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Attributes

Syntax
Attribute :
   InnerAttribute | OuterAttribute

InnerAttribute :
   #![ MetaItem ]

OuterAttribute :
   #[ MetaItem ]

MetaItem :
      IDENTIFIER
   | IDENTIFIER = LITERAL
   | IDENTIFIER ( LITERAL )
   | IDENTIFIER ( MetaSeq )

MetaSeq :
      EMPTY
   | MetaItem
   | MetaItem , MetaSeq

An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version. Attributes are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 (C#).

Attributes may appear as any of:

  • A single identifier, the attribute name
  • An identifier followed by the equals sign '=' and a literal, providing a key/value pair
  • An identifier followed by a parenthesized literal, providing a key/value pair
  • An identifier followed by a parenthesized list of sub-attribute arguments

Inner attributes, written with a bang ("!") after the hash ("#"), apply to the item that the attribute is declared within. Outer attributes, written without the bang after the hash, apply to the item or generic parameter that follow the attribute.

Attributes may be applied to many things in the language:

Some examples of attributes:


# #![allow(unused_variables)]
#fn main() {
// General metadata applied to the enclosing module or crate.
#![crate_type = "lib"]

// A function marked as a unit test
#[test]
fn test_foo() {
    /* ... */
}

// A conditionally-compiled module
#[cfg(target_os = "linux")]
mod bar {
    /* ... */
}

// A lint attribute used to suppress a warning/error
#[allow(non_camel_case_types)]
type int8_t = i8;

// Outer attribute applies to the entire function.
fn some_unused_variables() {
  #![allow(unused_variables)]
  
  let x = ();
  let y = ();
  let z = ();
}
#}

The rest of this page describes or links to descriptions of which attribute names have meaning.

Crate-only attributes

  • crate_name - specify the crate's crate name.
  • crate_type - see linkage.
  • no_builtins - disable optimizing certain code patterns to invocations of library functions that are assumed to exist
  • no_main - disable emitting the main symbol. Useful when some other object being linked to defines main.
  • no_start - disable linking to the native crate, which specifies the "start" language item.
  • no_std - disable linking to the std crate.
  • recursion_limit - Sets the maximum depth for potentially infinitely-recursive compile-time operations like auto-dereference or macro expansion. The default is #![recursion_limit="64"].
  • windows_subsystem - Indicates that when this crate is linked for a Windows target it will configure the resulting binary's subsystem via the linker. Valid values for this attribute are console and windows, corresponding to those two respective subsystems. More subsystems may be allowed in the future, and this attribute is ignored on non-Windows targets.

Module-only attributes

  • no_implicit_prelude - disable injecting use std::prelude::* in this module.
  • path - specifies the file to load the module from. #[path="foo.rs"] mod bar; is equivalent to mod bar { /* contents of foo.rs */ }. The path is taken relative to the directory that the current module is in.

Function-only attributes

  • test - indicates that this function is a test function, to only be compiled in case of --test.
    • ignore - indicates that this test function is disabled.
  • should_panic - indicates that this test function should panic, inverting the success condition.
  • cold - The function is unlikely to be executed, so optimize it (and calls to it) differently.

FFI attributes

On an extern block, the following attributes are interpreted:

  • link_args - specify arguments to the linker, rather than just the library name and type. This is feature gated and the exact behavior is implementation-defined (due to variety of linker invocation syntax).
  • link - indicate that a native library should be linked to for the declarations in this block to be linked correctly. link supports an optional kind key with three possible values: dylib, static, and framework. See external blocks for more about external blocks. Two examples: #[link(name = "readline")] and #[link(name = "CoreFoundation", kind = "framework")].
  • linked_from - indicates what native library this block of FFI items is coming from. This attribute is of the form #[linked_from = "foo"] where foo is the name of a library in either #[link] or a -l flag. This attribute is currently required to export symbols from a Rust dynamic library on Windows, and it is feature gated behind the linked_from feature.

On declarations inside an extern block, the following attributes are interpreted:

  • link_name - the name of the symbol that this function or static should be imported as.
  • linkage - on a static, this specifies the linkage type.

See type layout for documentation on the repr attribute which can be used to control type layout.

Macro-related attributes

  • macro_use on a mod — macros defined in this module will be visible in the module's parent, after this module has been included.

  • macro_use on an extern crate — load macros from this crate. An optional list of names #[macro_use(foo, bar)] restricts the import to just those macros named. The extern crate must appear at the crate root, not inside mod, which ensures proper function of the $crate macro variable.

  • macro_reexport on an extern crate — re-export the named macros.

  • macro_export - export a macro for cross-crate usage.

  • no_link on an extern crate — even if we load this crate for macros, don't link it into the output.

See the macros section of the first edition of the book for more information on macro scope.

Miscellaneous attributes

  • export_name - on statics and functions, this determines the name of the exported symbol.
  • link_section - on statics and functions, this specifies the section of the object file that this item's contents will be placed into.
  • no_mangle - on any item, do not apply the standard name mangling. Set the symbol for this item to its identifier.

