error[E0389]: cannot borrow data mutably in a `&` reference
  --> $DIR/issue-38147-4.rs:16:5
   |
15 | fn f(x: usize, f: &Foo) {
   |                   ---- use `&mut Foo` here to make mutable
16 |     f.s.push('x');
   |     ^^^ assignment into an immutable reference

error: aborting due to previous error