Trait bounds

We've seen two use cases for traits so far:

Unlocking "built-in" behaviour (e.g. operator overloading)
Adding new behaviour to existing types (i.e. extension traits)

There's a third use case: generic programming.

The problem

All our functions and methods, so far, have been working with concrete types.
Code that operates on concrete types is usually straightforward to write and understand. But it's also limited in its reusability.
Let's imagine, for example, that we want to write a function that returns true if an integer is even. Working with concrete types, we'd have to write a separate function for each integer type we want to support:

fn is_even_i32(n: i32) -> bool {
    n % 2 == 0
}

fn is_even_i64(n: i64) -> bool {
    n % 2 == 0
}

// Etc.

Alternatively, we could write a single extension trait and then different implementations for each integer type:

trait IsEven {
    fn is_even(&self) -> bool;
}

impl IsEven for i32 {
    fn is_even(&self) -> bool {
        self % 2 == 0
    }
}

impl IsEven for i64 {
    fn is_even(&self) -> bool {
        self % 2 == 0
    }
}

// Etc.

The duplication remains.

Generic programming

We can do better using generics.
Generics allow us to write code that works with a type parameter instead of a concrete type:

fn print_if_even<T>(n: T)
where
    T: IsEven + Debug
{
    if n.is_even() {
        println!("{n:?} is even");
    }
}

print_if_even is a generic function.
It isn't tied to a specific input type. Instead, it works with any type T that:

Implements the IsEven trait.
Implements the Debug trait.

This contract is expressed with a trait bound: T: IsEven + Debug.
The + symbol is used to require that T implements multiple traits. T: IsEven + Debug is equivalent to "where T implements IsEven and Debug".

Trait bounds

What purpose do trait bounds serve in print_if_even?
To find out, let's try to remove them:

fn print_if_even<T>(n: T) {
    if n.is_even() {
        println!("{n:?} is even");
    }
}

This code won't compile:

error[E0599]: no method named `is_even` found for type parameter `T` 
              in the current scope
 --> src/lib.rs:2:10
  |
1 | fn print_if_even<T>(n: T) {
  |                  - method `is_even` not found 
  |                    for this type parameter
2 |     if n.is_even() {
  |          ^^^^^^^ method not found in `T`

error[E0277]: `T` doesn't implement `Debug`
 --> src/lib.rs:3:19
  |
3 |         println!("{n:?} is even");
  |                   ^^^^^ 
  |   `T` cannot be formatted using `{:?}` because 
  |         it doesn't implement `Debug`
  |
help: consider restricting type parameter `T`
  |
1 | fn print_if_even<T: std::fmt::Debug>(n: T) {
  |                   +++++++++++++++++

Without trait bounds, the compiler doesn't know what T can do.
It doesn't know that T has an is_even method, and it doesn't know how to format T for printing. From the compiler point of view, a bare T has no behaviour at all.
Trait bounds restrict the set of types that can be used by ensuring that the behaviour required by the function body is present.

Syntax: inlining trait bounds

All the examples above used a where clause to specify trait bounds:

fn print_if_even<T>(n: T)
where
    T: IsEven + Debug
//  ^^^^^^^^^^^^^^^^^
//  This is a `where` clause
{
    // [...]
}

If the trait bounds are simple, you can inline them directly next to the type parameter:

fn print_if_even<T: IsEven + Debug>(n: T) {
    //           ^^^^^^^^^^^^^^^^^
    //           This is an inline trait bound
    // [...]
}

Syntax: meaningful names

In the examples above, we used T as the type parameter name. This is a common convention when a function has only one type parameter.
Nothing stops you from using a more meaningful name, though:

fn print_if_even<Number: IsEven + Debug>(n: Number) {
    // [...]
}

It is actually desirable to use meaningful names when there are multiple type parameters at play or when the name T doesn't convey enough information about the type's role in the function. Maximize clarity and readability when naming type parameters, just as you would with variables or function parameters. Follow Rust's conventions, though: use upper camel case for type parameter names.

The function signature is king

You may wonder why we need trait bounds at all. Can't the compiler infer the required traits from the function's body?
It could, but it won't.
The rationale is the same as for explicit type annotations on function parameters: each function signature is a contract between the caller and the callee, and the terms must be explicitly stated. This allows for better error messages, better documentation, less unintentional breakages across versions, and faster compilation times.

Exercise

The exercise for this section is located in 04_traits/05_trait_bounds

100 Exercises To Learn Rust