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The Ultimate Guide to Rust Programming

Trait Bounds

Explore how to apply trait bounds in Rust to restrict generic type parameters. Understand how to ensure functions only accept types implementing required traits, enhancing type safety and code flexibility.

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I want to quadruple my numbers. I could do * 4, but that’s so pedestrian. I’ve already got a double method. So why not simply call it twice?

Rust 1.40.0
fn quadruple(x: i32) -> i32 {
    x.double().double()
}
fn main() {
    println!("quadruple 5_i32 == {}", quadruple(5_i32));
}
trait Double {
    fn double(&self) -> Self;
}
impl Double for i32 {
    fn double(&self) -> Self {
        self * 2
    }
}
impl Double for i64 {
    fn double(&self) -> Self {
        self * 2
    }
}

That’s great, but it only works for an i32. If I try to call quadruple on an i64 I’ll get an error message about mismatched types. Fortunately, we’ve already learned all about type parameters. So, let’s say that our quadruple function works on any type:

Rust 1.40.0
fn quadruple<T>(x: T) -> T {
    x.double().double()
}
fn main() {
    println!("quadruple 5_i32 == {}", quadruple(5_i32));
    println!("quadruple 5_i64 == {}", quadruple(5_i64));
}

The compiler does not like this, but it gives us some ...