Lower attributes for functions without bodies by JonathanBrouwer · Pull Request #162761 · rust-lang/rust

JonathanBrouwer · 2026-09-14T11:37:56Z

This is a temporary solution to get the bug fixed.
I'm going to see if we can move the function hir::Params from hir::Body to hir::FnSig, this would cleanly fix this bug and would undo most of this PR. That is a large project which might take me some time to complete, so I'd prefer to get the bug fixed first with this workaround.

Maybe r? @jdonszelmann since you have context?
cc @mejrs cause you're probably interested

rustbot · 2026-09-14T11:38:02Z

jdonszelmann is currently at their maximum review capacity.
They may take a while to respond.

jdonszelmann · 2026-09-14T12:50:21Z


 warning: the `must_use` attribute cannot be used on function params
-  --> $DIR/param-attrs-builtin-attrs.rs:40:7
+  --> $DIR/param-attrs-builtin-attrs.rs:29:7


the check for these warnings should go, since it causes double emissions

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Let's fix this in a follow-up PR, it will be quite a big diff to remove this error and replace it by target checking consistently

JonathanBrouwer · 2026-09-14T12:51:16Z

@rust-lang/lang Is this a PR that you would want to take a look at?
This makes the following code, which was incorrectly allowed, no longer compile.
I believe there is some decision that for attribute fixes a ping was enough? I can't find that tho

fn help(x: fn(#[rustc_splat] usize)) {}

JonathanBrouwer · 2026-09-14T13:05:10Z

@rustbot ready

jdonszelmann · 2026-09-14T13:08:25Z

r=me then after some acknowledgement from t-lang. I think it falls under the same rule as earlier breaking changes in attrs (i.e. notify t-lang but do the change) but let them acknowledge that. Notably, this attribute is rather new, it's unlikely anyone depends on this.

JonathanBrouwer · 2026-09-14T13:56:21Z

Just in case
@bors try @rust-timer queue

Lower attributes for functions without bodies

teor2345

Apart from the comment tweak, this looks like what I'd expect to happen for splat. Thank you for this fix!

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teor2345

Did we also want to backport this to beta or stable?

Sorry for the multiple notifications 🙂

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teor2345 · 2026-09-14T14:37:29Z

    //~| ERROR allow, cfg, cfg_attr, deny, expect, forbid, and warn are the only allowed built-in attributes
 }

+trait Test {


I don't know if it's worth duplicating the trait, type, and extern tests in the splat feature gate test, up to you.

https://github.com/rust-lang/rust/blob/ada41e1ce81819f01577c0ee40ccbd6fc41384e8/tests/ui/feature-gates/feature-gate-splat.rs

I think it's not worth duplicating the test

rust-bors · 2026-09-14T14:55:52Z

☀️ Try build successful (CI)
Build commit: ed70b09 (ed70b094daccd48a86a95d8b0acdc0dfb125b3d0)
Base parent: ed77b7b (ed77b7b8699e342c2dc842c83cb5fe4025252ee8)

JonathanBrouwer · 2026-09-14T15:11:23Z

Don't think a stable backport is worth it. We can consider a beta backport, I opened a thread for it

rust-timer · 2026-09-14T16:08:54Z

Finished benchmarking commit (ed70b09): comparison URL.

Overall result: ❌✅ regressions and improvements - 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

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (primary 1.5%, secondary -8.5%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[0.8%, 2.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.5%	[-8.7%, -8.4%]	2
All ❌✅ (primary)	1.5%	[0.8%, 2.3%]	2

Binary size

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

Bootstrap: 495.332s -> 494.947s (-0.08%)
Artifact size: 406.96 MiB -> 406.95 MiB (-0.00%)

traviscross · 2026-09-15T18:52:12Z

I looked, and I could not find a delegation that would cover this.¹ The default rule is that breaking changes to the language require a lang FCP.

In this PR, we're breaking:

fn help(x: fn(#[rustc_splat] usize)) {}

That makes sense to me. I propose we do it.

@rfcbot fcp merge lang

As negative results, see, e.g., https://github.com/rust-lang/rust/issues/142838 and https://github.com/rust-lang/rust/pull/155193#issuecomment-4389641640. ↩

rust-rfcbot · 2026-09-15T18:52:15Z

@traviscross 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.

teor2345 · 2026-09-16T11:02:42Z

In this PR, we're breaking:
fn help(x: fn(#[rustc_splat] usize)) {}

For FCP context, this is an unstable incomplete feature, that was never meant to be usable on stable. (It currently ICEs on stable 1.98, and on 1.99 beta it works but isn't really usable, because splatted function items can only be declared on nightly.)

tmandry · 2026-09-16T15:34:19Z

This is a bugfix.

@rfcbot reviewed

rust-bors · 2026-09-23T07:14:01Z

☔ The latest upstream changes (presumably #163189) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rust-rfcbot · 2026-09-23T17:11:54Z

🔔 This is now entering its final comment period, as per the review above. 🔔

scottmcm · 2026-09-23T17:12:35Z

Agreed that this was clearly never supposed to be stable.

@rfcbot reviewed

teor2345 · 2026-09-23T18:18:08Z

Agreed that this was clearly never supposed to be stable.

@rfcbot reviewed

Triage note: after an hour, @scottmcm's box hasn't been ticked by the bot yet, so I ticket it manually.
(This doesn't change whether the FCP is over the threshold.)

JonathanBrouwer added 2 commits September 14, 2026 12:36

Route fn_hir_id into lower_fn_decl

d2bc932

Add regression tests

f7e96ba

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 14, 2026

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JonathanBrouwer force-pushed the lower-param-attrs branch 3 times, most recently from 29274ef to 5bf5ae2 Compare September 14, 2026 12:28

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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-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 14, 2026

JonathanBrouwer force-pushed the lower-param-attrs branch from 5bf5ae2 to e6552cd Compare September 14, 2026 13:05

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JonathanBrouwer added the S-waiting-on-t-lang Status: Awaiting decision from T-lang label Sep 14, 2026

JonathanBrouwer mentioned this pull request Sep 14, 2026

Remove the FnParamForbiddenAttr diagnostic #162770

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 14, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 14, 2026

Auto merge of #162761 - JonathanBrouwer:lower-param-attrs, r=<try>

ed70b09

Lower attributes for functions without bodies

teor2345 approved these changes Sep 14, 2026

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Lower attributes for functions without bodies

cad361c

JonathanBrouwer force-pushed the lower-param-attrs branch from e6552cd to cad361c Compare September 14, 2026 15:07

JonathanBrouwer added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Sep 14, 2026

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 14, 2026

rust-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 15, 2026

JonathanBrouwer removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Sep 16, 2026

rust-rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Sep 23, 2026

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rust-timer commented Sep 14, 2026

Overall result: ❌✅ regressions and improvements - no action needed

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