error: cannot borrow immutable borrowed content `*a` as mutable
  --> $DIR/mut-arg-hint.rs:18:5
   |
17 | pub fn foo<'a>(mut a: &'a String) {
   |                       ---------- use `&'a mut String` here to make mutable
18 |     a.push_str("foo");
   |     ^ cannot borrow as mutable

error: cannot borrow immutable borrowed content `*a` as mutable
  --> $DIR/mut-arg-hint.rs:13:9
   |
12 |     fn foo(mut a: &String) {
   |                   ------- use `&mut String` here to make mutable
13 |         a.push_str("bar");
   |         ^ cannot borrow as mutable

error: cannot borrow immutable borrowed content `*a` as mutable
  --> $DIR/mut-arg-hint.rs:25:9
   |
24 |     pub fn foo(mut a: &String) {
   |                       ------- use `&mut String` here to make mutable
25 |         a.push_str("foo");
   |         ^ cannot borrow as mutable

error: aborting due to 3 previous errors