Derive

The derive attribute allows new items to be automatically generated for data structures.

It uses the MetaListPaths syntax to specify a list of traits to implement or paths to derive macros to process.

For example, the following will create an impl item for the PartialEq and Clone traits for Foo, and the type parameter T will be given the PartialEq or Clone constraints for the appropriate impl:

#![allow(unused)]
fn main() {
#[derive(PartialEq, Clone)]
struct Foo<T> {
    a: i32,
    b: T,
}
}

The generated impl for PartialEq is equivalent to

#![allow(unused)]
fn main() {
struct Foo<T> { a: i32, b: T }
impl<T: PartialEq> PartialEq for Foo<T> {
    fn eq(&self, other: &Foo<T>) -> bool {
        self.a == other.a && self.b == other.b
    }
}
}

You can implement derive for your own traits through procedural macros.

The automatically_derived attribute

The automatically_derived attribute is automatically added to implementations created by the derive attribute for built-in traits. It has no direct effect, but it may be used by tools and diagnostic lints to detect these automatically generated implementations.