Attributes: metadata applied to some module, crate or item

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

When attributes apply to a whole crate, their syntax is #![crate_attribute],

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)]

#[attribute(value, value2)]


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