Deprecation

The deprecated attribute marks an item as deprecated. It has two optional fields, since and note.

  • since expects a version number, as in #[deprecated(since = "1.4.1")]
    • rustc doesn't know anything about versions, but external tools like clippy may check the validity of this field.
  • note is a free text field, allowing you to provide an explanation about the deprecation and preferred alternatives.

Only public items can be given the #[deprecated] attribute. #[deprecated] on a module is inherited by all child items of that module.

rustc will issue warnings on usage of #[deprecated] items. rustdoc will show item deprecation, including the since version and note, if available.

Here's an example.


# #![allow(unused_variables)]
#fn main() {
#[deprecated(since = "5.2", note = "foo was rarely used. Users should instead use bar")]
pub fn foo() {}

pub fn bar() {}
#}

The RFC contains motivations and more details.

Documentation

The doc attribute is used to document items and fields. Doc comments are transformed into doc attributes.

See The Rustdoc Book for reference material on this attribute.

Conditional compilation

Sometimes one wants to have different compiler outputs from the same code, depending on build target, such as targeted operating system, or to enable release builds.

Configuration options are boolean (on or off) and are named either with a single identifier (e.g. foo) or an identifier and a string (e.g. foo = "bar"; the quotes are required and spaces around the = are unimportant). Note that similarly-named options, such as foo, foo="bar" and foo="baz" may each be set or unset independently.

Configuration options are either provided by the compiler or passed in on the command line using --cfg (e.g. rustc main.rs --cfg foo --cfg 'bar="baz"'). Rust code then checks for their presence using the #[cfg(...)] attribute:


# #![allow(unused_variables)]
#fn main() {
// The function is only included in the build when compiling for macOS
#[cfg(target_os = "macos")]
fn macos_only() {
  // ...
}

// This function is only included when either foo or bar is defined
#[cfg(any(foo, bar))]
fn needs_foo_or_bar() {
  // ...
}

// This function is only included when compiling for a unixish OS with a 32-bit
// architecture
#[cfg(all(unix, target_pointer_width = "32"))]
fn on_32bit_unix() {
  // ...
}

// This function is only included when foo is not defined
#[cfg(not(foo))]
fn needs_not_foo() {
  // ...
}
#}

This illustrates some conditional compilation can be achieved using the #[cfg(...)] attribute. any, all and not can be used to assemble arbitrarily complex configurations through nesting.

The following configurations must be defined by the implementation:

  • target_arch = "..." - Target CPU architecture, such as "x86", "x86_64" "mips", "powerpc", "powerpc64", "arm", or "aarch64". This value is closely related to the first element of the platform target triple, though it is not identical.
  • target_os = "..." - Operating system of the target, examples include "windows", "macos", "ios", "linux", "android", "freebsd", "dragonfly", "bitrig" , "openbsd" or "netbsd". This value is closely related to the second and third element of the platform target triple, though it is not identical.
  • target_family = "..." - Operating system family of the target, e. g. "unix" or "windows". The value of this configuration option is defined as a configuration itself, like unix or windows.
  • unix - See target_family.
  • windows - See target_family.
  • target_env = ".." - Further disambiguates the target platform with information about the ABI/libc. Presently this value is either "gnu", "msvc", "musl", or the empty string. For historical reasons this value has only been defined as non-empty when needed for disambiguation. Thus on many GNU platforms this value will be empty. This value is closely related to the fourth element of the platform target triple, though it is not identical. For example, embedded ABIs such as gnueabihf will simply define target_env as "gnu".
  • target_endian = "..." - Endianness of the target CPU, either "little" or "big".
  • target_pointer_width = "..." - Target pointer width in bits. This is set to "32" for targets with 32-bit pointers, and likewise set to "64" for 64-bit pointers.
  • target_has_atomic = "..." - Set of integer sizes on which the target can perform atomic operations. Values are "8", "16", "32", "64" and "ptr".
  • target_vendor = "..." - Vendor of the target, for example apple, pc, or simply "unknown".
  • test - Enabled when compiling the test harness (using the --test flag).
  • debug_assertions - Enabled by default when compiling without optimizations. This can be used to enable extra debugging code in development but not in production. For example, it controls the behavior of the standard library's debug_assert! macro.

You can also set another attribute based on a cfg variable with cfg_attr:

#[cfg_attr(a, b)]

This is the same as #[b] if a is set by cfg, and nothing otherwise.

Lastly, configuration options can be used in expressions by invoking the cfg! macro: cfg!(a) evaluates to true if a is set, and false otherwise.

Lint check attributes

A lint check names a potentially undesirable coding pattern, such as unreachable code or omitted documentation, for the static entity to which the attribute applies.

For any lint check C:

  • allow(C) overrides the check for C so that violations will go unreported,
  • deny(C) signals an error after encountering a violation of C,
  • forbid(C) is the same as deny(C), but also forbids changing the lint level afterwards,
  • warn(C) warns about violations of C but continues compilation.

