# Attributes

Declarations can be annotated with ‘attributes’ in Rust. They look like this:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[test]
    # fn foo() {}

    #}

or like this:

    # #![allow(unused_variables)]
    #
    #fn main() {
    # mod foo {
    #![test]
    # }

    #}

The difference between the two is the `!`, which changes what the attribute applies to:

    #[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:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[test]
    fn check() {
        assert_eq!(2, 1 + 1);
    }

    #}

It is marked with `#[test]`. This means it’s special: when you run tests, this function will execute. When you compile as usual, it won’t even be included. This function is now a test function.

Attributes may also have additional data:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[inline(always)]
    fn super_fast_fn() {
    # }

    #}

Or even keys and values:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[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. Currently, you are not allowed to create your own attributes, the Rust compiler defines them.