error[E0658]: The use of std::mem::transmute() is gated in constants (see issue #53605)
  --> $DIR/feature-gate-const_transmute.rs:16:38
   |
LL | const TRANSMUTED_U32: u32 = unsafe { mem::transmute(Foo(3)) };
   |                                      ^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(const_transmute)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.