error[E0596]: cannot borrow `*self.s` as mutable, as it is behind a `&` reference
  --> $DIR/issue-38147-1.rs:17:9
   |
LL |     fn f(&self) {
   |          ----- help: consider changing this to be a mutable reference: `&mut self`
LL |         self.s.push('x');
   |         ^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.