warning: transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
  --> $DIR/allowed-cli-deny-by-default-lint.rs:8:17
   |
LL |         let y = std::mem::transmute::<&i32, &mut i32>(&5);
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: requested on the command line with `--force-warn mutable-transmutes`

warning: 1 warning emitted