100-exercises-to-learn-rust/book/src/04_traits/04_derive.md

Derive macros

Implementing PartialEq for Ticket was a bit tedious, wasn’t it? You had to manually compare each field of the struct.

Destructuring syntax

Furthermore, the implementation is brittle: if the struct definition changes (e.g. a new field is added), you have to remember to update the PartialEq implementation.

You can mitigate the risk by destructuring the struct into its fields:

impl PartialEq for Ticket {
    fn eq(&self, other: &Self) -> bool {
        let Ticket {
            title,
            description,
            status,
        } = self;
        // [...]
    }
}

If the definition of Ticket changes, the compiler will error out, complaining that your destructuring is no longer exhaustive.
You can also rename struct fields, to avoid variable shadowing:

impl PartialEq for Ticket {
    fn eq(&self, other: &Self) -> bool {
        let Ticket {
            title,
            description,
            status,
        } = self;
        let Ticket {
            title: other_title,
            description: other_description,
            status: other_status,
        } = other;
        // [...]
    }
}

Destructuring is a useful pattern to have in your toolkit, but there’s an even more convenient way to do this: derive macros.

Macros

You’ve already encountered a few macros in past exercises:

• assert_eq! and assert!, in the test cases
• println!, to print to the console

Rust macros are code generators.
They generate new Rust code based on the input you provide, and that generated code is then compiled alongside the rest of your program. Some macros are built into Rust’s standard library, but you can also write your own. We won’t be creating our own macro in this course, but you can find some useful pointers in the “Further reading” section.

Inspection

Some IDEs let you expand a macro to inspect the generated code. If that’s not possible, you can use cargo-expand.

Derive macros

A derive macro is a particular flavour of Rust macro. It is specified as an attribute on top of a struct.

#[derive(PartialEq)]
struct Ticket {
    title: String,
    description: String,
    status: String
}

Derive macros are used to automate the implementation of common (and “obvious”) traits for custom types. In the example above, the PartialEq trait is automatically implemented for Ticket. If you expand the macro, you’ll see that the generated code is functionally equivalent to the one you wrote manually, although a bit more cumbersome to read:

#[automatically_derived]
impl ::core::cmp::PartialEq for Ticket {
    #[inline]
    fn eq(&self, other: &Ticket) -> bool {
        self.title == other.title 
            && self.description == other.description
            && self.status == other.status
    }
}

The compiler will nudge you to derive traits when possible.

Further reading

• The little book of Rust macros
• Proc macro workshop