Trait Bounds

When working with generics, you often want to require the types to implement some trait, so that you can call this trait’s methods.

You can do this with T: Trait or impl Trait:

fn duplicate<T: Clone>(a: T) -> (T, T) {
    (a.clone(), a.clone())
}

// struct NotClonable;

fn main() {
    let foo = String::from("foo");
    let pair = duplicate(foo);
    println!("{pair:?}");
}
This slide should take about 8 minutes.

  • Try making a NonClonable and passing it to duplicate.

  • When multiple traits are necessary, use + to join them.

  • Show a where clause, students will encounter it when reading code.

    fn duplicate<T>(a: T) -> (T, T)
where
    T: Clone,
{
    (a.clone(), a.clone())
}
    • It declutters the function signature if you have many parameters.
    • It has additional features making it more powerful.
      • If someone asks, the extra feature is that the type on the left of “:” can be arbitrary, like Option<T>.

  • Note that Rust does not (yet) support specialization. For example, given the original duplicate, it is invalid to add a specialized duplicate(a: u32).