enough.

Erroneous code example:

This example deliberately fails to compile
fn foo() -> Box<Fn(u32) -> u32> {
    let x = 0u32;
    Box::new(|y| x + y)
}
Run

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 as shown in the previous code example.

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 behavior 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

CoerceUnsized was implemented on a struct which does not contain a field with an unsized type.

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

An unsized type is any type where the compiler does not know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

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

CoerceUnsized was implemented on a struct which contains more than one field with an unsized type.

Erroneous code example:

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

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.

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

CoerceUnsized was implemented on something that isn't a struct.

Erroneous code example:

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

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.

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.

E0378

The DispatchFromDyn trait was implemented on something which is not a pointer or a newtype wrapper around a pointer.

Erroneous code example:

This example deliberately fails to compile
#![feature(dispatch_from_dyn)]
use std::ops::DispatchFromDyn;

struct WrapperExtraField<T> {
    ptr: T,
    extra_stuff: i32,
}

impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
where
    T: DispatchFromDyn<U>,
{}
Run

The DispatchFromDyn trait currently can only be implemented for builtin pointer types and structs that are newtype wrappers around them — that is, the struct must have only one field (except forPhantomData), and that field must itself implement DispatchFromDyn.

#![feature(dispatch_from_dyn, unsize)]
use std::{
    marker::Unsize,
    ops::DispatchFromDyn,
};

struct Ptr<T: ?Sized>(*const T);

impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T>
where
    T: Unsize<U>,
{}
Run

Another example:

#![feature(dispatch_from_dyn)]
use std::{
    ops::DispatchFromDyn,
    marker::PhantomData,
};

struct Wrapper<T> {
    ptr: T,
    _phantom: PhantomData<()>,
}

impl<T, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T>
where
    T: DispatchFromDyn<U>,
{}
Run

E0379

A trait method was declared const.

Erroneous code example:

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

trait Foo {
    const fn bar() -> u32; // error!
}
Run

Trait methods cannot be declared const by design. For more information, see RFC 911.

E0380

An auto trait was declared with a method or an associated item.

Erroneous code example:

This example deliberately fails to compile
unsafe auto trait Trait {
    type Output; // error!
}
Run

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.

Erroneous code 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

A variable was used after its contents have been moved elsewhere.

Erroneous code 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 Understanding Ownership chapter in the Book.

E0383

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

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

An immutable variable was reassigned.

Erroneous code 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

E0386

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

This error occurs when an attempt is made to mutate the target of a mutable reference stored inside an immutable container.

For example, this can happen when storing a &mut inside an immutable Box:

let mut x: i64 = 1;
let y: Box<_> = Box::new(&mut x);
**y = 2; // error, cannot assign to data in an immutable container
Run

This error can be fixed by making the container mutable:

let mut x: i64 = 1;
let mut y: Box<_> = Box::new(&mut x);
**y = 2;
Run

It can also be fixed by using a type with interior mutability, such as Cell or RefCell:

use std::cell::Cell;

let x: i64 = 1;
let y: Box<Cell<_>> = Box::new(Cell::new(x));
y.set(2);
Run

E0387

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

This error occurs when an attempt is made to mutate or mutably reference data that a closure has captured immutably.

Erroneous code example:

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 in the API documentation for Cell.

E0388

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

E0389

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

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)).

Erroneous code example:

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

A method was implemented on 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` primitive
Run

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

A type dependency cycle has been encountered.

Erroneous code example:

This example deliberately fails to compile
trait FirstTrait : SecondTrait {

}

trait SecondTrait : FirstTrait {

}
Run

The previous example contains a circular dependency between two traits: FirstTrait depends on SecondTrait which itself depends on FirstTrait.

E0392

A type or lifetime parameter has been declared but is not actually used.

Erroneous code example:

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.

Erroneous code example:

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

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

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

You implemented a trait, overriding one or more of its associated types but did not reimplement its default methods.

