async-block diagnostics do not suggest async move when it may be needed

[WildCryptoFox]

(playground)

//* f hits lifetime errors
fn f(a: &usize) -> (impl '_ + Future<Output = ()>, &usize) {
    (async { *a; }, a)
}
// */

// g compiles as expected
fn g(a: &usize) -> (impl '_ + Future<Output = ()>, &usize) {
    async fn h(a: &usize) { *a; }
    (h(a), a)
}

error[E0515]: cannot return value referencing function parameter `a`
 --> src/main.rs:5:5
  |
5 |     (async { *a; }, a)
  |     ^^^^^^^^^^-^^^^^^^
  |     |         |
  |     |         `a` is borrowed here
  |     returns a value referencing data owned by the current function

This issue has been assigned to @gizmondo via this comment.

[nikomatsakis]

The error is correct, so what we are looking for here is a diagnostic improvement. The problem is that async { *a }, much like a closure, defaults to borrowing a in place -- so when we try to return, it has a reference to a as a is going out of scope.

To fix the error, you need async move (playground).

The situation is exactly analogous to closures and move closures. Note that in the analogous closure example (playground), we give a different error, complete with a suggestion:

error[E0373]: closure may outlive the current function, but it borrows `a`, which is owned by the current function
 --> src/main.rs:5:6
  |
5 |     (|| { *a; }, a)
  |      ^^    - `a` is borrowed here
  |      |
  |      may outlive borrowed value `a`
  |
note: closure is returned here
 --> src/main.rs:5:5
  |
5 |     (|| { *a; }, a)
  |     ^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `a` (and any other referenced variables), use the `move` keyword
  |
5 |     (move || { *a; }, a)
  |      ^^^^^^^

It seems like we should adapt the async { } case to be closer to the closure wording.

[nikomatsakis]

Marking this as OnDeck since it seems like this might be a common mistake, and I think the fix should be relatively straightforward.

[WildCryptoFox]

Ah thanks @nikomatsakis this makes sense.

[gilescope]

@rustbot claim

[tmandry]

Ping from triage. @gilescope are you working on this?

[tmandry]

Ping. @gilescope are you interested in continuing to work on this, or should I un-assign?

[nikomatsakis]

@rustbot claim

I'm going to "claim" this issue, but only to write mentoring instructions! If anybody is interested in taking a crack at it, I'd be happy to help.

[gilescope]

Sorry - if anyone else wants it that’s cool. I seem to have fingers in quite a few pies at the moment.

_{Sent with GitHawk}

[gizmondo]

@rustbot claim

[nikomatsakis]

@gizmondo thanks!!