1 struct Foo<'a> { 2 s: &'a mut String 3 } 4 f(x: usize, f: &Foo)5 fn f(x: usize, f: &Foo) { 6 f.s.push('x'); //~ ERROR cannot borrow `*f.s` as mutable, as it is behind a `&` reference 7 } 8 main()9 fn main() {} 10