fn main() {
    let mut x = String::from("aaa");
    let mut f = |z: &String| {
        x.push_str("ccc");
        z.len() == x.len()
    };

    let y = String::from("bbb");
    let a = f(&y);
    println!("{}", a);
    println!("{:?}", x);
    println!("{:?}", y);
}