interpret: ensure we check bool/char for validity when they are used in a cast

[RalfJung]

In general, Scalar::to_bits is a bit dangerous as it bypasses all type information. We should usually prefer matching on the type and acting according to that. So I also refactored unary_op handling of integers to do that. The remaining to_bits uses are operations that just fundamentally don't care about the sign (and only work on integers).

invalid_char_cast.rs is the key new test, the others already passed before this PR.

r? @oli-obk

[rustbot]

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

The Miri subtree was changed

cc @rust-lang/miri

[oli-obk]

Will this make

const fn cast(ptr: *const bool) { unsafe {
    let _val = *ptr as u32;
}}

const _: () = {
    let v = 3_u8;
    cast(&v as *const u8 as *const bool);
};

error? (it currently works on stable).

I'm aware we can do breaking changes around UB, and I do not expect anyone to be using this anywhere out there, but we should at least have a test and notify T-lang

[RalfJung]

Yes, that errors now -- I added a test. This clearly manifests a bool value out of the 3 so I think it should definitely be UB.

Cc @rust-lang/opsem @rust-lang/lang

[oli-obk]

Agreed.

@bors r+

[bors]

📌 Commit de4ac0c has been approved by oli-obk

It is now in the queue for this repository.

[scottmcm]

(Lang hat on; not speaking for the team)

This change seems clearly fine to me -- a by-value operation on a value of a type outside its validity invariant where that invariant has been clearly communicated for ages.

There might be some more subtle cases of UB where it'd be worth being more cautious, but I don't think this is one of them.