Miri approved way to rejoin the slices

[alice]

Ah, I misunderstood. I was looking at swapping the sub-buffer, but the issue was when swapping self. However, it seems to me that this could be fixed by checking whether self has been swapped for a different buffer. playground

@matklad

@alice

An unrelated point, but the backing storage should be &mut [MaybeUninit<u8>] to avoid this from being UB:

Oh wow! Do I understand this right? What happens here is that we change the buf itself to be uninitialized. That is, the API allows allocating whatever from the underlying bytes, and we can allocate, eg, MaybeUninit, through which we can mark original memory as no-longer initialized. So, by the time we drop mem, the original buffer contains radioactive garbage which we shouldn't be able to touch?

I wonder if MaybeUninit is even needed here? Like, what if we allocate T with some padding bytes, into &buf, and then look at those padding bytes? Would that be UB?

Yes, padding causes the same issue.