error[E0499]: cannot borrow `sc` as mutable more than once at a time
  --> tests/ui/scanout_double_back.rs:34:13
   |
33 |     let a = sc.back_mut();
   |             -- first mutable borrow occurs here
34 |     let b = sc.back_mut();
   |             ^^ second mutable borrow occurs here
35 |     let _ = (a, b);
   |              - first borrow later used here