Example of erroneous code:

#![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 or const 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 f<T, T>(s: T, u: T) {} // error: the name `T` is already used for a generic
                          //        parameter in this item's generic parameters
Run

Please verify that none of the type parameters are misspelled, and rename any clashing parameters. Example:

fn f<T, Y>(s: T, u: Y) {} // ok!
Run

Type parameters in an associated item also cannot shadow parameters from the containing item:

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

E0404

A type that is not a trait was used 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 trait
Run

Another erroneous code example:

This example deliberately fails to compile
struct Foo;

fn bar<T: Foo>(t: T) {} // error: `Foo` is not a trait
Run

Please verify that the trait's name was not misspelled or that the right identifier was used. 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 scope
Run

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, trait, or type definition.

Erroneous code example:

This example deliberately fails to compile
<Self>::foo; // error: use of `Self` outside of an impl, trait, or type
             // definition
Run

The Self keyword represents the current type, which explains why it can only be used inside an impl, trait, or type definition. 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

A used type name 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 scope
Run

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 list
Run

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

An identifier that is neither defined nor a struct was used.

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.

Erroneous code example:

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

let f = Foo();
// error: expected function, tuple struct or tuple variant, found `Foo`
// `Foo` is a struct name, but this expression uses it like a function name
Run

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, tuple struct or tuple variant,
// 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 inside of an associated function without a "self receiver" parameter.

Erroneous code example:

This example deliberately fails to compile
struct Foo;

impl Foo {
    // `bar` is a method, because it has a receiver parameter.
    fn bar(&self) {}

    // `foo` is not a method, because it has no receiver parameter.
    fn foo() {
        self.bar(); // error: `self` value is a keyword only available in
                    //        methods with a `self` parameter
    }
}
Run

The self keyword can only be used inside methods, which are associated functions (functions defined inside of a trait or impl block) that have a self receiver as its first parameter, like self, &self, &mut self or self: &mut Pin<Self> (this last one is an example of an "arbitrary self type").

Check if the associated function's parameter list should have contained a self receiver for it to be a method, and add it if so. Example:

struct Foo;

impl Foo {
    fn bar(&self) {}

    fn foo(self) { // `foo` is now a method.
        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 spelled or declared 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 { } list
Run

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 list
Run

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 prefix
Run

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

In Rust 2015, 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.

In Rust 2018, paths in use statements are relative to the current module unless they begin with the name of a crate or a literal crate::, in which case they start from the crate root. As in Rust 2015 code, the self:: and super:: prefixes refer to the current and parent modules 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

If you tried to use a module from an external crate and are using Rust 2015, you may have missed the extern crate declaration (which is usually placed in the crate root):

This code runs with edition 2015
extern crate core; // Required to use the `core` crate in Rust 2015.

use core::any;
Run

In Rust 2018 the extern crate declaration is not required and you can instead just use it:

This code runs with edition 2018
use core::any; // No extern crate required in Rust 2018.
Run

E0433

An undeclared crate, module, or type was used.

Erroneous code example:

This example deliberately fails to compile
let map = HashMap::new();
// error: failed to resolve: use of undeclared type `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

If you've expected to use a crate name:

This example deliberately fails to compile
use ferris_wheel::BigO;
// error: failed to resolve: use of undeclared crate or module `ferris_wheel`
Run

Make sure the crate has been added as a dependency in Cargo.toml.

To use a module from your current crate, add the crate:: prefix to the path.

E0434

A variable used inside an inner function comes from a dynamic 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

Inner functions do not have access to their containing environment. 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 constant
Run

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 was used on something other than a struct.

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

The functional record update syntax is only allowed for structs (struct-like enum variants don't qualify, for example). To fix the previous code, 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

An associated type whose name does not match any of the associated types in the trait was used when implementing the trait.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

impl Foo for i32 {
    type Bar = bool;
}
Run

Trait implementations can only implement associated types that are members of the trait in question.

The solution to this problem is to remove the extraneous associated type:

trait Foo {}

impl Foo for i32 {}
Run

E0438

An associated constant whose name does not match any of the associated constants in the trait was used when implementing the trait.

Erroneous code example:

This example deliberately fails to compile
trait Foo {}

impl Foo for i32 {
    const BAR: bool = true;
}
Run

Trait implementations can only implement associated constants that are members of the trait in question.

