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())

// Syntactic sugar for:
//   fn add_42_millions<T: Into<i32>>(x: T) -> i32 {
fn add_42_millions(x: impl Into<i32>) -> i32 {
    x.into() + 42_000_000

// struct NotClonable;

fn main() {
    let foo = String::from("foo");
    let pair = duplicate(foo);

    let many = add_42_millions(42_i8);
    let many_more = add_42_millions(10_000_000);

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

fn duplicate<T>(a: T) -> (T, T)
    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>.