`alloc::realloc` does not follow requirements of `Layout::from_size_align_unchecked`

[overlookmotel]

The docs for Layout::from_size_align_unchecked state that:

size, when rounded up to the nearest multiple of align, must not overflow isize (i.e., the rounded value must be less than or equal to isize::MAX)

alloc::realloc's docs state a different constraint:

new_size, when rounded up to the nearest multiple of layout.align(), must not overflow (i.e., the rounded value must be less than usize::MAX).

So this would be legal according to alloc::realloc's docs:

let layout = alloc::Layout::from_size_align(1, 1);
let ptr1 = alloc::alloc(layout);
let ptr2 = alloc::realloc(ptr1, layout, usize::MAX);

But this results in a call internally:

Layout::from_size_align_unchecked(usize::MAX, 1)

...which is meant to be UB.

I assume this is just that the docs are out of date after #95295.

[thomcc]

Yes, this seems like a documentation bug to me.

[cuviper]

This was noted in #95295 (comment), discussed at length, and deferred in #95295 (comment): "This PR does not make a decision about realloc." There is also an ACP: rust-lang/libs-team#61

[overlookmotel]

Apologies. I didn't read the whole (lengthy) thread. Just noticed the discrepancy in the docs, and was thoroughly confused why they didn't correspond.

Closing this in favour of rust-lang/libs-team#61

I'll leave the PR open as I see you've tagged it "waiting for team", but feel free to just close it if you prefer. It's a trivial change anyway.