Traits

Let's look again at our Ticket type:

pub struct Ticket {
    title: String,
    description: String,
    status: String,
}

All our tests, so far, have been making assertions using Ticket's fields.

assert_eq!(ticket.title(), "A new title");

What if we wanted to compare two Ticket instances directly?

let ticket1 = Ticket::new(/* ... */);
let ticket2 = Ticket::new(/* ... */);
ticket1 == ticket2

The compiler will stop us:

error[E0369]: binary operation `==` cannot be applied to type `Ticket`
  --> src/main.rs:18:13
   |
18 |     ticket1 == ticket2
   |     ------- ^^ ------- Ticket
   |     |
   |     Ticket
   |
note: an implementation of `PartialEq` might be missing for `Ticket`

Ticket is a new type. Out of the box, there is no behavior attached to it.
Rust doesn't magically infer how to compare two Ticket instances just because they contain Strings.

The Rust compiler is nudging us in the right direction though: it's suggesting that we might be missing an implementation of PartialEq. PartialEq is a trait!

What are traits?

Traits are Rust's way of defining interfaces.
A trait defines a set of methods that a type must implement to satisfy the trait's contract.

Defining a trait

The syntax for a trait definition goes like this:

trait <TraitName> {
    fn <method_name>(<parameters>) -> <return_type>;
}

We might, for example, define a trait named MaybeZero that requires its implementors to define an is_zero method:

trait MaybeZero {
    fn is_zero(self) -> bool;
}

Implementing a trait

To implement a trait for a type we use the impl keyword, just like we do for regular¹ methods, but the syntax is a bit different:

impl <TraitName> for <TypeName> {
    fn <method_name>(<parameters>) -> <return_type> {
        // Method body
    }
}

For example, to implement the MaybeZero trait for a custom number type, WrappingU32:

pub struct WrappingU32 {
    inner: u32,
}

impl MaybeZero for WrappingU32 {
    fn is_zero(self) -> bool {
        self.inner == 0
    }
}

Invoking a trait method

To invoke a trait method, we use the . operator, just like we do with regular methods:

let x = WrappingU32 { inner: 5 };
assert!(!x.is_zero());

To invoke a trait method, two things must be true:

The type must implement the trait.
The trait must be in scope.

To satisfy the latter, you may have to add a use statement for the trait:

use crate::MaybeZero;

This is not necessary if:

The trait is defined in the same module where the invocation occurs.
The trait is defined in the standard library's prelude. The prelude is a set of traits and types that are automatically imported into every Rust program. It's as if use std::prelude::*; was added at the beginning of every Rust module.

You can find the list of traits and types in the prelude in the Rust documentation.

A method defined directly on a type, without using a trait, is also known as an inherent method.

Exercise