error[E0499]: cannot borrow `r` as mutable more than once at a time
  --> $DIR/hold_ref.rs:9:15
   |
8  |     let s1 = r.read_str().unwrap();
   |              - first mutable borrow occurs here
9  |     let _s2 = r.read_str().unwrap();
   |               ^ second mutable borrow occurs here
10 |     black_box(s1);
   |               -- first borrow later used here