Code shared from the Rust Playground

playground.rs

fn main() {
    println!("Hello, world!");
}

fn test(holder: Holder) -> impl Animal {
    match holder {
        Holder::DogHolder(dog) => dog,
        Holder::CatHolder(cat) => cat,
    }
}

struct Dog;
struct Cat;
enum Holder {
    DogHolder(Dog),
    CatHolder(Cat),
}

impl Animal for Dog {
    fn hello(self) -> usize {
        1
    }
}

impl Animal for Cat {
    fn hello(self) -> usize {
        2
    }
}

trait Animal {
    fn hello(self) -> usize;
}