Detect attempt to use var-args in closure #146581

estebank · 2025-09-15T09:56:30Z

error: unexpected `...`
  --> $DIR/no-closure.rs:11:14
   |
LL |     let f = |...| {};
   |              ^^^ not a valid pattern
   |
   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

error: unexpected `...`
  --> $DIR/no-closure.rs:16:17
   |
LL |     let f = |_: ...| {};
   |                 ^^^
   |
   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

Fix #146489, when trying to use c-style var-args in a closure. We emit a more targeted message. We also silence inference errors when the pattern is PatKind::Err.

``` error: unexpected `...` --> $DIR/varargs-in-closure-isnt-supported.rs:5:20 | LL | let mut lol = |...| (); | ^^^ not a valid pattern | = note: C-variadic type `...` is not allowed here ```

rustbot · 2025-09-15T09:56:35Z

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rustbot · 2025-09-15T10:06:28Z

Some changes occurred in need_type_info.rs

cc @lcnr

lcnr · 2025-09-15T10:14:29Z

compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs

+            // We will have already emitted an error about this pattern.
+            err.downgrade_to_delayed_bug();


hmm, surprised we don't have a way to silence a lint by passing in an ErrorGuaranteed. We could then do

if let InferSourceKind::ClosureArg { kind: PatKind::Err(guar), .. } = kind { err.silence_via_error_guaranteed(guar); } }

though maybe even better... change this function to return Result<Diag, ErrorGuaranteed> and avoid creating a diagnostic at all edit: seems annoying to do 😅

I guess the rationale for downgrade_to_delayed_bug itself not taking an ErrorGuarantee is because itself is kind of an error guarantee already :)

The guarantee is more critical for anything that could cause a binary to be emitted otherwise.

lcnr · 2025-09-15T10:16:09Z

tests/ui/c-variadic/no-closure.stderr

-   |
+   = note: C-variadic type `...` is not allowed here

 error[E0743]: C-variadic type `...` may not be nested inside another type


existing, but what does "may not be nested inside another type" mean here 😅

Could we just emit the error like using ... in normal fn:

error: `...` is not supported for non-extern functions --> src/lib.rs:3:8 | 3 | fn foo(_: ...) {} | ^^^^^^ | = help: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

The reason this error triggers is because it is emitted by parse_ty_common, which was likely meant to catch uses like Vec<...>, but we didn't bother special casing the top-level case. It'll be hard to handle that in parse_ty_common because that doesn't know whether it is a top-level type being parsed, so it would have to be specially handled elsewhere. I'll look into it as a follow up.

Edit: scratch that. Curiosity got the best of me and fixed it quickly.

lcnr · 2025-09-15T10:17:24Z

r=me though still still doesn't seem as nice as it could be 🤔

could we explicitly state "closures must not be c-variadic?

When writing something like the expression `|_: ...| {}`, we now detect the `...` during parsing explicitly instead of relying on the detection in `parse_ty_common` so that we don't talk about "nested `...` are not supported". ``` error: unexpected `...` --> $DIR/no-closure.rs:6:35 | LL | const F: extern "C" fn(...) = |_: ...| {}; | ^^^ | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list ```

estebank · 2025-09-16T18:43:12Z

still doesn't seem as nice as it could be 🤔

could we explicitly state "closures must not be c-variadic?

Being that specific will require more special casing in the parser. The new output is at least an improvement :)

error: unexpected `...`
  --> $DIR/no-closure.rs:11:14
   |
LL |     let f = |...| {};
   |              ^^^ not a valid pattern
   |
   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

error: unexpected `...`
  --> $DIR/no-closure.rs:16:17
   |
LL |     let f = |_: ...| {};
   |                 ^^^
   |
   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

estebank · 2025-09-16T20:08:14Z

@bors r=lcnr

bors · 2025-09-16T20:08:18Z

📌 Commit e9270e3 has been approved by lcnr

It is now in the queue for this repository.

Detect attempt to use var-args in closure ``` error: unexpected `...` --> $DIR/no-closure.rs:11:14 | LL | let f = |...| {}; | ^^^ not a valid pattern | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list error: unexpected `...` --> $DIR/no-closure.rs:16:17 | LL | let f = |_: ...| {}; | ^^^ | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list ``` Fix rust-lang#146489, when trying to use c-style var-args in a closure. We emit a more targeted message. We also silence inference errors when the pattern is `PatKind::Err`.

Rollup of 9 pull requests Successful merges: - #144871 (Stabilize `btree_entry_insert` feature) - #145181 (remove FIXME block from `has_significant_drop`, it never encounters inference variables) - #145838 (don't apply temporary lifetime extension rules to non-extended `super let`) - #146259 (Suggest removing Box::new instead of unboxing it) - #146410 (Iterator repeat: no infinite loop for `last` and `count`) - #146460 (Add tidy readme) - #146581 (Detect attempt to use var-args in closure) - #146588 (tests/run-make: Update list of statically linked musl targets) - #146647 (Move `#[rustc_coherence_is_core]` to the `crate_level` file) r? `@ghost` `@rustbot` modify labels: rollup

Detect attempt to use var-args in closure ``` error: unexpected `...` --> $DIR/no-closure.rs:11:14 | LL | let f = |...| {}; | ^^^ not a valid pattern | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list error: unexpected `...` --> $DIR/no-closure.rs:16:17 | LL | let f = |_: ...| {}; | ^^^ | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list ``` Fix rust-lang#146489, when trying to use c-style var-args in a closure. We emit a more targeted message. We also silence inference errors when the pattern is `PatKind::Err`.

Rollup of 14 pull requests Successful merges: - #142807 (libtest: expose --fail-fast as an unstable command-line option) - #144871 (Stabilize `btree_entry_insert` feature) - #145071 (Update the minimum external LLVM to 20) - #145181 (remove FIXME block from `has_significant_drop`, it never encounters inference variables) - #145660 (initial implementation of the darwin_objc unstable feature) - #145838 (don't apply temporary lifetime extension rules to non-extended `super let`) - #146259 (Suggest removing Box::new instead of unboxing it) - #146410 (Iterator repeat: no infinite loop for `last` and `count`) - #146460 (Add tidy readme) - #146552 (StateTransform: Do not renumber resume local.) - #146564 (Remove Rvalue::Len again.) - #146581 (Detect attempt to use var-args in closure) - #146588 (tests/run-make: Update list of statically linked musl targets) - #146631 (cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 3)) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #146581 - estebank:issue-146489, r=lcnr Detect attempt to use var-args in closure ``` error: unexpected `...` --> $DIR/no-closure.rs:11:14 | LL | let f = |...| {}; | ^^^ not a valid pattern | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list error: unexpected `...` --> $DIR/no-closure.rs:16:17 | LL | let f = |_: ...| {}; | ^^^ | = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list ``` Fix #146489, when trying to use c-style var-args in a closure. We emit a more targeted message. We also silence inference errors when the pattern is `PatKind::Err`.

lcnr · 2025-09-17T11:12:50Z

The PR looks good. For future PRs, I do feel like the change was substantial enough (especially by introducing a check at a new place) that I would have liked to look at it again myself

Detect attempt to use var-args in closure

9405e76

