# Custom Derive

In Rust, you’ve always been able to automatically implement some traits through the derive attribute:

    # #![allow(unused_variables)]
    #fn main() {
    #[derive(Debug)]
    struct Pet {
        name: String,
    }
    #}

The `Debug` trait is then implemented for `Pet`, with vastly less boilerplate. For example, without `derive`, you'd have to write this:

    # #![allow(unused_variables)]
    #fn main() {
    use std::fmt;

    struct Pet {
        name: String,
    }

    impl fmt::Debug for Pet {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self {
                Pet { name } => {
                    let mut debug_trait_builder = f.debug_struct("Pet");

                    let _ = debug_trait_builder.field("name", name);

                    debug_trait_builder.finish()
                }
            }
        }
    }
    #}

Whew!

However, this only worked for traits provided as part of the standard library; it was not customizable. But now, you can tell Rust what to do when someone wants to derive your trait. This is used heavily in popular crates like serde and Diesel.

For more, including learning how to build your own custom derive, see The Rust Programming Language.