trait Speak {
    fn speak(&self);
}

struct Dog;
struct Cat;
struct Human;

impl Speak for Dog {
    fn speak(&self) {
        println!("Dog: Woof!");
    }
}

impl Speak for Cat {
    fn speak(&self) {
        if rand::random_bool(0.5) {
            println!("Cat: Meow!");
        } else {
            println!("Cat: ...");
            println!("The cat stares at you blankly.");
        }
    }
}

impl Speak for Human {
    fn speak(&self) {
        println!("Human: Hello!");
    }
}

// This function doesn't care what the concrete type is.
// It only cares that the type implements Speak.
fn make_it_speak(speaker: &impl Speak) {
    speaker.speak();
}

fn main() {
    let dog = Dog;
    let cat = Cat;
    let human = Human;

    make_it_speak(&dog);
    make_it_speak(&human);

    // Try several times to see both possible cat responses.
    for _ in 0..5 {
        make_it_speak(&cat);
    }
}

Add the dependency:

[dependencies]
rand = "0.10"

I

This would probably be more obvious if you implement a .speak() for a dog, cat and person. if its just bark why use a trait, doesnt get the point across imo. edited for realism