Improve diagnostics for references to closures by ozankenangungor · Pull Request #161400 · rust-lang/rust

ozankenangungor · 2026-08-20T14:02:05Z

Fixes #161327
When passing a reference to a closure that fails an Fn* bound (e.g. &mut c where c is an FnMut closure passed to a function expecting impl Fn, or an FnOnce closure passed where FnMut is expected), rustc currently suggests removing the leading &. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound.
This PR makes emit_specialized_closure_kind_error peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind.
To be conservative:

We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet).
We bail out to E0277 if a by-value capture has an unresolved type variable (like {integer}) that might turn out to be Copy after fallback.
If the closure actually satisfies the expected trait (e.g. a genuine Fn closure wrapped in &mut passed to Fn), found_kind.extends(expected_kind) holds, so we keep E0277 and the valid remove-ref suggestion.

rustbot · 2026-08-20T14:02:10Z

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davidtwco · 2026-09-04T13:00:44Z

@bors r+ rollup

rust-bors · 2026-09-04T13:00:47Z

📌 Commit 5db7af4 has been approved by davidtwco

It is now in the queue for this repository.

…re-ref-fn-kind-diagnostic, r=davidtwco Improve diagnostics for references to closures Fixes rust-lang#161327 When passing a reference to a closure that fails an `Fn*` bound (e.g. `&mut c` where `c` is an `FnMut` closure passed to a function expecting `impl Fn`, or an `FnOnce` closure passed where `FnMut` is expected), rustc currently suggests removing the leading `&`. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound. This PR makes `emit_specialized_closure_kind_error` peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind. To be conservative: - We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet). - We bail out to E0277 if a by-value capture has an unresolved type variable (like `{integer}`) that might turn out to be `Copy` after fallback. - If the closure actually satisfies the expected trait (e.g. a genuine `Fn` closure wrapped in `&mut` passed to `Fn`), `found_kind.extends(expected_kind)` holds, so we keep E0277 and the valid remove-ref suggestion.

Rollup of 25 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #161895 (std::sys::pal::sgx: fix mismatched alloc/free alignment) - #161900 (bootstrap: Include feature-gated items in bootstrap tool docs) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

…re-ref-fn-kind-diagnostic, r=davidtwco Improve diagnostics for references to closures Fixes rust-lang#161327 When passing a reference to a closure that fails an `Fn*` bound (e.g. `&mut c` where `c` is an `FnMut` closure passed to a function expecting `impl Fn`, or an `FnOnce` closure passed where `FnMut` is expected), rustc currently suggests removing the leading `&`. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound. This PR makes `emit_specialized_closure_kind_error` peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind. To be conservative: - We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet). - We bail out to E0277 if a by-value capture has an unresolved type variable (like `{integer}`) that might turn out to be `Copy` after fallback. - If the closure actually satisfies the expected trait (e.g. a genuine `Fn` closure wrapped in `&mut` passed to `Fn`), `found_kind.extends(expected_kind)` holds, so we keep E0277 and the valid remove-ref suggestion.

Rollup of 25 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #160745 (make closures act like MaybeDangling) - #161895 (std::sys::pal::sgx: fix mismatched alloc/free alignment) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

…re-ref-fn-kind-diagnostic, r=davidtwco Improve diagnostics for references to closures Fixes rust-lang#161327 When passing a reference to a closure that fails an `Fn*` bound (e.g. `&mut c` where `c` is an `FnMut` closure passed to a function expecting `impl Fn`, or an `FnOnce` closure passed where `FnMut` is expected), rustc currently suggests removing the leading `&`. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound. This PR makes `emit_specialized_closure_kind_error` peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind. To be conservative: - We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet). - We bail out to E0277 if a by-value capture has an unresolved type variable (like `{integer}`) that might turn out to be `Copy` after fallback. - If the closure actually satisfies the expected trait (e.g. a genuine `Fn` closure wrapped in `&mut` passed to `Fn`), `found_kind.extends(expected_kind)` holds, so we keep E0277 and the valid remove-ref suggestion.