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

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 error

fn main() {}
Run

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!

fn main() {}
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)]
struct Bar(u32);

mod foo {
    use crate::Bar;
    pub fn bar() -> Bar { // error: private type in public interface
        Bar(0)
    }
}

fn main() {}
Run

There are two ways to solve this error. The first is to make the public type signature only public to a module that also has access to the private type. This is done by using pub(crate) or pub(in crate::my_mod::etc) Example:

struct Bar(u32);

mod foo {
    use crate::Bar;
    pub(crate) fn bar() -> Bar { // only public to crate root
        Bar(0)
    }
}

fn main() {}
Run

The other way to solve this error is to make the private type public. Example:

pub struct Bar(u32); // we set the Bar type public
mod foo {
    use crate::Bar;
    pub fn bar() -> Bar { // ok!
        Bar(0)
    }
}

fn main() {}
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

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

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 private
Run

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 attribute
Run

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 deliberately fails to compile
#[link(name = "")] extern {}
// error: `#[link(name = "")]` given with empty name
Run

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 example
Run

To solve this error you can use conditional compilation:

#[cfg_attr(target="macos", link(name = "FooCoreServices", kind = "framework"))]
extern {}
Run

Learn more in the Conditional Compilation section of the Reference.

E0457

No description.

E0458

An unknown "kind" was specified for a link attribute.

Erroneous code example:

This example deliberately fails to compile
#[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 deliberately fails to compile
#[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 declaration was 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_crate
Run

If you would like to import all exported macros, write macro_use with no arguments.

E0468

A non-root module tried 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)] // ok!
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

The type does not fulfill the required lifetime.

Erroneous code example:

This example deliberately fails to compile
use std::sync::Mutex;

struct MyString<'a> {
    data: &'a str,
}

fn i_want_static_closure<F>(a: F)
    where F: Fn() + 'static {}

fn print_string<'a>(s: Mutex<MyString<'a>>) {

    i_want_static_closure(move || {     // error: this closure has lifetime 'a
                                        //        rather than 'static
        println!("{}", s.lock().unwrap().data);
    });
}
Run

In this example, the closure does not satisfy the 'static lifetime constraint. To fix this error, you need to double check the lifetime of the type. Here, we can fix this problem by giving s a static lifetime:

use std::sync::Mutex;

struct MyString<'a> {
    data: &'a str,
}

fn i_want_static_closure<F>(a: F)
    where F: Fn() + 'static {}

fn print_string(s: Mutex<MyString<'static>>) {

    i_want_static_closure(move || {     // ok!
        println!("{}", s.lock().unwrap().data);
    });
}
Run

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 'SnowWhite
                                          // must live longer than 'kiss.
    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 Foo<'a> {
    x: fn(&'a i32),
}

trait Trait<'a, 'b> {
    type Out;
}

impl<'a, 'b> Trait<'a, 'b> for usize {
    type Out = &'a Foo<'b>; // error!
}
Run

Here, the problem is that the compiler cannot be sure that the 'b lifetime will live longer than 'a, which should be mandatory in order to be sure that Trait::Out will always have a reference pointing to an existing type. So in this case, we just need to tell the compiler than 'b must outlive 'a:

struct Foo<'a> {
    x: fn(&'a i32),
}

trait Trait<'a, 'b> {
    type Out;
}

impl<'a, 'b: 'a> Trait<'a, 'b> for usize { // we added the lifetime enforcement
    type Out = &'a Foo<'b>; // it now works!
}
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;

const A: AtomicUsize = AtomicUsize::new(0);
static B: &'static AtomicUsize = &A;
// error: cannot borrow a constant which may contain interior mutability,
//        create a static instead
Run

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;

static A: AtomicUsize = AtomicUsize::new(0);
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: &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: &Cell<usize> = &D.a; // error

// or:
const F: &C = &D; // error
Run

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.

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

A value with a custom Drop implementation may be dropped during const-eval.

Erroneous code example:

This example deliberately fails to compile
enum DropType {
    A,
}

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

struct Foo {
    field1: DropType,
}

static FOO: Foo = Foo { field1: (DropType::A, DropType::A).1 }; // error!
Run

The problem here is that if the given type or one of its fields implements the Drop trait, this Drop implementation cannot be called within a const context since it may run arbitrary, non-const-checked code. To prevent this issue, ensure all values with custom a custom Drop implementation escape the initializer.

enum DropType {
    A,
}

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

struct Foo {
    field1: DropType,
}

static FOO: Foo = Foo { field1: DropType::A }; // We initialize all fields
                                               // by hand.
Run

E0495

A lifetime cannot be determined in the given situation.

