Drop tracking error message regression in issue-70935-complex-spans.rs

[eholk]

With -Zdrop-tracking enabled, issue-70935-complex-spans.rs gives the following stderr output:

error[E0277]: `Sender<i32>` cannot be shared between threads safely
  --> $DIR/issue-70935-complex-spans.rs:10:45
   |
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
   |                                             ^^^^^^^^^^^^^^^^^^ `Sender<i32>` cannot be shared between threads safely
   |
   = help: the trait `Sync` is not implemented for `Sender<i32>`
   = note: required because of the requirements on the impl of `Send` for `&Sender<i32>`
   = note: required because it appears within the type `[closure@$DIR/issue-70935-complex-spans.rs:13:13: 15:10]`
   = note: required because it appears within the type `[static generator@$DIR/issue-70935-complex-spans.rs:7:67: 8:2]`
   = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/issue-70935-complex-spans.rs:7:67: 8:2]>`
   = note: required because it appears within the type `impl Future<Output = ()>`
   = note: required because it appears within the type `impl Future<Output = ()>`
   = note: required because it appears within the type `for<'r, 's, 't0> {ResumeTy, impl Future<Output = ()>, ()}`
   = note: required because it appears within the type `[static generator@$DIR/issue-70935-complex-spans.rs:12:16: 16:6]`
   = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/issue-70935-complex-spans.rs:12:16: 16:6]>`
   = note: required because it appears within the type `impl Future<Output = ()>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

Without -Zdrop-tracking, we get the following error message:

error: future cannot be sent between threads safely
  --> src/test/ui/async-await/issue-70935-complex-spans.rs:10:45
   |
10 | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
   |                                             ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: the trait `Sync` is not implemented for `Sender<i32>`
note: future is not `Send` as this value is used across an await
  --> src/test/ui/async-await/issue-70935-complex-spans.rs:15:11
   |
13 |           baz(|| async{
   |  _____________-
14 | |             foo(tx.clone());
15 | |         }).await;
   | |         - ^^^^^^ await occurs here, with the value maybe used later
   | |_________|
   |           has type `[closure@src/test/ui/async-await/issue-70935-complex-spans.rs:13:13: 15:10]` which is not `Send`
note: the value is later dropped here
  --> src/test/ui/async-await/issue-70935-complex-spans.rs:15:17
   |
15 |         }).await;
   |                 ^

error: aborting due to previous error

The old message seems much better, so it'd be good to match the behavior with drop tracking.

[eholk]

Zulip thread with lots of discussion about this issue

/cc @jyn514

[jyn514]

@rustbot claim

[jyn514]

Noticed an existing weirdness with one of the diagnostics - this should point to tx, not the closure itself:

rust/src/test/ui/async-await/issue-70935-complex-spans.stderr

Lines 11 to 17 in ec21d7e

	LL | baz(|| async{
	| _____________-
	LL | | foo(tx.clone());
	LL | | }).await;
	| | - ^^^^^^ await occurs here, with the value maybe used later
	| |_________|
	| has type `[closure@$DIR/issue-70935-complex-spans.rs:13:13: 15:10]` which is not `Send`

[jyn514]

#98259 makes this significantly better but not the same as without -Zdrop-tracking. Not sure whether that's sufficient, or whether I should keep working on this. I'm not sure it's possible to make them identical without radical changes to how async-await diagnostics work: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/josh.20rambles.20about.20drop.20tracking.20.2397332/near/286660618

[eholk]

I'm happy with how this error message looks after the recent changes, so I'll go ahead and mark it as closed.