Detect (non-raw) borrows of null ZST pointers in CheckNull

[compiler-errors]

Fixes #136568. Ensures that we check that borrows of derefs are non-null in the CheckNull pass even if it's a ZST pointee.

I'm actually surprised that this is UB in Miri, but if it's certainly UB, then this PR modifies the null check to be stricter. I couldn't find anywhere in https://doc.rust-lang.org/reference/behavior-considered-undefined.html that discusses this case specifically, but I didn't read it too closely, or perhaps it's just missing a bullet point.

On the contrary, if this is actually erroneous UB in Miri, then I'm happy to close this (and perhaps fix the null check in Miri to exclude ZSTs?)

On the double contrary, if this is still an "open question", I'm also happy to close this and wait for a decision to be made.

r? @saethlin cc @RalfJung (perhaps you feel strongly about this change)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[theemathas]

I think this is definitely UB, because null references, even of ZSTs, already currently cause "unexpected" behavior.

For example, this code prints None, even in debug mode:

fn main() {
    let null_ref = unsafe { &*std::ptr::null::<()>() };
    println!("{:?}", Some(null_ref));
}

This code panics in debug mode, but prints None () in release mode.

fn main() {
    let null_ref = unsafe { &*std::ptr::null::<()>() };
    let x = Some(std::hint::black_box(null_ref));
    let y = x.unwrap();
    println!("{x:?} {y:?}");
}

[compiler-errors]

Oh, that is a good point. I forgot that null references of ZSTs are definitely UB bc of the niche.

[RalfJung]

References are non-null (validity/language invariant), and "creating an invalid value" is insta-UB. So yes, &*ptr::null::<T>() is UB for every T.

[saethlin]

r=me with comments tweaked ^

[rustbot]

This PR changes Stable MIR

cc @oli-obk, @celinval, @ouz-a

[compiler-errors]

I've also fixed the typo of occured -> occurred.

@bors r=saethlin

[bors]

📌 Commit a61537f has been approved by saethlin

It is now in the queue for this repository.