Erroneous code example:

This example deliberately fails to compile
fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
    match (&t,) { // error!
        ((u,),) => u,
    }
}

let y = Box::new((42,));
let x = transmute_lifetime(&y);
Run

In this code, you have two ways to solve this issue:

  1. Enforce that 'a lives at least as long as 'b.
  2. Use the same lifetime requirement for both input and output values.

So for the first solution, you can do it by replacing 'a with 'a: 'b:

fn transmute_lifetime<'a: 'b, 'b, T>(t: &'a (T,)) -> &'b T {
    match (&t,) { // ok!
        ((u,),) => u,
    }
}
Run

In the second you can do it by simply removing 'b so they both use 'a:

fn transmute_lifetime<'a, T>(t: &'a (T,)) -> &'a T {
    match (&t,) { // ok!
        ((u,),) => u,
    }
}
Run

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

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

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;
x;
// error: cannot borrow `i` as mutable more than once at a time
Run

Please note that in Rust, you can either have many immutable references, or one mutable reference. For more details you may want to read the References & Borrowing section of the Book.

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!
b;
a;
Run

E0500

A borrowed variable was used by a closure.

Erroneous code example:

This example deliberately fails to compile
fn you_know_nothing(jon_snow: &mut i32) {
    let nights_watch = &jon_snow;
    let starks = || {
        *jon_snow = 3; // error: closure requires unique access to `jon_snow`
                       //        but it is already borrowed
    };
    println!("{}", nights_watch);
}
Run

In here, jon_snow is already borrowed by the nights_watch reference, so it cannot be borrowed by the starks closure at the same time. To fix this issue, you can create the closure after the borrow has ended:

fn you_know_nothing(jon_snow: &mut i32) {
    let nights_watch = &jon_snow;
    println!("{}", nights_watch);
    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 starks = || {
        *jon_snow = 3;
    };
    println!("{}", jon_copy);
}
Run

E0501

A mutable variable is used but it is already captured by a closure.

Erroneous code example:

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 mut bar = || {
        inside_closure(a)
    };
    outside_closure(a); // error: cannot borrow `*a` as mutable because previous
                        //        closure requires unique access.
    bar();
}
Run

This error indicates that a mutable variable is used while it is still captured by a closure. Because the closure has borrowed the variable, it is not available 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 the chapter on Capturing in Rust By Example for more information.

To fix this error, you can finish using the closure before using the captured variable:

fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    let mut bar = || {
        inside_closure(a)
    };
    bar();
    // 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 mut 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 mut bar = || {
        inside_closure(a)
    };
    bar();
}
Run

E0502

A variable already borrowed as immutable was borrowed as mutable.

Erroneous code example:

This example deliberately fails to compile
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    let y = &a; // a is borrowed as immutable.
    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
            //        as immutable
    println!("{}", y);
}
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 y = &a; // ok!
    println!("{}", y);
}
Run

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.

E0503

A value was used after it was mutably borrowed.

Erroneous code example:

This example deliberately fails to compile
fn main() {
    let mut value = 3;
    // Create a mutable borrow of `value`.
    let borrow = &mut value;
    let _sum = value + 1; // error: cannot use `value` because
                          //        it was mutably borrowed
    println!("{}", borrow);
}
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 finishing using the borrow before the next use of the value:

fn main() {
    let mut value = 3;
    let borrow = &mut value;
    println!("{}", borrow);
    // 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.
    println!("{}", borrow);
}
Run

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.

E0504

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

This error occurs when an attempt is made to move a borrowed variable into a closure.

Erroneous code example:

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 borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    let _ref_to_val: &Value = &x;
    eat(x);
    borrow(_ref_to_val);
}
Run

Here, the function eat takes ownership of x. However, x cannot be moved because the borrow to _ref_to_val needs to last till the function borrow. To fix that you can do a few different things:

Examples:

struct Value {}

fn borrow(val: &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
    borrow(ref_to_val);
}
Run

Or:

struct Value {}

fn borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};

    let ref_to_val: &Value = &x;
    borrow(ref_to_val);
    // ref_to_val is no longer used.
    eat(x);
}
Run

Or:

#[derive(Clone, Copy)] // implement Copy trait
struct Value {}

fn borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    let ref_to_val: &Value = &x;
    eat(x); // it will be copied here.
    borrow(ref_to_val);
}
Run

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.

