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% Attributes
Declarations can be annotated with ‘attributes’ in Rust. They look like this:
```rust
#[test]
# fn foo() {}
```
or like this:
```rust
# mod foo {
#![test]
# }
```
The difference between the two is the `!`, which changes what the attribute
applies to:
```rust,ignore
#[foo]
struct Foo;
mod bar {
#![bar]
}
```
The `#[foo]` attribute applies to the next item, which is the `struct`
declaration. The `#![bar]` attribute applies to the item enclosing it, which is
the `mod` declaration. Otherwise, they’re the same. Both change the meaning of
the item they’re attached to somehow.
For example, consider a function like this:
```rust
#[test]
fn check() {
assert_eq!(2, 1 + 1);
}
```
It is marked with `#[test]`. This means it’s special: when you run
[tests][tests], this function will execute. When you compile as usual, it won’t
even be included. This function is now a test function.
[tests]: testing.html
Attributes may also have additional data:
```rust
#[inline(always)]
fn super_fast_fn() {
# }
```
Or even keys and values:
```rust
#[cfg(target_os = "macos")]
mod macos_only {
# }
```
Rust attributes are used for a number of different things. There is a full list
of attributes [in the reference][reference]. Currently, you are not allowed to
create your own attributes, the Rust compiler defines them.
[reference]: ../reference.html#attributes
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