error[E0499]: cannot borrow `*val` as mutable more than once at a time
 --> tests/ui/valueref.rs:5:21
  |
4 |     let ref_str = val.get_str()?;
  |                   ------------- first mutable borrow occurs here
5 |     let ref_str_2 = val.get_str()?;
  |                     ^^^^^^^^^^^^^ second mutable borrow occurs here
6 |     println!("{}, {}", ref_str, ref_str_2);
  |                        ------- first borrow later used here