error[E0596]: cannot borrow borrowed content `*self.s` of immutable binding as mutable
  --> $DIR/issue-38147-3.rs:17:9
   |
LL |     s: &'a String
   |        ---------- use `&'a mut String` here to make mutable
...
LL |         self.s.push('x');
   |         ^^^^^^ cannot borrow as mutable

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0596"