diagnostics: suggest type annotation for closure params on HRTB FnOnce mismatch by Rohan-Singla · Pull Request #158701 · rust-lang/rust

Rohan-Singla · 2026-07-02T20:38:20Z

Problem

When a closure passed to a function requiring for<'a> FnOnce(&'a mut [u8]) has
unannotated parameters (e.g. |buf|), the compiler emits the confusing error:

implementation of `FnOnce` is not general enough

without explaining how to fix it.

In many cases, simply adding an explicit type annotation to the closure
parameter (e.g. |buf: &mut [u8]|) resolves the lifetime ambiguity, but the
compiler does not currently provide any guidance.

What

This PR adds a diagnostic suggestion in placeholder_error.rs that detects
when:

the mismatched trait is an Fn-trait (FnOnce, FnMut, or Fn)
the self type is a local closure
one or more closure parameters do not have explicit type annotations

When these conditions are met, the compiler now emits the following help
message:

help: consider adding an explicit type annotation to the closure parameter
      to resolve the lifetime ambiguity
  |
  |     let outer_closure = |buf: &mut [u8]| {
  |                             +++++++++++

Testing

A UI test has been added covering the exact reproduction case from the
reported issue.

rustbot · 2026-07-02T20:38:25Z

r? @mejrs

rustbot has assigned @mejrs.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 75 candidates
Random selection from 20 candidates

…e mismatch

… suggestion

…n suggestion

mejrs

Thanks, this looks mostly good to me. Does this suggestion fix the errors in all the affected ui tests? Can you add rustfix tests for those that are?

Also it seems that you need to bless more tests.

@rustbot author

View changes since this review

mejrs · 2026-07-04T16:59:06Z

+                        err.multipart_suggestion(
+                            "consider adding an explicit type annotation to the closure \
+                             parameter to resolve the lifetime ambiguity",
+                            suggestions,


So the thing is that I'm not sure how often this actually fixes the error. This reads a lot like "if you add a type annotation, this will fix the error". We should not overpromise.

Suggested change

suggestions,

"consider adding an explicit type annotation to the closure's argument",

(also, should be argument/arguments depending on if there's more than one suggestion)

Thanks , changed the suggestion message added a .fixed file too!

rustbot · 2026-07-04T17:01:11Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

…ure HRTB annotation

Rohan-Singla · 2026-07-05T21:04:40Z

@rustbot ready

…estion

rust-log-analyzer · 2026-07-05T23:22:41Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/144)
.................................................. (100/144)
............................................       (144/144)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is detached from document: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 143 succeeded, 1 failed, 0 filtered out

Error: ()

mejrs

Thanks!

I think the CI failure is spurious?

@bors r+ rollup

View changes since this review

rust-bors · 2026-07-07T16:39:13Z

📌 Commit 049b3a3 has been approved by mejrs

It is now in the queue for this repository.

… r=mejrs diagnostics: suggest type annotation for closure params on HRTB FnOnce mismatch Fixes rust-lang#158393 ## Problem When a closure passed to a function requiring `for<'a> FnOnce(&'a mut [u8])` has unannotated parameters (e.g. `|buf|`), the compiler emits the confusing error: ```text implementation of `FnOnce` is not general enough ``` without explaining how to fix it. In many cases, simply adding an explicit type annotation to the closure parameter (e.g. `|buf: &mut [u8]|`) resolves the lifetime ambiguity, but the compiler does not currently provide any guidance. ## What This PR adds a diagnostic suggestion in `placeholder_error.rs` that detects when: - the mismatched trait is an Fn-trait (`FnOnce`, `FnMut`, or `Fn`) - the self type is a local closure - one or more closure parameters do not have explicit type annotations When these conditions are met, the compiler now emits the following help message: ```text help: consider adding an explicit type annotation to the closure parameter to resolve the lifetime ambiguity | | let outer_closure = |buf: &mut [u8]| { | +++++++++++ ``` ## Testing A UI test has been added covering the exact reproduction case from the reported issue.

…uwer Rollup of 18 pull requests Successful merges: - #157385 (Enable Enzyme on x86_64-apple) - #157561 (rustdoc: do not include extra stuff in span) - #158179 (std: unconditionally use `preadv`/`pwritev` on AArch64 macOS) - #158617 (allow mGCA const arguments to fall back to anon consts) - #158621 (disallow `extern "custom"` on wasm and spirv targets) - #158690 (delegation: support mapping of all arguments with `Self` type) - #158696 (Rename some `body_id` to `body_def_id`) - #158697 (Fixes for QNX SDP 8) - #158760 (Clarify that `LocalKey::try_with` may return `AccessError`) - #157801 (Rewrite safety requirements for `Allocator` impls) - #158333 (Fix typetree generation for differentiated functions) - #158646 (powerpc64le_unknown_freebsd.rs: link with -lgcc) - #158701 ( diagnostics: suggest type annotation for closure params on HRTB FnOnce mismatch) - #158791 (Avoid unused braces lint for macro generated arguments) - #158802 (Use `ci-mirrors` in `armhf-gnu` for {busybox, ubuntu rootfs} artifacts) - #158841 (Avoid final override ICE for RPITIT associated types) - #158889 (tests: catch up with LLVM returning f128 on the stack) - #158905 (delegation: add constraints to new generic args)

