Use NonNull

[SabrinaJewson]

This allows Option<AnyVec> to have the same size as AnyVec. But the main reason I did this was to fix the null pointer deref that can occur when allocation fails by using the type-system to force us to handle that case with handle_alloc_error. Additionally, it fixes the UB of taking multiple unique references to ZST_SLICE by using NonNull::dangling() instead.

[tower120]

This allows Option to have the same size as AnyVec

I know that Option<NonZero/NonNull> have the same size as NonZero/NonNull, but struct AnyVec have other members...
I didn't know that making one member NonNull will make struct Option -friendly.

Additionally, it fixes the UB of taking multiple unique references to ZST_SLICE by using NonNull::dangling() instead.

You mean taking multiple as_slice ? If so, why with NonNull::dangling() this is not the case?

[SabrinaJewson]

The issue was that if with_capacity was called in parallel from multiple threads, it could end up creating multiple co-existing unique references to the ZST_SLICE constant, which is UB.

[tower120]

Ok. But what about calling as_slice with mem = NonNull::dangling()? Aren't that crate reference from invalid memory pointer?

[SabrinaJewson]

No, that's fine — even though it's invalid you're not reading any bytes so it's OK. Box does it for allocating Boxes around ZSTs.

[tower120]

But if follow that logic, aren't multiple references to ZST_SLICE is OK too? Since we never touch it.

[SabrinaJewson]

No, because a unique reference asserts that no other unique references exist to the same location, whether or not you use the reference.

fn unsound() {
    static mut X: [u8; 1] = [0];
    // It is not sound to create multiple unique references to the same
    // location if it is not a ZST.
    unsafe { &mut X };
}
fn sound() {
    // It is sound to create multiple unique references to the same
    // location if they come from different places and are ZSTs
    unsafe { <NonNull<[u8; 0]>>::dangling().as_mut() };
}

[tower120]

// It is not sound to create multiple unique references to the same
// location if it is not a ZST.

Wait a minute.
So if instead of:

static mut ZST_ARRAY: [u8;1] = [0];

It was:

struct empty{};
static mut X: [empty; 1] = [empty{}];

That would be OK?

Were I can read about multi &mut ZST references?

[SabrinaJewson]

That would be OK?

I am not quite sure on that, but it should definitely be avoided since we have a better alternative that is definitely sound.

Were I can read about multi &mut ZST references?

I found this issue which might be interesting.

[tower120]

Thank you for this PR!