trait C {}

impl C for A {}

impl C for B {}

pub struct A {}

pub struct B {}

fn main() {
    println!("Hello, world!");
        let x = "a";
        let s: Box<dyn C> = match x {
        "a" => Box::new(A {}),
        "b" => Box::new(B {}),
        _ => panic!()
    };
}