Fix diagnostics for async block cloning

[wutchzone]

Closes #121547

r? diagnostics

[wutchzone]

I am not sure what is a more fitting span, whether the path_span or the capture_kind_span. Both seem to be working correctly with the provided test.

[compiler-errors]

This seems fine, but I kind of wish that the message noted that the semantics change due to the clone.

[compiler-errors]

I feel like this error message is not right in spirit. Really we should be saying something like "cloning the value [if this is the behavior that you intended]".

Cloning here could have different semantics than actually moving the value into the async block, for example.

[wutchzone]

Just to make sure, are you suggesting we should change the help message in general for cloning or only for this particular case in the async block?

[compiler-errors]

I don't actually know :/ I guess this is a more general problem than what this aims to fix

[wutchzone]

May I suggest creating an issue where we can talk about the appropriate wording? I would like to also share my opinion, however, I think this is not the right PR for that.

[bors]

☔ The latest upstream changes (presumably #121652) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

Does this work for cases like:

let x = async {
    drop(value);
}.await;
use(value);

Outside of a loop?

[wutchzone]

FYI: due to the #121652, the suggestion is now (on nightly) even more funnier 😀

error[E0382]: use of moved value: `value`
 --> src/main.rs:3:9
  |
1 |   async fn clone_async_block(value: String) {
  |                              ----- move occurs because `value` has type `String`, which does not implement the `Copy` trait
2 |       for _ in 0..10 {
  |       -------------- inside of this loop
3 | /         async { //~ ERROR: use of moved value: `value` [E0382]
4 | |             drop(value);
  | |                  ----- use occurs due to use in coroutine
5 | |             //~^ HELP: consider cloning the value if the performance cost is acceptable
6 | |         }.await
  | |_________^ value moved here, in previous iteration of loop
  |
help: consider moving the expression out of the loop so it is only moved once
  |
2 ~     let mut value = await;
3 ~     for _ in 0..10 {
4 |         async { //~ ERROR: use of moved value: `value` [E0382]
5 |             drop(value);
6 |             //~^ HELP: consider cloning the value if the performance cost is acceptable
7 ~         }.value
  |

[wutchzone]

Does this work for cases like:

let x = async {
   drop(value);
}.await;
use(value);

Outside of a loop?

nightly:

error[E0382]: use of moved value: `value`
 --> src/main.rs:8:13
  |
4 |   async fn clone_async_block(value: String) {
  |                              ----- move occurs because `value` has type `String`, which does not implement the `Copy` trait
5 |       let _x = async {
  |  ______________-
6 | |         drop(value);
  | |              ----- variable moved due to use in coroutine
7 | |     }.await;
  | |_____- value moved here
8 |       consume(value);
  |               ^^^^^ value used here after move
  |
help: consider cloning the value if the performance cost is acceptable
  |
7 |     }.clone().await;
  |      ++++++++

after my patch:

error[E0382]: use of moved value: `value`
 --> src/main.rs:8:13
  |
4 |   async fn clone_async_block(value: String) {
  |                              ----- move occurs because `value` has type `String`, which does not implement the `Copy` trait
5 |       let _x = async {
  |  ______________-
6 | |         drop(value);
  | |              ----- variable moved due to use in coroutine
7 | |     }.await;
  | |_____- value moved here
8 |       consume(value);
  |               ^^^^^ value used here after move
  |
help: consider cloning the value if the performance cost is acceptable
  |
6 |         drop(value.clone());
  |                   ++++++++

I guess it works even for this case.

[bors]

☔ The latest upstream changes (presumably #122947) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r+

[bors]

📌 Commit 2c433d0 has been approved by compiler-errors

It is now in the queue for this repository.