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

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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%)

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

rustbot assigned jdonszelmann Sep 14, 2026

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Comment thread compiler/rustc_ast_lowering/src/lib.rs Outdated

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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

jdonszelmann requested changes Sep 14, 2026

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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

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 14, 2026

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

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rust-timer commented Sep 14, 2026

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

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