22.4 derive macros

The derive macro is a procedural macro that generates automatically an implementation for selected traits.

In the following example, both Debug and Clone are implemented for User with generated default implementations:

:clear

#[derive(Debug, Clone)]
struct User {
  id: u32,
  name: String,
}

The Clone trait allows us to make an explicit copy of an existing instance:

let u = User { id: 12, name: String::from("John"), };
let v = u.clone();
println!("id={}, name={}", v.id, v.name);

Couldn't automatically determine type of variable `v`.
Please give it an explicit type.

And the Debug trait allows to automatically convert an instance into a string and print it:

v