Attributes

Throughout this course, we've been using attributes, but we haven't delved into them yet. Let's briefly discuss the attributes you've encountered and provide some useful resources for further exploration.

At a high level, an attribute is metadata applied to a crate, module, or item. This metadata serves various purposes. Attributes follow this syntax:

InnerAttribute :
   # ! [ Attr ]

OuterAttribute :
   # [ Attr ]

Attr :
      SimplePath AttrInput?
   | unsafe ( SimplePath AttrInput? )

AttrInput :
      DelimTokenTree
   | = Expression

Attributes come in two forms: InnerAttribute and OuterAttribute. The difference between them is that an InnerAttribute includes a bang (!) between the # and the [. An InnerAttribute applies to the item it is declared within, while an OuterAttribute applies to whatever follows it.

Inner Attributes

For example, in our grep program, we've used the inner attribute #![allow(unused_imports)]. By default, the Rust compiler issues warnings if your program imports packages via use but doesn't actually utilize them. Our program imports the File module with use std::fs::File;, but since we are using Cursor for development and have commented out the file I/O code, the compiler complains about the unused import. This attribute disables that warning for the crate.

Outer Attributes

Recall in the last section, we derived the Debug and PartialEq traits. We achieved this using the #[derive(...)] attribute. Notice how this attribute is placed directly before the definition of the Interval struct. This is an example of an OuterAttribute.

#[derive(Debug, PartialEq)]
pub struct Interval<T> {
    pub start: T,
    pub end: T,
}

Classification of Attributes

Attributes are commonly grouped into the following categories:

• Built-in attributes
• Macro attributes
• Derive macro helper attributes
• Tool attributes

Summary

The Rust Reference provides an in-depth look at attributes, and you are encouraged to explore it for a comprehensive understanding of their uses. Additionally, Rust By Example offers extra material and illustrates various use cases.

Next

Attributes offer even greater utility, which we'll explore through command line argument parsing!