…uwer Rollup of 19 pull requests Successful merges: - #156016 (view-types: store view types in the AST) - #157385 (Enable Enzyme on x86_64-apple) - #158179 (std: unconditionally use `preadv`/`pwritev` on AArch64 macOS) - #158621 (disallow `extern "custom"` on wasm and spirv targets) - #158690 (delegation: support mapping of all arguments with `Self` type) - #158696 (Rename some `body_id` to `body_def_id`) - #158697 (Fixes for QNX SDP 8) - #158760 (Clarify that `LocalKey::try_with` may return `AccessError`) - #157801 (Rewrite safety requirements for `Allocator` impls) - #158085 (rustdoc: Fix sidebar heading order) - #158333 (Fix typetree generation for differentiated functions) - #158646 (powerpc64le_unknown_freebsd.rs: link with -lgcc) - #158701 ( diagnostics: suggest type annotation for closure params on HRTB FnOnce mismatch) - #158791 (Avoid unused braces lint for macro generated arguments) - #158802 (Use `ci-mirrors` in `armhf-gnu` for {busybox, ubuntu rootfs} artifacts) - #158841 (Avoid final override ICE for RPITIT associated types) - #158889 (tests: catch up with LLVM returning f128 on the stack) - #158905 (delegation: add constraints to new generic args) - #158922 (tests: clean up over-constraint on LLVM feature count) Failed merges: - #158617 (allow mGCA const arguments to fall back to anon consts)

Rohan-Singla · 2026-07-07T21:51:35Z

Thanks!

I think the CI failure is spurious?

@bors r+ rollup

View changes since this review

Yeah i believe so think so i suspected its due to all tests are not blessed but i did so now , thanks for the rollup!

Rollup merge of #158701 - Rohan-Singla:fix/#158393, r=mejrs diagnostics: suggest type annotation for closure params on HRTB FnOnce mismatch Fixes #158393 ## Problem When a closure passed to a function requiring `for<'a> FnOnce(&'a mut [u8])` has unannotated parameters (e.g. `|buf|`), the compiler emits the confusing error: ```text implementation of `FnOnce` is not general enough ``` without explaining how to fix it. In many cases, simply adding an explicit type annotation to the closure parameter (e.g. `|buf: &mut [u8]|`) resolves the lifetime ambiguity, but the compiler does not currently provide any guidance. ## What This PR adds a diagnostic suggestion in `placeholder_error.rs` that detects when: - the mismatched trait is an Fn-trait (`FnOnce`, `FnMut`, or `Fn`) - the self type is a local closure - one or more closure parameters do not have explicit type annotations When these conditions are met, the compiler now emits the following help message: ```text help: consider adding an explicit type annotation to the closure parameter to resolve the lifetime ambiguity | | let outer_closure = |buf: &mut [u8]| { | +++++++++++ ``` ## Testing A UI test has been added covering the exact reproduction case from the reported issue.

jamesmunns · 2026-07-08T00:08:02Z

Hell yeah, thanks @Rohan-Singla!

rustbot assigned mejrs Jul 2, 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 Jul 2, 2026

This comment has been minimized.

Sign in to view

Rohan-Singla added 2 commits July 3, 2026 03:46

diagnostics: suggest type annotation for closure params on HRTB FnOnc…

a00bdac

…e mismatch

test: bless existing UI tests affected by new closure type annotation…

155ead1

… suggestion

Rohan-Singla force-pushed the fix/#158393 branch from 5421311 to 155ead1 Compare July 2, 2026 22:16

test: bless remaining UI tests affected by new closure type annotatio…

5817f14

…n suggestion

This comment has been minimized.

Sign in to view

mejrs reviewed Jul 4, 2026

View reviewed changes

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 Jul 4, 2026

diagnostics: improve suggestion message and add rustfix test for clos…

0e0b98a

…ure HRTB annotation

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 Jul 5, 2026

This comment has been minimized.

Sign in to view

test: bless nll UI tests affected by new closure type annotation sugg…

049b3a3

…estion

mejrs approved these changes Jul 7, 2026

View reviewed changes

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 Jul 7, 2026

JonathanBrouwer mentioned this pull request Jul 7, 2026

Rollup of 7 pull requests #158919

Closed

JonathanBrouwer mentioned this pull request Jul 7, 2026

Rollup of 18 pull requests #158923

Closed

JonathanBrouwer mentioned this pull request Jul 7, 2026

Rollup of 19 pull requests #158924

Merged

rust-bors Bot merged commit b8df403 into rust-lang:main Jul 8, 2026
9 of 13 checks passed

rustbot added this to the 1.99.0 milestone Jul 8, 2026

	suggestions,
	"consider adding an explicit type annotation to the closure's argument",

Uh oh!

Uh oh!

Conversation

Rohan-Singla commented Jul 2, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Problem

What

Testing

Uh oh!

rustbot commented Jul 2, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

mejrs left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

mejrs Jul 4, 2026

Choose a reason for hiding this comment

Uh oh!

Rohan-Singla Jul 5, 2026

Choose a reason for hiding this comment

Uh oh!

rustbot commented Jul 4, 2026

Uh oh!

Rohan-Singla commented Jul 5, 2026

Uh oh!

This comment has been minimized.

rust-log-analyzer commented Jul 5, 2026

Uh oh!

mejrs left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

rust-bors Bot commented Jul 7, 2026

Uh oh!

Rohan-Singla commented Jul 7, 2026

Uh oh!

Uh oh!

jamesmunns commented Jul 8, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Rohan-Singla commented Jul 2, 2026 •

edited by rustbot

Loading

mejrs left a comment •

edited by rustbot

Loading

mejrs left a comment •

edited by rustbot

Loading