Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error by estebank · Pull Request #144558 · rust-lang/rust

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Jul 28, 2025

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…caused a move error

When encountering a move error involving a closure because the captured value isn't `Copy`, and the obligation comes from a bound on a type parameter that requires `Fn` or `FnMut`, we point at it and explain that an `FnOnce` wouldn't cause the move error.

```
error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
  --> f111.rs:15:25
   |
14 | fn do_stuff(foo: Option<Foo>) {
   |             ---  ----------- move occurs because `foo` has type `Option<Foo>`, which does not implement the `Copy` trait
   |             |
   |             captured outer variable
15 |     require_fn_trait(|| async {
   |                      -- ^^^^^ `foo` is moved here
   |                      |
   |                      captured by this `Fn` closure
16 |         if foo.map_or(false, |f| f.foo()) {
   |            --- variable moved due to use in coroutine
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
  --> f111.rs:12:53
   |
12 | fn require_fn_trait<F: Future<Output = ()>>(_: impl Fn() -> F) {}
   |                                                     ^^^^^^^^^
help: consider cloning the value if the performance cost is acceptable
   |
16 |         if foo.clone().map_or(false, |f| f.foo()) {
   |               ++++++++
```

@estebank

@estebank

@bors bors added S-waiting-on-bors

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and removed S-waiting-on-review

Status: Awaiting review from the assignee but also interested parties.

labels

Aug 10, 2025

samueltardieu added a commit to samueltardieu/rust that referenced this pull request

Aug 10, 2025
Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error

When encountering a move error involving a closure because the captured value isn't `Copy`, and the obligation comes from a bound on a type parameter that requires `Fn` or `FnMut`, we point at it and explain that an `FnOnce` wouldn't cause the move error.

```
error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
  --> f111.rs:15:25
   |
14 | fn do_stuff(foo: Option<Foo>) {
   |             ---  ----------- move occurs because `foo` has type `Option<Foo>`, which does not implement the `Copy` trait
   |             |
   |             captured outer variable
15 |     require_fn_trait(|| async {
   |                      -- ^^^^^ `foo` is moved here
   |                      |
   |                      captured by this `Fn` closure
16 |         if foo.map_or(false, |f| f.foo()) {
   |            --- variable moved due to use in coroutine
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
  --> f111.rs:12:53
   |
12 | fn require_fn_trait<F: Future<Output = ()>>(_: impl Fn() -> F) {}
   |                                                     ^^^^^^^^^
help: consider cloning the value if the performance cost is acceptable
   |
16 |         if foo.clone().map_or(false, |f| f.foo()) {
   |               ++++++++
```

Fix rust-lang#68119, by pointing at `Fn` and `FnMut` bounds involved in move errors.

bors added a commit that referenced this pull request

Aug 10, 2025
Rollup of 3 pull requests

Successful merges:

 - #135846 (Detect struct construction with private field in field with default)
 - #144558 (Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error)
 - #145149 (Make config method invoke inside parse use dwn_ctx)

r? `@ghost`
`@rustbot` modify labels: rollup

Zalathar added a commit to Zalathar/rust that referenced this pull request

Aug 11, 2025
Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error

When encountering a move error involving a closure because the captured value isn't `Copy`, and the obligation comes from a bound on a type parameter that requires `Fn` or `FnMut`, we point at it and explain that an `FnOnce` wouldn't cause the move error.

```
error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
  --> f111.rs:15:25
   |
14 | fn do_stuff(foo: Option<Foo>) {
   |             ---  ----------- move occurs because `foo` has type `Option<Foo>`, which does not implement the `Copy` trait
   |             |
   |             captured outer variable
15 |     require_fn_trait(|| async {
   |                      -- ^^^^^ `foo` is moved here
   |                      |
   |                      captured by this `Fn` closure
16 |         if foo.map_or(false, |f| f.foo()) {
   |            --- variable moved due to use in coroutine
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
  --> f111.rs:12:53
   |
12 | fn require_fn_trait<F: Future<Output = ()>>(_: impl Fn() -> F) {}
   |                                                     ^^^^^^^^^
help: consider cloning the value if the performance cost is acceptable
   |
16 |         if foo.clone().map_or(false, |f| f.foo()) {
   |               ++++++++
```

Fix rust-lang#68119, by pointing at `Fn` and `FnMut` bounds involved in move errors.

bors added a commit that referenced this pull request

Aug 11, 2025
Rollup of 7 pull requests

Successful merges:

 - #143949 (Constify remaining traits/impls for `const_ops`)
 - #144330 (document assumptions about `Clone` and `Eq` traits)
 - #144350 (std: sys: io: io_slice: Add UEFI types)
 - #144558 (Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error)
 - #145149 (Make config method invoke inside parse use dwn_ctx)
 - #145227 (Tweak spans providing type context on errors when involving macros)
 - #145228 (Remove unnecessary parentheses in `assert!`s)

r? `@ghost`
`@rustbot` modify labels: rollup

rust-timer added a commit that referenced this pull request

Aug 11, 2025
Rollup merge of #144558 - estebank:issue-68119, r=lcnr

Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error

When encountering a move error involving a closure because the captured value isn't `Copy`, and the obligation comes from a bound on a type parameter that requires `Fn` or `FnMut`, we point at it and explain that an `FnOnce` wouldn't cause the move error.

```
error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
  --> f111.rs:15:25
   |
14 | fn do_stuff(foo: Option<Foo>) {
   |             ---  ----------- move occurs because `foo` has type `Option<Foo>`, which does not implement the `Copy` trait
   |             |
   |             captured outer variable
15 |     require_fn_trait(|| async {
   |                      -- ^^^^^ `foo` is moved here
   |                      |
   |                      captured by this `Fn` closure
16 |         if foo.map_or(false, |f| f.foo()) {
   |            --- variable moved due to use in coroutine
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
  --> f111.rs:12:53
   |
12 | fn require_fn_trait<F: Future<Output = ()>>(_: impl Fn() -> F) {}
   |                                                     ^^^^^^^^^
help: consider cloning the value if the performance cost is acceptable
   |
16 |         if foo.clone().map_or(false, |f| f.foo()) {
   |               ++++++++
```

Fix #68119, by pointing at `Fn` and `FnMut` bounds involved in move errors.