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Rollup merge of #132423 - RalfJung:const-eval-align-offset, r=dtolnay

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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 https://github.com/rust-lang/rust/issues/90962
Closes https://github.com/rust-lang/rust/issues/104203

Cc `@rust-lang/wg-const-eval` `@rust-lang/libs-api`
This commit is contained in:
Jubilee
2024-11-03 20:08:13 -08:00
committed by GitHub
parent f35433e250 19e287060d
commit 3313e760d0
16 changed files with 185 additions and 1022 deletions
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2
library/core/src/lib.rs
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@@ -112,7 +112,6 @@
#![feature(asm_experimental_arch)]
#![feature(const_align_of_val)]
#![feature(const_align_of_val_raw)]
#![feature(const_align_offset)]
#![feature(const_alloc_layout)]
#![feature(const_black_box)]
#![feature(const_char_encode_utf16)]
@@ -123,7 +122,6 @@
#![feature(const_nonnull_new)]
#![feature(const_option_ext)]
#![feature(const_pin_2)]
#![feature(const_pointer_is_aligned)]
#![feature(const_ptr_is_null)]
#![feature(const_ptr_sub_ptr)]
#![feature(const_raw_ptr_comparison)]