Finish stabilization of `result_ffi_guarantees` #130628

workingjubilee · 2024-09-20T16:31:05Z

The internal linting has been changed, so all that is left is making sure we stabilize what we want to stabilize.

rustbot · 2024-09-20T16:31:13Z

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workingjubilee · 2024-09-20T16:38:55Z

Also cc @rust-lang/lang for your information, as y'all already FCPed this in rust-lang/rfcs#3391 (comment)

compiler/rustc_feature/src/accepted.rs

Noratrieb · 2024-09-20T18:00:28Z

tests/ui/rfcs/rfc-3391-result-ffi-guarantees.rs

@@ -1,3 +1,4 @@
+//@ build-pass


can you add a test for your own custom result-like type here to ensure that it's not special cased to Result?
additionally, do we have tests that guarantee that, you know, this guarantee actually holds for various examples? would be good to have them :)

The RFC does not actually extend beyond Result, actually.

well then improper_ctypes is implemented incorrectly, because it allows all result-like enums. the comment above the feature declaration also says result-like

I'm happy to write more tests anyways, mind.

well, we'd need to make sure what we actually want to guarantee before writing tests or changing the lint^^

well then improper_ctypes is implemented incorrectly, because it allows all result-like enums. the comment above the feature declaration also says result-like

Hm.

i have been informed and see in the code that it also allows all option-like enums so... yolo?

The feature description should certainly at least be correct here.

The logic about "Option-like types" is ooooold, it exists at least Since Rust 1.5 where it got moved in this PR.

(There's also a "FIXME: This duplicates code in trans" there, which still exists, and which I have no idea if it is still true -- it is unclear to me where that duplicate code would be and what it would do.)

I think the original logic for recognizing Option-like types was added when the lint was originally implemented, in 25f9534. It was just always carried over since then.

So... I think for Result we don't want this, we want to specifically recognize that type. And probably for Option, too, but that's a different PR.

Noratrieb · 2024-09-20T18:22:30Z

https://doc.rust-lang.org/stable/std/primitive.fn.html#abi-compatibility, our "most canonical" place to put ABI docs, isn't updated yet, it only mentions Option<T>

workingjubilee · 2024-09-20T18:38:06Z

Oh, good catch!

ehuss · 2024-09-20T19:14:24Z

@ehuss has raised the concern that the documentation updates are unfinished.

To repeat my answer in the issue, I think if std's Result docs are correct¹, then that should be fine for now. The Reference docs are lagging and work is being done to catch up, but doesn't need to block anything here.

Though it seems like those docs were missing a caveat that the ABI was not guaranteed for FFI, which I would expect this PR to then remove. ↩

workingjubilee · 2024-09-20T20:02:38Z

current TODOs:

I want to write another test for sizedness checking of some types, at least
~~Correct feature description~~
Update std docs for fn(...T) -> R: https://doc.rust-lang.org/stable/std/primitive.fn.html#abi-compatibility
Fix comment
??? stabilization report???

workingjubilee · 2024-09-20T20:03:05Z

@rustbot author

tmandry · 2024-09-20T23:38:50Z

@rfcbot fcp merge

This completes stabilization of RFC 3391, which guarantees the layout of Option<T>, Result<T, ()>, and Result<(), T> where T is a nonzero type like Nonnull, function pointer, etc. to be the same as the underlying type.

These semantics were already documented prematurely without lang FCP in #124870. The effect of this RFC appears to be to stop linting on such types in extern "C" fn declarations as not being FFI safe.

This FCP serves as signoff that we are fine with fully stabilizing the RFC.

@rustbot label I-lang-nominated

rfcbot · 2024-09-20T23:38:52Z

Team member @tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

compiler/rustc_lint/src/types.rs

RalfJung · 2024-09-21T07:20:42Z

Update std docs for fn(...T) -> R: https://doc.rust-lang.org/stable/std/primitive.fn.html#abi-compatibility

Ah, I now have a separate PR for that before I saw you had this on your list: #130653.

Feel free to cherry-pick that commit into your PR an close mine.

workingjubilee · 2024-09-21T14:29:23Z

I'm happy-enough to just let that land.

rust-timer · 2024-10-21T12:28:14Z

Finished benchmarking commit (3e33bda): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 3.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 8.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	8.1%	[8.1%, 8.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 780.893s -> 782.557s (0.21%)
Artifact size: 333.66 MiB -> 333.66 MiB (0.00%)

rustbot assigned BoxyUwU Sep 20, 2024

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workingjubilee mentioned this pull request Sep 20, 2024

Tracking Issue for result_ffi_guarantees (RFC 3391) #110503

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cuviper reviewed Sep 20, 2024

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-blocked Status: Blocked on something else such as an RFC or other implementation work. labels Sep 20, 2024

tmandry added T-lang Relevant to the language team and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 20, 2024

rustbot added the I-lang-nominated Nominated for discussion during a lang team meeting. label Sep 20, 2024

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Sep 20, 2024

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traviscross mentioned this pull request Sep 21, 2024

ABI compatibility: mention Result guarantee #130653

Merged

rustbot added this to the 1.84.0 milestone Oct 21, 2024

This was referenced Oct 21, 2024

abi/compatibility: also test Option-like types #132002

Merged

ABI checks do not accept Option-"like" types rust-lang/miri#3983

Closed

update ABI compatibility docs for new option-like rules #132003

Merged

chorman0773 mentioned this pull request Oct 21, 2024

Spec abi chapter rust-lang/reference#1545

Open

workingjubilee deleted the clean-up-result-ffi-guarantees branch October 21, 2024 21:16

RalfJung mentioned this pull request Oct 23, 2024

miri: update ABI compat checks to accept Option-like types #132057

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Imberflur mentioned this pull request Nov 15, 2024

RFC: Unsafe Set Enum Discriminants rust-lang/rfcs#3727

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apiraino removed the to-announce Announce this issue on triage meeting label Apr 30, 2025

zachs18 mentioned this pull request Jun 3, 2025

Specify that "option-like" enums must be #[repr(Rust)] to be ABI-compatible with their non-1ZST field. #141947

Merged

RalfJung mentioned this pull request Sep 13, 2025

Result/Option layout guarantee clarifications #146509

Open

Finish stabilization of result_ffi_guarantees #130628

Finish stabilization of result_ffi_guarantees #130628

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Noratrieb commented Sep 20, 2024

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ehuss commented Sep 20, 2024

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rust-timer commented Oct 21, 2024

Overall result: ✅ improvements - no action needed

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Finish stabilization of `result_ffi_guarantees` #130628

Finish stabilization of `result_ffi_guarantees` #130628

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