% 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.