Declarations can be annotated with ‘attributes’ in Rust. They look like this:
#[test]Run
or like this:
#![test]Run
The difference between the two is the !
, which changes what the attribute
applies to:
#[foo] struct Foo; mod bar { #![bar] }Run
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); }Run
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() {Run
Or even keys and values:
#[cfg(target_os = "macos")] mod macos_only {Run
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.