E0623 highlights wrong parameter in async fn

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.

related: #63499

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

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!

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.

@SNCPlay42

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

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

@rustbot

@rustbot claim

@rustbot

@rustbot label +AsyncAwait-Polish

Rollup merge of rust-lang#92183 - tmandry:issue-74256, r=estebank

There is a remaining issue not fixed by #92183 which I'll attempt to elucidate here.

In try_report_anon_anon_conflict, we are reporting an error with two anonymous lifetimes. We rely on the fact that anonymous lifetimes can only be introduced in parameters, and the only other place they can appear is the return type. To check if the conflict occurs in the return type, we check to see if either lifetime appears in the return type. If it does, we highlight the return type and the parameter corresponding to the other lifetime.

This is a little broken. Just because a lifetime appears in the return type does not necessarily mean the return type had anything to do with the current error. We want to know what types the failing lifetime obligation came from, and the span of those types. For example, for each lifetime it could be

The return type
The type of the parameter that introduced the lifetime
An associated type embedded in either of the above (e.g., impl Future<Output = &'a i32>)

Current output:

error: lifetime may not live long enough
 --> src/main.rs:5:9
  |
4 |     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`
5 |         f
  |         ^ method was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
  |
help: consider introducing a named lifetime parameter and update trait if needed
  |
4 |     async fn async_fn<'a>(self: &'a Struct, f: &'a u32) -> &u32 {
  |                      ++++        ++             ++

error: lifetime may not live long enough
 --> src/main.rs:9:9
  |
8 |     fn sync_fn(self: &Struct, f: &u32) -> &u32 {
  |                      -           - let's call the lifetime of this reference `'1`
  |                      |
  |                      let's call the lifetime of this reference `'2`
9 |         f
  |         ^ method was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
  |
help: consider introducing a named lifetime parameter and update trait if needed
  |
8 |     fn sync_fn<'a>(self: &'a Struct, f: &'a u32) -> &u32 {
  |               ++++        ++             ++

this works now.

Navigation Menu

Repository navigation

E0623 highlights wrong parameter in async fn #74256

6 remaining items

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

E0623 highlights wrong parameter in async fn #74256

Description

Activity

ayazhafiz commented on Jul 12, 2020

tmandry commented on Jul 14, 2020

SNCPlay42 commented on Jul 22, 2020

SNCPlay42 commented on Jul 22, 2020

eholk commented on Sep 8, 2021

tmandry commented on Sep 9, 2021

eholk commented on Sep 16, 2021

6 remaining items

eholk commented on Oct 18, 2021

tmandry commented on Apr 14, 2022

estebank commented on Aug 3, 2023

compiler-errors commented on Dec 31, 2024

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions