An attribute is metadata applied to some module, crate or item. This metadata can be used to/for:
When attributes apply to a whole crate, their syntax is #![crate_attribute],
and when they apply to a module or item, the syntax is #[item_attribute]
(notice the missing bang !).
Attributes can take arguments with different syntaxes:
#[attribute = "value"]#[attribute(key = "value")]#[attribute(value)]