Use infer tys for synthetic params when lowering const paths point to fns by mu001999 · Pull Request #157625 · rust-lang/rust

mu001999 · 2026-06-08T17:50:47Z

#![feature(min_generic_const_args)]

trait Trait {}
impl<'t> Trait for [(); N] {}
fn N(arg: impl Trait) {}

arg: impl Trait implies a synthetic param <impl Trait> and the obligation _: Trait;
when matching impl Trait for [(); N], lowering const arg N fills in that synthetic param, so the impl header contains <[(); N::<impl Trait>] as Trait>;
then instantiating this header with the impl's args causing a param/arg mismatch and ICE.

This PR uses infer tys for synthetic params. Although this will emit the error in ItemCtxt because infer tys are not allowed here. I think that is reasonable, because N omits the synthetic parameter, though we cannot write the ty explicitly anywhere.

Fixes #155834

rustbot · 2026-06-09T02:44:49Z

HIR ty lowering was modified

cc @fmease

rustbot · 2026-06-09T02:44:51Z

r? @JohnTitor

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JohnTitor

LGTM! @bors r+ rollup

View changes since this review

rust-bors · 2026-06-27T01:21:01Z

📌 Commit 0805fc2 has been approved by JohnTitor

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.

Use infer tys for synthetic params when lowering const paths point to fns ```rust #![feature(min_generic_const_args)] trait Trait {} impl<'t> Trait for [(); N] {} fn N(arg: impl Trait) {} ``` 1. `arg: impl Trait` implies a synthetic param `<impl Trait>` and the obligation `_: Trait`; 2. when matching `impl Trait for [(); N]`, lowering const arg `N` fills in that synthetic param, so the impl header contains `<[(); N::<impl Trait>] as Trait>`; 3. then instantiating this header with the impl's args causing a param/arg mismatch and ICE. This PR uses infer tys for synthetic params. Although this will emit the error in `ItemCtxt` because infer tys are not allowed here. I think that is reasonable, because `N` omits the synthetic parameter, though we cannot write the ty explicitly anywhere. Fixes rust-lang#155834

Rollup of 5 pull requests Successful merges: - #158234 (Cross-referencing tuple_trait tracking issue, source and the Unstable Book) - #157625 (Use infer tys for synthetic params when lowering const paths point to fns) - #158306 (tests: modify s390x vector test to be robust to instruction scheduling) - #158431 (More lint cleanups) - #158452 (Add missing links in integer docs)

jhpratt · 2026-06-27T05:34:50Z

This is the only PR in the rollup touching const generics, so I presume this is the cause.

error[E0121]: the placeholder `_` is not allowed within types on item signatures for implementations
##[error]  --> /checkout/tests/ui/const-generics/mgca/synth-gen-arg-ice-158152.rs:6:16
   |
LL | impl A<[usize; fn_item]> for () {}
   |                ^^^^^^^ not allowed in type signatures

error[E0747]: type provided when a constant was expected
##[error]  --> /checkout/tests/ui/const-generics/mgca/synth-gen-arg-ice-158152.rs:8:26
   |
LL | fn fn_item(_: impl Trait<usize>) {}
   |                          ^^^^^
   |
help: if this generic argument was intended as a const parameter, surround it with braces
   |
LL | fn fn_item(_: impl Trait<{ usize }>) {}
   |                          +       +

error: aborting due to 2 previous errors

#158465 (comment)

@bors r-

rust-bors · 2026-06-27T05:34:56Z

This pull request was unapproved.

This PR was contained in a rollup (#158465), which was unapproved.

