error[E0499]: cannot borrow `back` as mutable more than once at a time
  --> tests/ui/inflight_cpu_access.rs:29:27
   |
25 |     let dst = back.dma_dst();
   |               ---- first mutable borrow occurs here
...
29 |     let _slice = unsafe { back.cpu_slice() };
   |                           ^^^^ second mutable borrow occurs here
30 |     // Force NLL to keep `dst` alive past `cpu_slice()`.
31 |     let _ = dst.dma_addr();
   |             --- first borrow later used here
