error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
  --> tests/ui-nightly/transmute-mut-size-decrease.rs:17:32
   |
17 | const DECREASE_SIZE: &mut u8 = transmute_mut!(&mut [0u8; 2]);
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: source type: `[u8; 2]` (16 bits)
   = note: target type: `u8` (8 bits)
   = note: this error originates in the macro `$crate::assert_size_eq` which comes from the expansion of the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)