Error message in async fn refers to lifetime `'1` without defining it

[SNCPlay42]

pub async fn f(x: &mut i32) -> &i32 {
    let y = &*x;
    *x += 1;
    y
}

Output on 1.46.0-nightly (2020-07-04 0cd7ff7) (Playground)

   Compiling playground v0.0.1 (/playground)
error[E0506]: cannot assign to `*x` because it is borrowed
 --> src/lib.rs:3:5
  |
2 |     let y = &*x;
  |             --- borrow of `*x` occurs here
3 |     *x += 1;
  |     ^^^^^^^ assignment to borrowed `*x` occurs here
4 |     y
  |     - returning this value requires that `*x` is borrowed for `'1`
5 | }
  | - return type of generator is &'1 i32

error: aborting due to previous error

For more information about this error, try `rustc --explain E0506`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

Without async, a note like this is emitted, and should also be emitted with async:

1 | pub fn f(x: &mut i32) -> &i32 {
  |             - let's call the lifetime of this reference `'1`

Output on stable 1.44.1 (with async) is different:

   Compiling playground v0.0.1 (/playground)
error[E0506]: cannot assign to `*x` because it is borrowed
 --> src/lib.rs:3:5
  |
2 |     let y = &*x;
  |             --- borrow of `*x` occurs here
3 |     *x += 1;
  |     ^^^^^^^ assignment to borrowed `*x` occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0506`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[SNCPlay42]

This note

5 | }
  | - return type of generator is &'1 i32

Is the one that is supposed to define '1 (it's produced by highlight_region_name), but it's not clear that it's doing so - it appears after the mention of '1, not before, uses "generator" instead of "async function", and has a poor span.

[tmandry]

Discussed in @rust-lang/wg-async-foundations. We think rewording the message to something like "let's call the return type of the async function &'1 i32" is a good way to improve this message. But further improvements will be needed to bring it up to par with the non-async version.

[nikomatsakis]

In particular, we would really like to cite the function parameter, not the return type, which is kind of random. But I guess I'd have to dig into the code to figure out how we can make that association.

[SNCPlay42]

@rustbot claim

I've got as far as

error[E0506]: cannot assign to `*x` because it is borrowed
  --> issue-74072.rs:5:5
   |
LL | pub async fn f(x: &mut i32) -> &i32 {
   |                                ---- return type of async function is &'1 i32
LL |     let y = &*x;
   |             --- borrow of `*x` occurs here
LL |     *x += 1;
   |     ^^^^^^^ assignment to borrowed `*x` occurs here
LL |     y
   |     - returning this value requires that `*x` is borrowed for `'1`

I think I can make further improvements.