struct S<'a> {
    s: &'a mut String,
}
#[allow(unused_variables)]
fn main() {
    let s = &mut "hello".to_string();
    let t = S { s };
    let u = &t;

    //let w: &mut String = u.s;
    //w.push_str(" ya");
    //assert_eq!("hello ya", u.s);
    //assert_eq!("hello ya", w);

    let w: &String = u.s;

    let mut x: String = "hi".to_string();
    let y: &String = &x;
    let z: &mut String = &mut x;
}