% Attributes
Declarations can be annotated with ‘attributes’ in Rust. They look like this:
#[test]
# fn foo() {}
or like this:
# 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:
#[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:
#[inline(always)]
fn super_fast_fn() {
# }
Or even keys and values:
#[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.
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