error[E0499]: cannot borrow `circ` as mutable more than once at a time
  --> tests/ui/dca_half_guard_double.rs:25:15
   |
20 |     let g1 = circ.half_guard(&mut ctx, Half::First);
   |              ---- first mutable borrow occurs here
...
25 |     let _g2 = circ.half_guard(&mut ctx, Half::Second);
   |               ^^^^ second mutable borrow occurs here
26 |     let _ = g1.half();
   |             -- first borrow later used here