``` error: unexpected `...` --> $DIR/varargs-in-closure-isnt-supported.rs:5:20 | LL | let mut lol = |...| (); | ^^^ not a valid pattern | = note: C-variadic type `...` is not allowed here ```

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 15, 2025

rustbot assigned nnethercote Sep 15, 2025

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Sep 15, 2025

Silence inference error on PatKind::Err

0e290e4

Fix existing test

a84c8de

lcnr reviewed Sep 15, 2025

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estebank added 4 commits September 16, 2025 11:08

fmt

c916e88

remove redundant test

ed85f98

Reword note

8306a2f

bors 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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 16, 2025

jhpratt mentioned this pull request Sep 17, 2025

Rollup of 9 pull requests #146662

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This was referenced Sep 17, 2025

Rollup of 15 pull requests #146665

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Rollup of 14 pull requests #146666

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bors merged commit 61f5358 into rust-lang:master Sep 17, 2025
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rustbot added this to the 1.92.0 milestone Sep 17, 2025

github-actions bot mentioned this pull request Sep 18, 2025

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Detect attempt to use var-args in closure #146581

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		// We will have already emitted an error about this pattern.
		err.downgrade_to_delayed_bug();

Detect attempt to use var-args in closure #146581

Detect attempt to use var-args in closure #146581

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