Code shared from the Rust Playground

playground.rs

// Unfortunately no polymorphism

struct Alpha {}

trait Beta {
    fn p(&self, z: &Beta) -> bool;
}

trait Gamma: Beta {}

impl Beta for Alpha {
    fn p(&self, z: &Beta) -> bool {
        true
    }
}

impl Gamma for Alpha {}

fn main() {
    let x: &Gamma = &Alpha {};
    let y: &Beta = x;
    println!("{}", x.p(x));
}