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:

1. fn duplicate<T: Clone>(a: T) -> (T, T) {
2. (a.clone(), a.clone())
3. }

5. // Syntactic sugar for:
6. //   fn add_42_millions<T: Into<i32>>(x: T) -> i32 {
7. fn add_42_millions(x: impl Into<i32>) -> i32 {
8. x.into() + 42_000_000
9. }

11. // struct NotClonable;

13. fn main() {
14. let foo = String::from("foo");
15. let pair = duplicate(foo);
16. println!("{pair:?}");

18. let many = add_42_millions(42_i8);
19. println!("{many}");
20. let many_more = add_42_millions(10_000_000);
21. println!("{many_more}");
22. }

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>.