Rollup of 27 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #160745 (make closures act like MaybeDangling) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162030 (Prevent `--test` to be used in `rustdoc-html` testsuite) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

…uwer Rollup of 25 pull requests Successful merges: - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162255 (let people change rustc-dev-guide in-tree) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1) - #162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

…re-ref-fn-kind-diagnostic, r=davidtwco Improve diagnostics for references to closures Fixes rust-lang#161327 When passing a reference to a closure that fails an `Fn*` bound (e.g. `&mut c` where `c` is an `FnMut` closure passed to a function expecting `impl Fn`, or an `FnOnce` closure passed where `FnMut` is expected), rustc currently suggests removing the leading `&`. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound. This PR makes `emit_specialized_closure_kind_error` peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind. To be conservative: - We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet). - We bail out to E0277 if a by-value capture has an unresolved type variable (like `{integer}`) that might turn out to be `Copy` after fallback. - If the closure actually satisfies the expected trait (e.g. a genuine `Fn` closure wrapped in `&mut` passed to `Fn`), `found_kind.extends(expected_kind)` holds, so we keep E0277 and the valid remove-ref suggestion.

…uwer Rollup of 25 pull requests Successful merges: - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162255 (let people change rustc-dev-guide in-tree) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1) - #162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

Rollup merge of #161400 - ozankenangungor:issue-161327-closure-ref-fn-kind-diagnostic, r=davidtwco Improve diagnostics for references to closures Fixes #161327 When passing a reference to a closure that fails an `Fn*` bound (e.g. `&mut c` where `c` is an `FnMut` closure passed to a function expecting `impl Fn`, or an `FnOnce` closure passed where `FnMut` is expected), rustc currently suggests removing the leading `&`. If the underlying closure doesn't implement the required trait in the first place, that suggestion is misleading because removing the reference still won't make it satisfy the bound. This PR makes `emit_specialized_closure_kind_error` peel references to closures so we can emit E0525 and point out the mutation/move that caused the mismatch. Since closure kind inference hasn't run yet when checking the call arguments, we run a quick capture pass on-demand to figure out the actual closure kind. To be conservative: - We bail out to E0277 if there are nested closures in the body (since we don't know their captures yet). - We bail out to E0277 if a by-value capture has an unresolved type variable (like `{integer}`) that might turn out to be `Copy` after fallback. - If the closure actually satisfies the expected trait (e.g. a genuine `Fn` closure wrapped in `&mut` passed to `Fn`), `found_kind.extends(expected_kind)` holds, so we keep E0277 and the valid remove-ref suggestion.

rust-timer · 2026-09-06T09:20:39Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (132155e): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -7.5%)

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)	-7.5%	[-11.9%, -3.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-7.5%	[-11.9%, -3.1%]	2

Cycles

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

Binary size

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Bootstrap: missing data
Artifact size: 403.29 MiB -> 403.37 MiB (0.02%)

…uwer Rollup of 25 pull requests Successful merges: - rust-lang/rust#162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - rust-lang/rust#162179 (type system const items via direct rhs) - rust-lang/rust#162255 (let people change rustc-dev-guide in-tree) - rust-lang/rust#162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - rust-lang/rust#162277 (Introduce `rustc_middle::middel::resolve`) - rust-lang/rust#162285 (box: fixup map/try_map deallocate calls) - rust-lang/rust#162286 (string: don't unwind prematurely) - rust-lang/rust#162289 (alloc: a bunch of safety comments) - rust-lang/rust#162290 (abby test DSL: AliasTyOutlivesViaEnv) - rust-lang/rust#162292 (Update `askama` version to `0.16.1`) - rust-lang/rust#160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - rust-lang/rust#160906 (Suggest usize instead of placeholder type for array length constants) - rust-lang/rust#160936 (traits: Represent live alias arguments as bitsets) - rust-lang/rust#161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - rust-lang/rust#161400 (Improve diagnostics for references to closures) - rust-lang/rust#161656 (Suggest mutable references for FnMut closure arguments) - rust-lang/rust#161711 (Add more splat fn type tests) - rust-lang/rust#161786 (Make `tcx.def_id_partial_cmp` public) - rust-lang/rust#161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - rust-lang/rust#162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - rust-lang/rust#162212 (Implement `Rng` for `Box`) - rust-lang/rust#162246 (Fix incorrect meta span) - rust-lang/rust#162266 (std: fix typo) - rust-lang/rust#162291 (Add regression test from 1.98.1) - rust-lang/rust#162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

Improve diagnostics for references to closures

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rustbot assigned davidtwco Aug 20, 2026

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 Aug 20, 2026

davidtwco approved these changes Sep 4, 2026

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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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 4, 2026

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Rollup of 25 pull requests #162310

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rust-bors Bot merged commit 9da14bb into rust-lang:main Sep 5, 2026
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Improve diagnostics for references to closures - #161400

Improve diagnostics for references to closures#161400
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ozankenangungor:issue-161327-closure-ref-fn-kind-diagnostic

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