Attributes - The Rust Reference

# Attributes

Any item declaration may have an attribute applied to it. Attributes in Rust are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 (C#). An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version. 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 list of sub-attribute arguments

Attributes with a bang ("!") after the hash ("#") apply to the item that the attribute is declared within. Attributes that do not have a bang after the hash apply to the item that follows the attribute.

An example 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;

    #}

> Note: At some point in the future, the compiler will distinguish between language-reserved and user-available attributes. Until then, there is effectively no difference between an attribute handled by a loadable syntax extension and the compiler.

## Crate-only attributes

- `crate_name` - specify the crate's crate name.
- `crate_type` - see linkage.
- `feature` - see compiler features.
- `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.
- `plugin` - load a list of named crates as compiler plugins, e.g. `#![plugin(foo, bar)]`. Optional arguments for each plugin, i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's registrar function. The `plugin` feature gate is required to use this attribute.
- `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

- `main` - indicates that this function should be passed to the entry point, rather than the function in the crate root named `main`.
- `plugin_registrar` - mark this function as the registration point for [compiler plugins][plugin], such as loadable syntax extensions.
- `start` - indicates that this function should be used as the entry point, overriding the "start" language item. See the "start" language item for more details.
- `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.
- `naked` - The function utilizes a custom ABI or custom inline ASM that requires epilogue and prologue to be skipped.

## Static-only attributes

- `thread_local` - on a `static mut`, this signals that the value of this static may change depending on the current thread. The exact consequences of this are implementation-defined.

## 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.

On `enum`s:

- `repr` - on C-like enums, this sets the underlying type used for representation. Takes one argument, which is the primitive type this enum should be represented for, or `C`, which specifies that it should be the default `enum` size of the C ABI for that platform. Note that enum representation in C is undefined, and this may be incorrect when the C code is compiled with certain flags.

On `struct`s:

- `repr` - specifies the representation to use for this struct. Takes a list of options. The currently accepted ones are `C` and `packed`, which may be combined. `C` will use a C ABI compatible struct layout, and `packed` will remove any padding between fields (note that this is very fragile and may break platforms which require aligned access).

## 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 book for more information on macro scope.

## Miscellaneous attributes

- `deprecated` - mark the item as deprecated; the full attribute is `#[deprecated(since = "crate version", note = "...")`, where both arguments are optional.
- `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.
- `simd` - on certain tuple structs, derive the arithmetic operators, which lower to the target's SIMD instructions, if any; the `simd` feature gate is necessary to use this attribute.
- `unsafe_destructor_blind_to_params` - on `Drop::drop` method, asserts that the destructor code (and all potential specializations of that code) will never attempt to read from nor write to any references with lifetimes that come in via generic parameters. This is a constraint we cannot currently express via the type system, and therefore we rely on the programmer to assert that it holds. Adding this to a Drop impl causes the associated destructor to be considered "uninteresting" by the Drop-Check rule, and thus it can help sidestep data ordering constraints that would otherwise be introduced by the Drop-Check rule. Such sidestepping of the constraints, if done incorrectly, can lead to undefined behavior (in the form of reading or writing to data outside of its dynamic extent), and thus this attribute has the word "unsafe" in its name. To use this, the `unsafe_destructor_blind_to_params` feature gate must be enabled.
- `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
- `rustc_on_unimplemented` - Write a custom note to be shown along with the error when the trait is found to be unimplemented on a type. You may use format arguments like `{T}`, `{A}` to correspond to the types at the point of use corresponding to the type parameters of the trait of the same name. `{Self}` will be replaced with the type that is supposed to implement the trait but doesn't. You can also use the trait's name which will be replaced with the full path for the trait, for example for the trait `Foo` in module `Bar`, `{Foo}` can be used and will show up as `Bar::Foo`. To use this, the `on_unimplemented` feature gate must be enabled.
- `must_use` - on structs and enums, will warn if a value of this type isn't used or assigned to a variable. You may also include an optional message by using `#[must_use = "message"]` which will be given alongside the warning.

### 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.

    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:

    #[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 }
    }

### Language items

Some primitive Rust operations are defined in Rust code, rather than being implemented directly in C or assembly language. The definitions of these operations have to be easy for the compiler to find. The `lang` attribute makes it possible to declare these operations. For example, the `str` module in the Rust standard library defines the string equality function:

    #[lang = "str_eq"]
    pub fn eq_slice(a: &str, b: &str) -> bool {
        // details elided
    }

The name `str_eq` has a special meaning to the Rust compiler, and the presence of this definition means that it will use this definition when generating calls to the string equality function.

The set of language items is currently considered unstable. A complete list of the built-in language items will be added in the future.

### Inline attributes

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.

### Compiler Features

Certain aspects of Rust may be implemented in the compiler, but they're not necessarily ready for every-day use. These features are often of "prototype quality" or "almost production ready", but may not be stable enough to be considered a full-fledged language feature.

For this reason, Rust recognizes a special crate-level attribute of the form:

    #![feature(feature1, feature2, feature3)]

This directive informs the compiler that the feature list: `feature1`, `feature2`, and `feature3` should all be enabled. This is only recognized at a crate-level, not at a module-level. Without this directive, all features are considered off, and using the features will result in a compiler error.

The currently implemented features of the reference compiler are documented in The Unstable Book.

If a feature is promoted to a language feature, then all existing programs will start to receive compilation warnings about `#![feature]` directives which enabled the new feature (because the directive is no longer necessary). However, if a feature is decided to be removed from the language, errors will be issued (if there isn't a parser error first). The directive in this case is no longer necessary, and it's likely that existing code will break if the feature isn't removed.

If an unknown feature is found in a directive, it results in a compiler error. An unknown feature is one which has never been recognized by the compiler.