View changes since this unapproval

jhpratt · 2026-06-27T05:36:20Z

Just to be sure

@bors try jobs=x86_64-gnu-llvm-21

rust-bors · 2026-06-27T05:36:26Z

⌛ Trying commit 0805fc2 with merge 0ac3a15…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/28280079330

Use infer tys for synthetic params when lowering const paths point to fns try-job: x86_64-gnu-llvm-21

mu001999 · 2026-06-27T05:57:36Z

This is the cause, I will rebase and update the test.

jhpratt · 2026-06-27T05:58:47Z

@bors try cancel

rust-bors · 2026-06-27T05:58:50Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/28280079330

… fns

rustbot · 2026-06-27T06:04:35Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

jhpratt · 2026-06-27T06:07:08Z

@bors r=JohnTitor

rust-bors · 2026-06-27T06:07:11Z

📌 Commit 4bbd8fc has been approved by JohnTitor

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

Use infer tys for synthetic params when lowering const paths point to fns ```rust #![feature(min_generic_const_args)] trait Trait {} impl<'t> Trait for [(); N] {} fn N(arg: impl Trait) {} ``` 1. `arg: impl Trait` implies a synthetic param `<impl Trait>` and the obligation `_: Trait`; 2. when matching `impl Trait for [(); N]`, lowering const arg `N` fills in that synthetic param, so the impl header contains `<[(); N::<impl Trait>] as Trait>`; 3. then instantiating this header with the impl's args causing a param/arg mismatch and ICE. This PR uses infer tys for synthetic params. Although this will emit the error in `ItemCtxt` because infer tys are not allowed here. I think that is reasonable, because `N` omits the synthetic parameter, though we cannot write the ty explicitly anywhere. Fixes rust-lang#155834

Rollup of 5 pull requests Successful merges: - #158234 (Cross-referencing tuple_trait tracking issue, source and the Unstable Book) - #157625 (Use infer tys for synthetic params when lowering const paths point to fns) - #158306 (tests: modify s390x vector test to be robust to instruction scheduling) - #158431 (More lint cleanups) - #158452 (Add missing links in integer docs)

Use infer tys for synthetic params when lowering const paths point to fns ```rust #![feature(min_generic_const_args)] trait Trait {} impl<'t> Trait for [(); N] {} fn N(arg: impl Trait) {} ``` 1. `arg: impl Trait` implies a synthetic param `<impl Trait>` and the obligation `_: Trait`; 2. when matching `impl Trait for [(); N]`, lowering const arg `N` fills in that synthetic param, so the impl header contains `<[(); N::<impl Trait>] as Trait>`; 3. then instantiating this header with the impl's args causing a param/arg mismatch and ICE. This PR uses infer tys for synthetic params. Although this will emit the error in `ItemCtxt` because infer tys are not allowed here. I think that is reasonable, because `N` omits the synthetic parameter, though we cannot write the ty explicitly anywhere. Fixes rust-lang#155834

Rollup of 6 pull requests Successful merges: - #158234 (Cross-referencing tuple_trait tracking issue, source and the Unstable Book) - #157625 (Use infer tys for synthetic params when lowering const paths point to fns) - #158306 (tests: modify s390x vector test to be robust to instruction scheduling) - #158431 (More lint cleanups) - #158452 (Add missing links in integer docs) - #158472 (Add regression test for unexpected pointer dereference issue)

Use infer tys for synthetic params when lowering const paths point to fns ```rust #![feature(min_generic_const_args)] trait Trait {} impl<'t> Trait for [(); N] {} fn N(arg: impl Trait) {} ``` 1. `arg: impl Trait` implies a synthetic param `<impl Trait>` and the obligation `_: Trait`; 2. when matching `impl Trait for [(); N]`, lowering const arg `N` fills in that synthetic param, so the impl header contains `<[(); N::<impl Trait>] as Trait>`; 3. then instantiating this header with the impl's args causing a param/arg mismatch and ICE. This PR uses infer tys for synthetic params. Although this will emit the error in `ItemCtxt` because infer tys are not allowed here. I think that is reasonable, because `N` omits the synthetic parameter, though we cannot write the ty explicitly anywhere. Fixes rust-lang#155834

