Comment by ynik

You are not testing what you think you are testing.

"let &mut a2 = &mut a;" is pattern-matching away the reference, so it's equivalent to "let a2 = a;". You're not actually casting a mutable reference to a pointer, you're casting the integer 13 to a pointer. Dereferencing that obviously produces UB.

If you fix the program ("let a2 = &mut a;"), then Miri accepts it just fine.