The lint checks supported by the compiler can be found via rustc -W help, along with their default settings. Compiler plugins can provide additional lint checks.


# #![allow(unused_variables)]
#fn main() {
pub mod m1 {
    // Missing documentation is ignored here
    #[allow(missing_docs)]
    pub fn undocumented_one() -> i32 { 1 }

    // Missing documentation signals a warning here
    #[warn(missing_docs)]
    pub fn undocumented_too() -> i32 { 2 }

    // Missing documentation signals an error here
    #[deny(missing_docs)]
    pub fn undocumented_end() -> i32 { 3 }
}
#}

This example shows how one can use allow and warn to toggle a particular check on and off:


# #![allow(unused_variables)]
#fn main() {
#[warn(missing_docs)]
pub mod m2{
    #[allow(missing_docs)]
    pub mod nested {
        // Missing documentation is ignored here
        pub fn undocumented_one() -> i32 { 1 }

        // Missing documentation signals a warning here,
        // despite the allow above.
        #[warn(missing_docs)]
        pub fn undocumented_two() -> i32 { 2 }
    }

    // Missing documentation signals a warning here
    pub fn undocumented_too() -> i32 { 3 }
}
#}

This example shows how one can use forbid to disallow uses of allow for that lint check:


# #![allow(unused_variables)]
#fn main() {
#[forbid(missing_docs)]
pub mod m3 {
    // Attempting to toggle warning signals an error here
    #[allow(missing_docs)]
    /// Returns 2.
    pub fn undocumented_too() -> i32 { 2 }
}
#}

must_use Attribute

The must_use attribute can be used on user-defined composite types (structs, enums, and unions) and functions.

When used on user-defined composite types, if the expression of an expression statement has that type, then the unused_must_use lint is violated.

#[must_use]
struct MustUse {
  // some fields
}

# impl MustUse {
#   fn new() -> MustUse { MustUse {} }
# }
#
fn main() {
  // Violates the `unused_must_use` lint.
  MustUse::new();
}

When used on a function, if the expression of an expression statement is a call expression to that function, then the unused_must_use lint is violated. The exceptions to this is if the return type of the function is (), !, or a zero-variant enum, in which case the attribute does nothing.

#[must_use]
fn five() -> i32 { 5i32 }

fn main() {
  // Violates the unused_must_use lint.
  five();
}

When used on a function in a trait declaration, then the behavior also applies when the call expression is a function from an implementation of the trait.

trait Trait {
  #[must_use]
  fn use_me(&self) -> i32;
}

impl Trait for i32 {
  fn use_me(&self) -> i32 { 0i32 }
}

fn main() {
  // Violates the `unused_must_use` lint.
  5i32.use_me();
}

When used on a function in an implementation, the attribute does nothing.

Note: Trivial no-op expressions containing the value will not violate the lint. Examples include wrapping the value in a type that does not implement Drop and then not using that type and being the final expression of a block expression that is not used.

#[must_use]
fn five() -> i32 { 5i32 }

fn main() {
  // None of these violate the unused_must_use lint.
  (five(),);
  Some(five());
  { five() };
  if true { five() } else { 0i32 };
  match true {
    _ => five()
  };
}

Note: It is idiomatic to use a let statement with a pattern of _ when a must-used value is purposely discarded.

#[must_use]
fn five() -> i32 { 5i32 }

fn main() {
  // Does not violate the unused_must_use lint.
  let _ = five();
}

The must_use attribute may also include a message by using #[must_use = "message"]. The message will be given alongside the warning.

Inline attribute

The inline attribute suggests that the compiler should place a copy of the function or static in the caller, rather than generating code to call the function or access the static where it is defined.

The compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can actually make the program slower, so it should be used with care.

#[inline] and #[inline(always)] always cause the function to be serialized into the crate metadata to allow cross-crate inlining.

There are three different types of inline attributes:

  • #[inline] hints the compiler to perform an inline expansion.
  • #[inline(always)] asks the compiler to always perform an inline expansion.
  • #[inline(never)] asks the compiler to never perform an inline expansion.

derive

The derive attribute allows certain traits to be automatically implemented for data structures. For example, the following will create an impl for the PartialEq and Clone traits for Foo, the type parameter T will be given the PartialEq or Clone constraints for the appropriate impl:


# #![allow(unused_variables)]
#fn main() {
#[derive(PartialEq, Clone)]
struct Foo<T> {
    a: i32,
    b: T,
}
#}

The generated impl for PartialEq is equivalent to


# #![allow(unused_variables)]
#fn main() {
# struct Foo<T> { a: i32, b: T }
impl<T: PartialEq> PartialEq for Foo<T> {
    fn eq(&self, other: &Foo<T>) -> bool {
        self.a == other.a && self.b == other.b
    }

    fn ne(&self, other: &Foo<T>) -> bool {
        self.a != other.a || self.b != other.b
    }
}
#}

You can implement derive for your own type through procedural macros.