Lifetime error points to wrong reference in async fn return

@rustbot

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

@Aaron1011

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

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.

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.

@Aaron1011

@Aaron1011 Are you still planning to work on this?

Prefer regions with an `external_name` in `approx_universal_upper_bound`

Rollup merge of rust-lang#78164 - Aaron1011:fix/async-region-name, r=…

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Lifetime error points to wrong reference in async fn return #75785

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Lifetime error points to wrong reference in async fn return #75785

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SNCPlay42 commented on Aug 21, 2020

Aaron1011 commented on Aug 21, 2020

SNCPlay42 commented on Aug 21, 2020

tmandry commented on Oct 20, 2020

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