E0506

An attempt was made to assign to a borrowed value.

Erroneous code example:

This example deliberately fails to compile
struct FancyNum {
    num: u8,
}

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,
}

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,
}

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 print_fancy_ref(fancy_ref: &FancyNum){
    println!("Ref: {}", fancy_ref.num);
}

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);
Run

E0507

A borrowed value was moved out.

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:

This can also happen when using a type implementing Fn or FnMut, as neither allows moving out of them (they usually represent closures which can be called more than once). Much of the text following applies equally well to non-FnOnce closure bodies.

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

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.

E0508

A value was moved out of a non-copy fixed-size array.

Erroneous code example:

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.

Erroneous code example:

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

E0510

The matched value was assigned in a match guard.

Erroneous code example:

This example deliberately fails to compile
let mut x = Some(0);
match x {
    None => {}
    Some(_) if { x = None; false } => {} // error!
    Some(_) => {}
}
Run

When matching on a variable it cannot be mutated in the match guards, as this could cause the match to be non-exhaustive.

Here executing x = None would modify the value being matched and require us to go "back in time" to the None arm. To fix it, change the value in the match arm:

let mut x = Some(0);
match x {
    None => {}
    Some(_) => {
        x = None; // ok!
    }
}
Run

E0511

Invalid monomorphization of an intrinsic function was used.

Erroneous code example:

This example deliberately fails to compile
#![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: cannot transmute between types of different sizes,
    //        or dependently-sized types
}
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.

E0515

A reference to a local variable was returned.

Erroneous code example:

This example deliberately fails to compile
fn get_dangling_reference() -> &'static i32 {
    let x = 0;
    &x
}
Run
This example deliberately fails to compile
use std::slice::Iter;
fn get_dangling_iterator<'a>() -> Iter<'a, i32> {
    let v = vec![1, 2, 3];
    v.iter()
}
Run

Local variables, function parameters and temporaries are all dropped before the end of the function body. So a reference to them cannot be returned.

Consider returning an owned value instead:

use std::vec::IntoIter;

fn get_integer() -> i32 {
    let x = 0;
    x
}

fn get_owned_iterator() -> IntoIter<i32> {
    let v = vec![1, 2, 3];
    v.into_iter()
}
Run

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

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

An #[inline(..)] attribute was incorrectly placed on something other than a function or method.

Example 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

E0521

No description.

E0522

The lang attribute was used in an invalid context.

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

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).

E0523

No description.

E0524

A variable which requires unique access is being used in more than one closure at the same time.

Erroneous code example:

This example deliberately fails to compile
fn set(x: &mut isize) {
    *x += 4;
}

fn dragoooon(x: &mut isize) {
    let mut c1 = || set(x);
    let mut c2 = || set(x); // error!

    c2();
    c1();
}
Run

To solve this issue, multiple solutions are available. First, is it required for this variable to be used in more than one closure at a time? If it is the case, use reference counted types such as Rc (or Arc if it runs concurrently):

use std::rc::Rc;
use std::cell::RefCell;

fn set(x: &mut isize) {
    *x += 4;
}

fn dragoooon(x: &mut isize) {
    let x = Rc::new(RefCell::new(x));
    let y = Rc::clone(&x);
    let mut c1 = || { let mut x2 = x.borrow_mut(); set(&mut x2); };
    let mut c2 = || { let mut x2 = y.borrow_mut(); set(&mut x2); }; // ok!

    c2();
    c1();
}
Run

If not, just run closures one at a time:

fn set(x: &mut isize) {
    *x += 4;
}

fn dragoooon(x: &mut isize) {
    { // This block isn't necessary since non-lexical lifetimes, it's just to
      // make it more clear.
        let mut c1 = || set(&mut *x);
        c1();
    } // `c1` has been dropped here so we're free to use `x` again!
    let mut c2 = || set(&mut *x);
    c2();
}
Run

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 better understand how these work in Rust, read the Closures chapter of the Book.

