Supertraits

A trait can require that types implementing it also implement other traits, called supertraits. Here, any type implementing Pet must implement Animal.

trait Animal {
    fn leg_count(&self) -> u32;
}
trait Pet: Animal {
    fn name(&self) -> String;
}
struct Dog(String);
impl Animal for Dog {
    fn leg_count(&self) -> u32 {
        4
    }
}
impl Pet for Dog {
    fn name(&self) -> String {
        self.0.clone()
    }
}
fn main() {
    let puppy = Dog(String::from("Rex"));
    println!("{} has {} legs", puppy.name(), puppy.leg_count());
}

This is sometimes called “trait inheritance” but students should not expect this to behave like OO inheritance. It just specifies an additional requirement on implementations of a trait.