Attributes

An attribute is metadata applied to some module, crate or item. This metadata can be used to/for:

• conditional compilation of code
• disable lints (warnings)
• enable compiler features
• mark functions as unit tests
• [attribute like macros][macros]

Attributes look like #[outer_attribute]. They apply to the [item][item] immediately following it. Some examples of items are: a function, a module declaration, a constant, a structure, an enum. Here is an example where attribute #[derive(Debug)] applies to the struct Rectangle:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

Attributes can take arguments with different syntaxes:

• #[attribute(key: "value")]
• #[attribute(value)]

Attributes can have multiple values and can be separated over multiple lines, too:

#[attribute(value, value2)]


#[attribute(value, value2, value3,
            value4, value5)]