struct S {
    a: String,
}

fn library_fn(s: &String) -> String {
    let mut r = String::from("foo");
    r.push_str(&s[..]);
    r
}

impl S {
    fn change_a(&mut self) {
        self.a = library_fn(&self.a);
    }
}

fn main () {
    let mut s = S { a: String::from("bar") };
    s.change_a();
    println!("{:?}",s.a);
}