remove const-support for align_offset and is_aligned #132423
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Operations like is_aligned would return actively wrong results at compile-time, i.e. calling it on the same pointer at compiletime and runtime could yield different results. That's no good. Instead of having hacks to make align_offset kind-of work in const-eval, just use const_eval_select in the few places where it makes sense, which also ensures those places are all aware they need to make sure the fallback behavior is consistent.
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…kingjubilee Rollup of 4 pull requests Successful merges: - rust-lang#131222 (Generate correct symbols.o for sparc-unknown-none-elf) - rust-lang#132423 (remove const-support for align_offset and is_aligned) - rust-lang#132565 (Reduce dependence on the target name) - rust-lang#132576 (remove attribute ids from hir stats (they're simply not needed)) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#132423 - RalfJung:const-eval-align-offset, r=dtolnay remove const-support for align_offset and is_aligned As part of the recent discussion to stabilize `ptr.is_null()` in const context, the general vibe was that it's okay for a const function to panic when the same operation would work at runtime (that's just a case of "dynamically detecting that something is not supported as a const operation"), but it is *not* okay for a const function to just return a different result. Following that, `is_aligned` and `is_aligned_to` have their const status revoked in this PR, since they do return actively wrong results at const time. In the future we can consider having a new intrinsic or so that can check whether a pointer is "guaranteed to be aligned", but the current implementation based on `align_offset` does not have the behavior we want. In fact `align_offset` itself behaves quite strangely in const, and that support needs a bunch of special hacks. That doesn't seem worth it. Instead, the users that can fall back to a different implementation should just use const_eval_select directly, and everything else should not be made const-callable. So this PR does exactly that, and entirely removes const support for align_offset. Closes some tracking issues by removing the associated features: Closes rust-lang#90962 Closes rust-lang#104203 Cc `@rust-lang/wg-const-eval` `@rust-lang/libs-api`
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As part of the recent discussion to stabilize
ptr.is_null()in const context, the general vibe was that it's okay for a const function to panic when the same operation would work at runtime (that's just a case of "dynamically detecting that something is not supported as a const operation"), but it is not okay for a const function to just return a different result.Following that,
is_alignedandis_aligned_tohave their const status revoked in this PR, since they do return actively wrong results at const time. In the future we can consider having a new intrinsic or so that can check whether a pointer is "guaranteed to be aligned", but the current implementation based onalign_offsetdoes not have the behavior we want.In fact
align_offsetitself behaves quite strangely in const, and that support needs a bunch of special hacks. That doesn't seem worth it. Instead, the users that can fall back to a different implementation should just use const_eval_select directly, and everything else should not be made const-callable. So this PR does exactly that, and entirely removes const support for align_offset.Closes some tracking issues by removing the associated features:
Closes #90962
Closes #104203
Cc @rust-lang/wg-const-eval @rust-lang/libs-api