Improve move error diagnostic for `AsyncFn` closures #150572

heathdutton · 2026-01-01T22:25:57Z

When an async closure captures a variable by move but is constrained to AsyncFn or AsyncFnMut, the error message now explains that the closure kind is the issue and points to the trait bound, similar to the existing diagnostic for Fn/FnMut closures.

Before:

error[E0507]: cannot move out of `foos` which is behind a shared reference
  --> src/lib.rs:12:20
   |
11 | async fn foo(foos: &mut [&mut Foo]) -> Result<(), ()> {
   |              ---- move occurs because `foos` has type...
12 |     in_transaction(async || -> Result<(), ()> {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ `foos` is moved here
13 |         for foo in foos {
   |                    ---- variable moved due to use in coroutine

After:

error[E0507]: cannot move out of `y`, a captured variable in an `AsyncFn` closure
  --> src/lib.rs:9:10
   |
LL |     let y = vec![format!("World")];
   |         - captured outer variable
LL |     call(async || {
   |          ^^^^^^^^ captured by this `AsyncFn` closure
...
help: `AsyncFn` and `AsyncFnMut` closures require captured values to be able
      to be consumed multiple times, but `AsyncFnOnce` closures may consume
      them only once
  --> src/lib.rs:5:27
   |
LL | fn call<F>(_: F) where F: AsyncFn() {}
   |                           ^^^^^^^^^

Fixes #150174

rustbot · 2026-01-01T22:26:03Z

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compiler/rustc_borrowck/src/diagnostics/move_errors.rs

mati865 · 2026-01-12T10:09:38Z

@rustbot reroll

Hopefully another reviewer will have better knowledge about #150572 (comment)

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

rustbot · 2026-01-21T18:51:19Z

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.

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

lcnr

nit, then r=me

View changes since this review

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

lcnr · 2026-01-28T17:51:13Z

after nit

@bors delegate+

rust-bors · 2026-01-28T17:51:17Z

✌️ @heathdutton, you can now approve this pull request!

If @lcnr told you to "r=me" after making some further change, then please make that change and post @bors r=lcnr.

When an async closure captures a variable by move but is constrained to `AsyncFn` or `AsyncFnMut`, the error message now explains that the closure kind is the issue and points to the trait bound, similar to the existing diagnostic for `Fn`/`FnMut` closures.

heathdutton · 2026-01-28T21:00:28Z

@bors r=lcnr

rust-bors · 2026-01-28T21:00:31Z

📌 Commit f5f2ca0 has been approved by lcnr

It is now in the queue for this repository.

…iagnostic-v2, r=lcnr Improve move error diagnostic for `AsyncFn` closures When an async closure captures a variable by move but is constrained to `AsyncFn` or `AsyncFnMut`, the error message now explains that the closure kind is the issue and points to the trait bound, similar to the existing diagnostic for `Fn`/`FnMut` closures. **Before:** ``` error[E0507]: cannot move out of `foos` which is behind a shared reference --> src/lib.rs:12:20 | 11 | async fn foo(foos: &mut [&mut Foo]) -> Result<(), ()> { | ---- move occurs because `foos` has type... 12 | in_transaction(async || -> Result<(), ()> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^ `foos` is moved here 13 | for foo in foos { | ---- variable moved due to use in coroutine ``` **After:** ``` error[E0507]: cannot move out of `y`, a captured variable in an `AsyncFn` closure --> src/lib.rs:9:10 | LL | let y = vec![format!("World")]; | - captured outer variable LL | call(async || { | ^^^^^^^^ captured by this `AsyncFn` closure ... help: `AsyncFn` and `AsyncFnMut` closures require captured values to be able to be consumed multiple times, but `AsyncFnOnce` closures may consume them only once --> src/lib.rs:5:27 | LL | fn call<F>(_: F) where F: AsyncFn() {} | ^^^^^^^^^ ``` Fixes rust-lang#150174

