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