…uwer Rollup of 13 pull requests Successful merges: - #157871 ([rustdoc] Update `doc_cfg` hide/show syntax) - #158234 (Cross-referencing tuple_trait tracking issue, source and the Unstable Book) - #158480 (add smoketest for std::net::hostname) - #157625 (Use infer tys for synthetic params when lowering const paths point to fns) - #158290 (add crashtests [1/N]) - #158306 (tests: modify s390x vector test to be robust to instruction scheduling) - #158313 (Move `check_target_feature` into the attribute parser) - #158431 (More lint cleanups) - #158452 (Add missing links in integer docs) - #158467 (Add proc macro for unused assignments and corresponding test) - #158472 (Add regression test for unexpected pointer dereference issue) - #158475 (Fix doc comment on get_debug_as_hex.) - #158476 (Fix doc comment on FormattingOptions::new().)

Rollup merge of #157625 - mu001999:fix-155834, r=JohnTitor Use infer tys for synthetic params when lowering const paths point to fns ```rust #![feature(min_generic_const_args)] trait Trait {} impl<'t> Trait for [(); N] {} fn N(arg: impl Trait) {} ``` 1. `arg: impl Trait` implies a synthetic param `<impl Trait>` and the obligation `_: Trait`; 2. when matching `impl Trait for [(); N]`, lowering const arg `N` fills in that synthetic param, so the impl header contains `<[(); N::<impl Trait>] as Trait>`; 3. then instantiating this header with the impl's args causing a param/arg mismatch and ICE. This PR uses infer tys for synthetic params. Although this will emit the error in `ItemCtxt` because infer tys are not allowed here. I think that is reasonable, because `N` omits the synthetic parameter, though we cannot write the ty explicitly anywhere. Fixes #155834

JonathanBrouwer · 2026-06-27T20:25:33Z

@rust-timer build 0ce5a1c
Perf run for #158487

rust-timer · 2026-06-27T21:44:31Z

Finished benchmarking commit (0ce5a1c): 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

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Results (primary 9.4%, secondary 6.6%)

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	mean	range	count
Regressions ❌ (primary)	9.4%	[9.4%, 9.4%]	1
Regressions ❌ (secondary)	6.6%	[6.4%, 6.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	9.4%	[9.4%, 9.4%]	1

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Results (secondary -2.7%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-3.2%, -2.2%]	2
All ❌✅ (primary)	-	-	0

Binary size

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Bootstrap: 485.171s -> 486.984s (0.37%)
Artifact size: 393.14 MiB -> 393.13 MiB (-0.00%)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 8, 2026

mu001999 marked this pull request as ready for review June 9, 2026 02:44

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 Jun 9, 2026

rustbot assigned JohnTitor Jun 9, 2026

JohnTitor approved these changes Jun 27, 2026

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jhpratt mentioned this pull request Jun 27, 2026

Rollup of 5 pull requests #158465

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rust-bors Bot 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jun 27, 2026

rust-bors Bot pushed a commit that referenced this pull request Jun 27, 2026

Auto merge of #157625 - mu001999:fix-155834, r=<try>

0ac3a15

Use infer tys for synthetic params when lowering const paths point to fns try-job: x86_64-gnu-llvm-21

Use infer tys for synthetic params when lowering const paths point to…

4bbd8fc

… fns

mu001999 force-pushed the fix-155834 branch from 0805fc2 to 4bbd8fc Compare June 27, 2026 06:04

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 27, 2026

jhpratt mentioned this pull request Jun 27, 2026

Rollup of 5 pull requests #158471

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jhpratt mentioned this pull request Jun 27, 2026

Rollup of 6 pull requests #158474

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This was referenced Jun 27, 2026

Rollup of 10 pull requests #158481

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Rollup of 13 pull requests #158487

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rust-bors Bot merged commit ad69585 into rust-lang:main Jun 27, 2026
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rustbot added this to the 1.98.0 milestone Jun 27, 2026

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