error: transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
  --> $DIR/transmute-imut-to-mut.rs:6:32
   |
LL |     let _a: &mut u8 = unsafe { transmute(&1u8) };
   |                                ^^^^^^^^^
   |
   = note: `#[deny(mutable_transmutes)]` on by default

error: aborting due to 1 previous error