Rust Traits and Enums

trait Shape {
    fn area(&self) -> f64;
}

struct Circle(f64);
struct Rect(f64, f64);

impl Shape for Circle {
    fn area(&self) -> f64 { std::f64::consts::PI * self.0 * self.0 }
}
impl Shape for Rect {
    fn area(&self) -> f64 { self.0 * self.1 }
}

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
}

fn show(m: &Message) -> String {
    match m {
        Message::Quit => "quit".to_string(),
        Message::Move { x, y } => format!("move to {},{}", x, y),
        Message::Write(t) => format!("write {}", t),
    }
}

fn main() {
    let shapes: Vec<Box<dyn Shape>> = vec![Box::new(Circle(1.0)), Box::new(Rect(2.0, 3.0))];
    for s in &shapes {
        println!("{:.2}", s.area());
    }
    println!("{}", show(&Message::Move { x: 1, y: 2 }));
    println!("{}", show(&Message::Write("hi".to_string())));
    println!("{}", show(&Message::Quit));
}

3.14
6.00
move to 1,2
write hi
quit