Can references inside an enum transiently set an invalid enum discriminant?

[nia-e]

Per #414 (comment), opsem consensus appeared to be that a &mut T can write an invalid value of T "temporarily", so long as it's restored to a valid form before returning into safe code. Does this also mean that, for a T with niches, it is valid to overwrite the discriminant of a wrapping enum (e.g. could a &mut NonNull<T> transiently write 0 or uninit even if the said NonNull was part of an Option<NonNull<T>>)?

Relevant to rust-lang/rust#161452

[RalfJung]

Per #414 (comment), opsem consensus appeared to be that a &mut T can write an invalid value of T "temporarily", so long as it's restored to a valid form before returning into safe code

That's not what that discussion was about. That discussion was about creating &mut that point to bad things.

Putting bad things into an &mut that someone else created and gave you is a different question.

I think you are asking the same question as #618.

[RalfJung]

@rust-lang/opsem I'd like to see if we have consensus for making code like this sound:

use std::mem::MaybeUninit;

pub fn foo(x: &mut bool) {
    let ptr = x as *mut bool;
    unsafe { ptr.cast::<MaybeUninit<bool>>().write(MaybeUninit::uninit()) };
    // We must not unwind here! That would leak the invalid state of the `bool` to surrounding code.
    unsafe { ptr.write(true) };
}

This puts limitations on what we do in #84: we cannot make it immediate UB to use a reference to a field of an enum to change the discriminant of the enum. Instead, we have to wait until that reference gets invalidated before we declare UB.

See rust-lang/reference#2338 for Reference-level wording of this.
@rustbot fcp merge

[RalfJung]

Wrong bot...
@rfcbot fcp merge

[RalfJung]

Wrong bot...
@rfcbot fcp merge opsem

[rust-rfcbot]

@RalfJung has proposed to merge this. The next step is review by the rest of the tagged team members:

• @CAD97
• @RalfJung
• @digama0
• @joshlf
• @saethlin

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