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 ..:

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
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 ..:

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

An unknown tuple struct/variant has been used.

Erroneous code example:

This example deliberately fails to compile
let Type(x) = Type(12); // error!
match Bar(12) {
    Bar(x) => {} // error!
    _ => {}
}
Run

In most cases, it's either a forgotten import or a typo. However, let's look at how you can have such a type:

This code runs with edition 2018
struct Type(u32); // this is a tuple struct

enum Foo {
    Bar(u32), // this is a tuple variant
}

use Foo::*; // To use Foo's variant directly, we need to import them in
            // the scope.
Run

Either way, it should work fine with our previous code:

This code runs with edition 2018
struct Type(u32);

enum Foo {
    Bar(u32),
}
use Foo::*;

let Type(x) = Type(12); // ok!
match Type(12) {
    Type(x) => {} // ok!
    _ => {}
}
Run

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, unit 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

An item which isn't a unit struct, a variant, nor a constant has been used as a match pattern.

Erroneous code example:

This example deliberately fails to compile
struct Tortoise;

impl Tortoise {
    fn turtle(&self) -> u32 { 0 }
}

match 0u32 {
    Tortoise::turtle => {} // Error!
    _ => {}
}
if let Tortoise::turtle = 0u32 {} // Same error!
Run

If you want to match against a value returned by a method, you need to bind the value first:

struct Tortoise;

impl Tortoise {
    fn turtle(&self) -> u32 { 0 }
}

match 0u32 {
    x if x == Tortoise.turtle() => {} // Bound into `x` then we compare it!
    _ => {}
}
Run

E0534

The inline attribute was malformed.

Erroneous code example:

This example deliberately fails to compile
#[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 see the inline attribute section of the Reference.

E0535

An unknown argument was given to the inline attribute.

Erroneous code example:

This example deliberately fails to compile
#[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 see the inline Attribute section of the Reference.

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 the section on Conditional Compilation in the Reference.

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 the section on Conditional Compilation in the Reference.

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

An invalid meta-item was used inside an attribute.

Erroneous code example:

This example deliberately fails to compile
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

#[rustc_deprecated(reason)] // error!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}

#[unstable(feature = "unstable_struct", issue)] // error!
struct Unstable;

#[rustc_const_unstable(feature)] // error!
const fn unstable_fn() {}

#[stable(feature = "stable_struct", since)] // error!
struct Stable;

#[rustc_const_stable(feature)] // error!
const fn stable_fn() {}
Run

Meta items are the key-value pairs inside of an attribute. To fix these issues you need to give required key-value pairs.

#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

#[rustc_deprecated(since = "1.39.0", reason = "reason")] // ok!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}

#[unstable(feature = "unstable_struct", issue = "123")] // ok!
struct Unstable;

#[rustc_const_unstable(feature = "unstable_fn", issue = "124")] // ok!
const fn unstable_fn() {}

#[stable(feature = "stable_struct", since = "1.39.0")] // ok!
struct Stable;

#[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
const fn stable_fn() {}
Run

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.

E0549

No description.

E0550

More than one deprecated attribute has been put on an item.

Erroneous code example:

This example deliberately fails to compile
#[deprecated(note = "because why not?")]
#[deprecated(note = "right?")] // error!
fn the_banished() {}
Run

The deprecated attribute can only be present once on an item.

#[deprecated(note = "because why not, right?")]
fn the_banished() {} // ok!
Run

E0551

An invalid meta-item was used inside an attribute.

Erroneous code example:

This example deliberately fails to compile
#[deprecated(note)] // error!
fn i_am_deprecated() {}
Run

Meta items are the key-value pairs inside of an attribute. To fix this issue, you need to give a value to the note key. Example:

#[deprecated(note = "because")] // ok!
fn i_am_deprecated() {}
Run

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.

Erroneous code example:

This example is not tested
#![feature(non_ascii_idents)] // error: `#![feature]` may not be used on the
                              //        stable release channel
Run

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).

E0556

The feature attribute was badly formed.

Erroneous code example:

This example deliberately fails to compile
#![feature(foo_bar_baz, foo(bar), foo = "baz", foo)] // error!
#![feature] // error!
#![feature = "foo"] // error!
Run

The feature attribute only accept a "feature flag" and can only be used on nightly. Example:

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

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 removed
Run

Delete the offending feature attribute.

