# Attributes

> **<sup>Syntax</sup>**  
> _Attribute_ :  
> &nbsp;&nbsp; _InnerAttribute_ | _OuterAttribute_  
>  
> _InnerAttribute_ :  
> &nbsp;&nbsp; `#![` MetaItem `]`  
>   
> _OuterAttribute_ :  
> &nbsp;&nbsp; `#[` MetaItem `]`  
>   
> _MetaItem_ :  
> &nbsp;&nbsp; &nbsp;&nbsp; IDENTIFIER  
> &nbsp;&nbsp; | IDENTIFIER `=` LITERAL  
> &nbsp;&nbsp; | IDENTIFIER `(` _MetaSeq_ `)`  
> &nbsp;&nbsp; | IDENTIFIER `(` _MetaSeq_ `,` `)`  
>   
> _MetaSeq_ :  
> &nbsp;&nbsp; &nbsp;&nbsp; EMPTY  
> &nbsp;&nbsp; | _MetaItem_  
> &nbsp;&nbsp; | _MetaSeq_ `,` _MetaItem_  

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:

```rust
// 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;
```

## Crate-only attributes

- `crate_name` - specify the crate's crate name.
- `crate_type` - see [linkage](linkage.html).
- `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.

[subsystem]: https://msdn.microsoft.com/en-us/library/fcc1zstk.aspx

### 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`.
- `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](items/external-blocks.html) 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](http://llvm.org/docs/LangRef.html#linkage-types).

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).

On `union`s:

- `repr` - Same as per `struct`.

## 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](../book/first-edition/macros.html#the-variable-crate).

- `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](../book/first-edition/macros.html#scoping-and-macro-importexport) 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.
- `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.

### 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:

```rust
// 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`:

```rust,ignore
#[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][unstable book plugin] can provide additional lint checks.

```rust,ignore
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:

```rust
#[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:

```rust,ignore
#[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 }
}
```

### 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`:

```rust
#[derive(PartialEq, Clone)]
struct Foo<T> {
    a: i32,
    b: T,
}
```

The generated `impl` for `PartialEq` is equivalent to

```rust
# 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].

[Doc comments]: comments.html#doc-comments
[The Rustdoc Book]: ../rustdoc/the-doc-attribute.html
[procedural macros]: procedural-macros.html