Sandbox::https://github.com/rust-lang/rust/pull/165481

asquared31415 · 2026-11-05T02:01:01Z

This properly gives bool its own ABI, shared only with bool-like enums that have no explicit repr. #[repr($int)] enums now always use the ABI of $int.

This PR makes no language guarantees, however it will help facilitate such guarantees, if they're decided on: rust-lang/unsafe-code-guidelines#628

Additionally, it seems to give LLVM slightly more information about the tags of certain enums, especially when it comes to variants with different sizes.

Previously, bools were identified for the purposes of ABI as being any u8 primitive that had a valid range of 0..=1. An enum with #[repr(u8)] and with discriminants 0 and 1 would satisfy that condition and be treated like a bool. This is somewhat sketchy, and while no miscompilation has been found, I would not be surprised if the i1 Extension Situation could cause problems on some targets.

fixes #165026

mu001999 · 2026-11-05T02:29:05Z

@bors try @rust-timer queue

Separate `Primitive::Bool` and make `#[repr($int)]` enums always use the ABI of $int

rust-bors · 2026-11-05T03:27:15Z

☀️ Try build successful (CI)
Build commit: 309247f (309247f2d061081730fed50fb52ae20610509cc8)
Base parent: 2d20b42 (2d20b42b883dcfc014f3c2c56081eeb6091c2179)

rust-timer · 2026-11-05T04:16:15Z

Finished benchmarking commit (309247f): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 3.2%, secondary 0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.2%	[2.1%, 3.8%]	3
Regressions ❌ (secondary)	2.5%	[2.1%, 3.2%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-5.3%, -2.5%]	3
All ❌✅ (primary)	3.2%	[2.1%, 3.8%]	3

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 480.462s -> 480.743s (0.06%)
Artifact size: 406.22 MiB -> 406.31 MiB (0.02%)

asquared31415 · 2026-11-06T02:35:37Z

This file is one of two meaningful test changes, the rest are debug layouts changing as expected.

This function and the corresponding function converting the other way are "regressed" in that now there will be an explicit trunc or zext. They were testing for the bad behavior (a repr(u8) that's actually a i1).

The trunc/ext should go away at the assembly level though.

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asquared31415 · 2026-11-06T02:37:19Z

This file is the second meaningful test change.

The changes here are scary because I'm not quite sure of the requirements, but I asked on zulip for a second opinion.

I believe this is a correct change, with LLVM using more knowledge about the enum tag to generate a different instruction.

View changes since the review

Yeah, this looks fine to me. It makes sense that LLVM now exploits the known 0..=1 range of the discriminant.

workingjubilee · 2026-11-07T02:20:49Z

 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "nightly", derive(StableHash))]
 pub enum Primitive {
+    Bool,


I had been wondering when this would happen. Thanks.

View changes since the review

I'm still not so sure about it, between the number of things that were silently non-exhaustively matching on Primitive for the integer type and discarding the rest, and the fact that bool really is a Fancy Integer, I almost want to try and integrate it into the Integer variant.

But also, bool clearly acts differently in several places, so it sort of is special.

Hmm. I have not scrutinized this PR closely. I think putting it in Integer would also make sense to me, but I'm going to wildly guess that it actually goes less well.

It, at the very least, implies the existence of (bool, is_signed), which im sure explodes things if that accidentally leaks through. Integer also seems to be about being the byte-level "storage" type, so that probably isn't the best idea.

There probably exists a better version of Primitive, with a complete refactor, but this PR seems to at least disentangle bool Enough(tm) to get it to be its own ABI. We can delegate further things to "Future Work".

rustbot · 2026-11-07T16:38:31Z

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

rustbot · 2026-11-07T16:38:33Z

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workingjubilee · 2026-11-12T22:35:05Z

I should probably try to make sure I look at this.

asquared31415 · 2026-11-12T23:03:02Z

Given the discussions in I think that the best thing for now is to add a "FIXME: this should work but doesn't" for test_abi_newtype::<bool>, and I don't think this makes anything worse in practice and is part of the path forward to improving the ABI Situations.

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Auto merge of #165481 - asquared31415:enum-int-repr, r=<try>

309247f

Separate `Primitive::Bool` and make `#[repr($int)]` enums always use the ABI of $int

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asquared31415 marked this pull request as ready for review November 7, 2026 16:38

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Comment thread compiler/rustc_abi/src/lib.rs Resolved

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RalfJung mentioned this pull request Nov 11, 2026

bool and MaybeUninit<bool> are not actually ABI compatible #165800

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workingjubilee self-assigned this Nov 12, 2026

ensure repr(int) on enums uses the same ABI as int

aa642a9

rust-cloud-vms Bot force-pushed the enum-int-repr branch from 034fa54 to aa642a9 Compare November 12, 2026 23:32

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