error[E0389]: cannot borrow data mutably in a `&` reference
  --> $DIR/issue-38147-4.rs:6:5
   |
LL | fn f(x: usize, f: &Foo) {
   |                   ---- use `&mut Foo` here to make mutable
LL |     f.s.push('x'); //~ ERROR cannot borrow data mutably
   |     ^^^ assignment into an immutable reference

error: aborting due to previous error

For more information about this error, try `rustc --explain E0389`.