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

A non-ident or non-wildcard pattern has been used as a parameter of a function pointer type.

Erroneous code example:

This example deliberately fails to compile
type A1 = fn(mut param: u8); // error!
type A2 = fn(&param: u32); // error!
Run

When using an alias over a function type, you cannot e.g. denote a parameter as being mutable.

To fix the issue, remove patterns (_ is allowed though). Example:

type A1 = fn(param: u8); // ok!
type A2 = fn(_: u32); // ok!
Run

You can also omit the parameter name:

type A3 = fn(i16); // ok!
Run

E0562

Abstract return types (written impl Trait for some trait Trait) are only allowed as function and inherent impl 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.

E0565

A literal was used in a built-in attribute that doesn't support literals.

Erroneous code example:

This example deliberately fails to compile
#[repr("C")] // error: meta item in `repr` must be an identifier
struct Repr {}

fn main() {}
Run

Literals in attributes are new and largely unsupported in built-in attributes. Work to support literals where appropriate is ongoing. Try using an unquoted name instead:

#[repr(C)] // ok!
struct Repr {}

fn main() {}
Run

E0566

Conflicting representation hints have been used on a same item.

Erroneous code example:

This example deliberately fails to compile
#[repr(u32, u64)]
enum Repr { A }
Run

In most cases (if not all), using just one representation hint is more than enough. If you want to have a representation hint depending on the current architecture, use cfg_attr. Example:

#[cfg_attr(linux, repr(u32))]
#[cfg_attr(not(linux), repr(u64))]
enum Repr { A }
Run

E0567

Generics have been used on an auto trait.

Erroneous code example:

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

auto trait Generic<T> {} // error!
Run

Since an auto trait is implemented on all existing types, the compiler would not be able to infer the types of the trait's generic parameters.

To fix this issue, just remove the generics:

#![feature(optin_builtin_traits)]

auto trait Generic {} // ok!
Run

E0568

A super trait has been added to an auto trait.

Erroneous code example:

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

auto trait Bound : Copy {} // error!

fn main() {}
Run

Since an auto trait is implemented on all existing types, adding a super trait would filter out a lot of those types. In the current example, almost none of all the existing types could implement Bound because very few of them have the Copy trait.

To fix this issue, just remove the super trait:

#![feature(optin_builtin_traits)]

auto trait Bound {} // ok!

fn main() {}
Run

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 list of unsupported ABIs 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 { // This is now a "loop" loop.
    if satisfied(i) {
        break 2 * i; // ok!
    }
    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

Something other than a type has been used when one was expected.

Erroneous code examples:

This example deliberately fails to compile
enum Dragon {
    Born,
}

fn oblivion() -> Dragon::Born { // error!
    Dragon::Born
}

const HOBBIT: u32 = 2;
impl HOBBIT {} // error!

enum Wizard {
    Gandalf,
    Saruman,
}

trait Isengard {
    fn wizard(_: Wizard::Saruman); // error!
}
Run

In all these errors, a type was expected. For example, in the first error, if we want to return the Born variant from the Dragon enum, we must set the function to return the enum and not its variant:

enum Dragon {
    Born,
}

fn oblivion() -> Dragon { // ok!
    Dragon::Born
}
Run

In the second error, you can't implement something on an item, only on types. We would need to create a new type if we wanted to do something similar:

struct Hobbit(u32); // we create a new type

const HOBBIT: Hobbit = Hobbit(2);
impl Hobbit {} // ok!
Run

In the third case, we tried to only expect one variant of the Wizard enum, which is not possible. To make this work, we need to using pattern matching over the Wizard enum:

enum Wizard {
    Gandalf,
    Saruman,
}

trait Isengard {
    fn wizard(w: Wizard) { // ok!
        match w {
            Wizard::Saruman => {
                // do something
            }
            _ => {} // ignore everything else
        }
    }
}
Run

E0574

Something other than a struct, variant or union has been used when one was expected.

