Implement some more checks in ptr_guaranteed_cmp.

[zachs18]

• Pointers with different residues modulo their allocations' least common alignment are never equal.
• Pointers to the same static allocation are equal if and only if they have the same offset.
• Pointers to different non-zero-sized static allocations are unequal if both point within their allocation, and not on opposite ends.

Tracking issue for const_raw_ptr_comparison: #53020

This should not affect is_null, the only usage of this intrinsic on stable.

Closes #144584

[rustbot]

r? @davidtwco

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[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[theemathas]

Are two different immutable statics required to have distinct addresses? In other words, is the compiler allowed to optimize two immutable statics with identical contents to be stored at the same place?

[RalfJung]

I don't think it is, but this also restricts people doing their own custom linking shenanigans, so we should have some stable docs to point to before having logic of this sort in const-eval. That sounds like something t-lang should sign off on.

[RalfJung]

If you just want to fix #144584, please reduce the PR to only affect the case where both pointers are based on the same static.

[zachs18]

Are two different immutable statics required to have distinct addresses? In other words, is the compiler allowed to optimize two immutable statics with identical contents to be stored at the same place?

Yes, they are required to have distinct addresses (if non-zero-sized). No, the compiler is not allowed to coalesce two immutable statics with the same contents.

we should have some stable docs to point to before having logic of this sort in const-eval.

This is guaranteed for non-zero-sized statics, in the Reference:

https://doc.rust-lang.org/nightly/reference/items/static-items.html#r-items.static.storage-disjointness

If the static has a size of at least 1 byte, this allocated object is disjoint from all other such static objects as well as heap allocations and stack-allocated variables. However, the storage of immutable static items can overlap with objects that do not themselves have a unique address, such as promoteds and const items.

This (non-zero-sized statics not overlapping) has been FCP'd by T-lang here (though not anything about consteval specifically)