13 Attributes

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

• conditional compilation of code
• set crate name, version and type (binary or library)
• disable lints (warnings)
• enable compiler features (macros, glob imports, etc.)
• link to a foreign library
• mark functions as unit tests
• mark functions that will be part of a benchmark
• attribute like macros

Attributes look like #[outer_attribute] or #![inner_attribute], with the difference between them being where they apply.

• #[outer_attribute] applies to the 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,
  }

• #![inner_attribute] applies to the enclosing item (typically a module or a crate). In other words, this attribute is interpreted as applying to the entire scope in which it’s placed. Here is an example where #![allow(unused_variables)] applies to the whole crate (if placed in main.rs):

  #![allow(unused_variables)]
  
  fn main() {
      let x = 3; // This would normally warn about an unused variable.
  }

Attributes can take arguments with different syntaxes:

• #[attribute = "value"]
• #[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)]