error[E0658]: The use of std::mem::transmute() is gated in constants
  --> $DIR/feature-gate-const_transmute.rs:6:38
   |
LL | const TRANSMUTED_U32: u32 = unsafe { mem::transmute(Foo(3)) };
   |                                      ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/53605
   = 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`.