Erroneous code example:

This example deliberately fails to compile
mod Mordor {}

let sauron = Mordor { x: () }; // error!

enum Jak {
    Daxter { i: isize },
}

let eco = Jak::Daxter { i: 1 };
match eco {
    Jak { i } => {} // error!
}
Run

In all these errors, a type was expected. For example, in the first error, we tried to instantiate the Mordor module, which is impossible. If you want to instantiate a type inside a module, you can do it as follow:

mod Mordor {
    pub struct TheRing {
        pub x: usize,
    }
}

let sauron = Mordor::TheRing { x: 1 }; // ok!
Run

In the second error, we tried to bind the Jak enum directly, which is not possible: you can only bind one of its variants. To do so:

enum Jak {
    Daxter { i: isize },
}

let eco = Jak::Daxter { i: 1 };
match eco {
    Jak::Daxter { i } => {} // ok!
}
Run

E0575

Something other than a type or an associated type was given.

Erroneous code example:

This example deliberately fails to compile
enum Rick { Morty }

let _: <u8 as Rick>::Morty; // error!

trait Age {
    type Empire;
    fn Mythology() {}
}

impl Age for u8 {
    type Empire = u16;
}

let _: <u8 as Age>::Mythology; // error!
Run

In both cases, we're declaring a variable (called _) and we're giving it a type. However, <u8 as Rick>::Morty and <u8 as Age>::Mythology aren't types, therefore the compiler throws an error.

<u8 as Rick>::Morty is an enum variant, you cannot use a variant as a type, you have to use the enum directly:

enum Rick { Morty }

let _: Rick; // ok!
Run

<u8 as Age>::Mythology is a trait method, which is definitely not a type. However, the Age trait provides an associated type Empire which can be used as a type:

trait Age {
    type Empire;
    fn Mythology() {}
}

impl Age for u8 {
    type Empire = u16;
}

let _: <u8 as Age>::Empire; // ok!
Run

E0576

An associated item wasn't found in the given type.

Erroneous code example:

This example deliberately fails to compile
trait Hello {
    type Who;

    fn hello() -> <Self as Hello>::You; // error!
}
Run

In this example, we tried to use the non-existent associated type You of the Hello trait. To fix this error, use an existing associated type:

trait Hello {
    type Who;

    fn hello() -> <Self as Hello>::Who; // ok!
}
Run

E0577

Something other than a module was found in visibility scope.

Erroneous code example:

This example deliberately fails to compile
pub struct Sea;

pub (in crate::Sea) struct Shark; // error!

fn main() {}
Run

Sea is not a module, therefore it is invalid to use it in a visibility path. To fix this error we need to ensure Sea is a module.

Please note that the visibility scope can only be applied on ancestors!

This code runs with edition 2018
pub mod Sea {
    pub (in crate::Sea) struct Shark; // ok!
}

fn main() {}
Run

E0578

A module cannot be found and therefore, the visibility cannot be determined.

Erroneous code example:

This example deliberately fails to compile
foo!();

pub (in ::Sea) struct Shark; // error!

fn main() {}
Run

Because of the call to the foo macro, the compiler guesses that the missing module could be inside it and fails because the macro definition cannot be found.

To fix this error, please be sure that the module is in scope:

This code runs with edition 2018
pub mod Sea {
    pub (in crate::Sea) struct Shark;
}

fn main() {}
Run

E0579

A lower range wasn't less than the upper range.

Erroneous code example:

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

fn main() {
    match 5u32 {
        // This range is ok, albeit pointless.
        1 .. 2 => {}
        // This range is empty, and the compiler can tell.
        5 .. 5 => {} // error!
    }
}
Run

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.

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

The problem here is that the lifetime isn't contrained by any of the arguments, making it impossible to determine how long it's supposed to live.

To fix this issue, either use the lifetime in the arguments, or use the 'static lifetime. 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 is only present 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 deliberately fails to compile
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

A doc comment that is not attached to anything has been encountered.

Erroneous code example:

This example deliberately fails to compile
trait Island {
    fn lost();

    /// I'm lost!
}
Run

A little reminder: a doc comment has to be placed before the item it's supposed to document. So if you want to document the Island trait, you need to put a doc comment before it, not inside it. Same goes for the lost method: the doc comment needs to be before it:

/// I'm THE island!
trait Island {
    /// I'm lost!
    fn lost();
}
Run

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() {}