Lifetime error points to wrong reference in async fn return

[SNCPlay42]

pub async fn async_fn(x: &mut i32) -> (&i32, &i32) {
    let y = &*x;
    *x += 1;
    (&32, y)
}

Output (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 |     (&32, y)
  |     -------- returning this value requires that `*x` is borrowed for `'1`
5 | }
  | - return type of generator is (&'1 i32, &i32)

The '1 lifetime name should be given to the second reference in the tuple.

@rustbot modify labels: C-bug A-async-await A-diagnostics A-lifetimes T-compiler

[SNCPlay42]

On stable the output is different:

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

Per bisect-rustc, the commit that changed the output is f781bab - almost definitely #73806. @Aaron1011 is there something that can be done to improve this case?

Bisect output

searched nightlies: from nightly-2020-06-01 to nightly-2020-07-15
regressed nightly: nightly-2020-07-02
searched commits: from 16957bd to f781bab
regressed commit: f781bab

bisected with cargo-bisect-rustc v0.5.2

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --regress success --script ./test.sh --start 2020-06-01 --end 2020-07-15 --preserve 

[Aaron1011]

Here's the output with async removed:

pub fn async_fn(x: &mut i32) -> (&i32, &i32) {
    let y = &*x;
    *x += 1;
    (&32, y)
}

error[E0506]: cannot assign to `*x` because it is borrowed
 --> src/lib.rs:3:5
  |
1 | pub fn async_fn(x: &mut i32) -> (&i32, &i32) {
  |                    - let's call the lifetime of this reference `'1`
2 |     let y = &*x;
  |             --- borrow of `*x` occurs here
3 |     *x += 1;
  |     ^^^^^^^ assignment to borrowed `*x` occurs here
4 |     (&32, y)
  |     -------- returning this value requires that `*x` is borrowed for `'1`

It looks like we're missing the "let's call the lifetime of this reference" note in the async case.

[SNCPlay42]

It looks like we're missing the "let's call the lifetime of this reference" note in the async case.

The current equivalent of that annotation is the note at the bottom: (see #74072)

- return type of generator is (&'1 i32, &i32)

I actually ran into this while testing a potential fix for that issue - it would make the output

LL | pub async fn async_fn(x: &mut i32) -> (&i32, &i32) {
   |                                        - let's call the lifetime of this reference `'1`
LL |     let y = &*x;
   |             --- borrow of `*x` occurs here
LL |     *x += 1;
   |     ^^^^^^^ assignment to borrowed `*x` occurs here
LL |     (&32, y)
   |     -------- returning this value requires that `*x` is borrowed for `'1`

Which still points at the wrong reference.

[tmandry]

@Aaron1011 Are you still planning to work on this?