Rollup of 8 pull requests Successful merges: - #150474 (Tidy: detect ui tests subdirectory changes so `tests/ui/README.md` stays in sync) - #150572 (Improve move error diagnostic for `AsyncFn` closures) - #151596 (Fix -Zmir-enable-passes to detect unregistered enum names in declare_passes macro) - #151802 (Make `QueryDispatcher::Qcx` an associated type) - #151559 ([rustdoc] Add a marker to tell users that there are hidden (deprecated) items in the search results) - #151665 (Fix contrast ratio for `Since` element in rustodoc dark theme) - #151798 (Update `askama` to `0.15.3`) - #151800 (Subscribe myself to translation diagnostics)

Rollup of 12 pull requests Successful merges: - #150474 (Tidy: detect ui tests subdirectory changes so `tests/ui/README.md` stays in sync) - #150572 (Improve move error diagnostic for `AsyncFn` closures) - #151596 (Fix -Zmir-enable-passes to detect unregistered enum names in declare_passes macro) - #151802 (Make `QueryDispatcher::Qcx` an associated type) - #149110 (Implement `cast_slice` for raw pointer types) - #151559 ([rustdoc] Add a marker to tell users that there are hidden (deprecated) items in the search results) - #151665 (Fix contrast ratio for `Since` element in rustdoc dark theme) - #151785 (Stabilize feature(push_mut)) - #151798 (Update `askama` to `0.15.3`) - #151800 (Subscribe myself to translation diagnostics) - #151804 (Document, sort, and tweak spellcheck entries in `typos.toml`) - #151805 (Fix grammar in `env::current_exe()#Security`)

Rollup merge of #150572 - heathdutton:issue-150174-asyncfn-diagnostic-v2, r=lcnr Improve move error diagnostic for `AsyncFn` closures When an async closure captures a variable by move but is constrained to `AsyncFn` or `AsyncFnMut`, the error message now explains that the closure kind is the issue and points to the trait bound, similar to the existing diagnostic for `Fn`/`FnMut` closures. **Before:** ``` error[E0507]: cannot move out of `foos` which is behind a shared reference --> src/lib.rs:12:20 | 11 | async fn foo(foos: &mut [&mut Foo]) -> Result<(), ()> { | ---- move occurs because `foos` has type... 12 | in_transaction(async || -> Result<(), ()> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^ `foos` is moved here 13 | for foo in foos { | ---- variable moved due to use in coroutine ``` **After:** ``` error[E0507]: cannot move out of `y`, a captured variable in an `AsyncFn` closure --> src/lib.rs:9:10 | LL | let y = vec![format!("World")]; | - captured outer variable LL | call(async || { | ^^^^^^^^ captured by this `AsyncFn` closure ... help: `AsyncFn` and `AsyncFnMut` closures require captured values to be able to be consumed multiple times, but `AsyncFnOnce` closures may consume them only once --> src/lib.rs:5:27 | LL | fn call<F>(_: F) where F: AsyncFn() {} | ^^^^^^^^^ ``` Fixes #150174

rustbot assigned mati865 Jan 1, 2026

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heathdutton force-pushed the issue-150174-asyncfn-diagnostic-v2 branch from d65f71a to 9ef09ef Compare January 2, 2026 03:47

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heathdutton marked this pull request as draft January 19, 2026 15:49

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 Jan 19, 2026

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heathdutton force-pushed the issue-150174-asyncfn-diagnostic-v2 branch 2 times, most recently from 48c7d9e to 0f81d39 Compare January 21, 2026 18:51

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Zalathar mentioned this pull request Jan 29, 2026

Rollup of 8 pull requests #151803

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Rollup of 12 pull requests #151809

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Toolchain upgrade to nightly-2026-01-30 flux-rs/flux#1345

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Improve move error diagnostic for AsyncFn closures #150572

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Improve move error diagnostic for `AsyncFn` closures #150572

Improve move error diagnostic for `AsyncFn` closures #150572

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