Constify pointer::is_aligned{,to}

[lukas-code]

In this PR i've made <*const T>::is_aligned and <*const T>::is_aligned_to unstably const.
The main motivation here is to allow something like debug_assert!(is_aligned_and_not_null(ptr)) in const fns without any const_eval_select hacks.

If we don't know the address of a pointer in const eval, this function only returns true if we know for sure that the pointer is aligned. This means that in some cases a pointer can never be aligned, which matches the behavior of align_offset¹.

Tracking issue for is_aligned: #96284

Footnotes

1. Note that currently align_offset will always return usize::MAX in CTFE, but it could return a useful value for some alignments in the future. ↩

[rust-highfive]

r? @joshtriplett

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[RalfJung]

Cc @rust-lang/wg-const-eval

I am not entirely sure about this... pointers are wonky in const and this function does not behave the way people will expect it to behave.

Also I think we should keep this and align_offset consistent, and we don't need a new intrinsic for this if we just do align_offset == 0.

[oli-obk]

Also I think we should keep this and align_offset consistent, and we don't need a new intrinsic for this if we just do align_offset == 0.

yea, it should not add a new intrinsic, but be implemented in terms of the existing one.

[oli-obk]

I am not entirely sure about this... pointers are wonky in const and this function does not behave the way people will expect it to behave.

the same is true for align_offset and [T]::align_to... which kind makes me want to just declare them "never gonna be const" and instead push the obligation to the caller who can use const_eval_select to skip the check

[saethlin]

I think doing that would make those interfaces more popular, people should be using the standard library instead of re-implementing these. Right now I know of a few people who specifically avoid them because of the vague caveat in the docs. If we remove that, there should be a proper announcement of the change in direction we're taking in designing pointer interfaces.

And I for one would like pressure for const_eval_select to be more usable.

[RalfJung]

Pressure doesn't help. People doing the work help. :)

[oli-obk]

And I for one would like pressure for const_eval_select to be more usable.

I'm on it 😆

[saethlin]

I think I meant a different kind of pressure. If I can tell a compelling story, it's easy to motivate community members to pitch in.

[lukas-code]

I've put up an alternate PR which doesn't need a new intrinsic and uses align_offset instead: #102795

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This PR changes MIR

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[lukas-code]

Whoops, sorry for the massive ping. Pushed something here that I shouldn't have. Anyway, closing this in favor of #102795.