error: &mut return type is not allowed unless there is a &mut argument
  --> tests/ui/mut_return.rs:10:9
   |
10 |         fn f(t: &Thing) -> Pin<&mut CxxString>;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: &mut return type is not allowed unless there is a &mut argument
  --> tests/ui/mut_return.rs:14:9
   |
14 |         fn j(t: &Thing) -> &mut [u8];
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^