E0623 highlights wrong parameter in async fn

[SNCPlay42]

struct Struct { }

impl Struct {
    async fn async_fn(self: &Struct, f: &u32) -> &u32 {
        f
    }
    
    fn sync_fn(self: &Struct, f: &u32) -> &u32 {
        f
    }
}

Output (playground)

   Compiling playground v0.0.1 (/playground)
error[E0623]: lifetime mismatch
 --> src/main.rs:5:9
  |
4 |     async fn async_fn(self: &Struct, f: &u32) -> &u32 {
  |                             -------              ----
  |                             |
  |                             this parameter and the return type are declared with different lifetimes...
5 |         f
  |         ^ ...but data from `f` is returned here

error[E0623]: lifetime mismatch
 --> src/main.rs:9:9
  |
8 |     fn sync_fn(self: &Struct, f: &u32) -> &u32 {
  |                                  ----     ----
  |                                  |
  |                                  this parameter and the return type are declared with different lifetimes...
9 |         f
  |         ^ ...but data from `f` is returned here

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0623`.
error: could not compile `playground`.

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

The error in sync_fn is correct, but the one for async_fn incorrectly claims the return type lifetime is different from self's lifetime, when they are in fact the same per the elision rules.

[ayazhafiz]

related: #63499

[tmandry]

Seems related to #74072. Triaging as On Deck since this is an incorrect diagnostic which can cause confusion.

[SNCPlay42]

Another case:

async fn async_fn(x: &i32, y: &mut &i32) {
    *y = x;
}

fn sync_fn(x: &i32, y: &mut &i32) {
    *y = x;
}

Output (playground)

error[E0623]: lifetime mismatch
 --> src/main.rs:2:10
  |
1 | async fn async_fn(x: &i32, y: &mut &i32) {
  |                                    ----  -
  |                                    |
  |                                    this parameter and the return type are declared with different lifetimes...
2 |     *y = x;
  |          ^ ...but data from `x` is returned here

error[E0623]: lifetime mismatch
 --> src/main.rs:6:10
  |
5 | fn sync_fn(x: &i32, y: &mut &i32) {
  |               ----          ----
  |               |
  |               these two types are declared with different lifetimes...
6 |     *y = x;
  |          ^ ...but data from `x` flows into `y` here

The error message for async_fn refers to returning when no returning is involved!

[SNCPlay42]

Output for the async functions in both cases with #![feature(nll)]: (playground)

error: lifetime may not live long enough
 --> src/main.rs:4:5
  |
3 | async fn async_fn(x: &i32, y: &mut &i32) {
  |                      -             - let's call the lifetime of this reference `'2`
  |                      |
  |                      let's call the lifetime of this reference `'1`
4 |     *y = x;
  |     ^^^^^^ assignment requires that `'1` must outlive `'2`

error: lifetime may not live long enough
  --> src/main.rs:11:9
   |
10 |     async fn async_fn(self: &Struct, f: &u32) -> &u32 {
   |                             -           - let's call the lifetime of this reference `'1`
   |                             |
   |                             let's call the lifetime of this reference `'2`
11 |         f
   |         ^ associated function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`

Looks like this can be tagged as fixed by NLL.

[eholk]

@SNCPlay42 - since this is tagged as fixed by NLL, do you think we can go ahead and close the whole issue?

[tmandry]

This still reproduces for me. Adding #![feature(nll)] does fix the issue, I'm not sure what we're using that feature for nowadays.

[eholk]

@rustbot claim