error: Undefined Behavior: constructing invalid value at .<enum-tag>: encountered $HEX, but expected a valid enum tag
  --> $DIR/invalid_enum_tag.rs:LL:CC
   |
LL |     let _f = unsafe { std::mem::transmute::<i32, Foo>(42) };
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-tag>: encountered $HEX, but expected a valid enum tag
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `main` at $DIR/invalid_enum_tag.rs:LL:CC

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error