Rustc test exhibits UB? (or MIR optimizations are broken)

[oli-obk]

https://github.com/rust-lang/rust/blob/b1363a73ede57ae595f3a1be2bb75d308ba4f7f6/src/test/run-pass/cast-rfc0401-vtable-kinds.rs prints nothing if run without mir optimizations and triggers assert_eq!(fl, (43+4)); if run with mir optimizations.

The critical code is

unsafe fn round_trip_and_call<'a>(t: *const (Foo<u32>+'a)) -> u32 {
    let foo_e : *const Foo<u16> = t as *const _;
    let r_1 = foo_e as *mut Foo<u32>;

    (&*r_1).foo(0)
}

Found by miri

cc @arielb1

[oli-obk]

dupe of #40473