Confusing error message with borrows and dyn Error

[insanitybit]

Given the following code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=06baecf9bfb364dbe030e6009f2316f9

use std::sync::Arc;

async fn do_stuff(boop: Arc<dyn std::error::Error>, beep: &str, borp: &str){
}

The current output is:

error[[E0700]](https://doc.rust-lang.org/stable/error-index.html#E0700): hidden type for `impl Trait` captures lifetime that does not appear in bounds
 [--> src/lib.rs:3:76
](https://play.rust-lang.org/#)  |
3 | async fn do_stuff(boop: Arc<dyn std::error::Error>, beep: &str, borp: &str){
  |                                                                            ^
  |
  = note: hidden type `impl Future<Output = [async output]>` captures lifetime '_#14r

Ideally the output should look like:

I'm not really sure. I actually don't get why this is an error? If I remove any one of those arguments it works. I think this is a combination of dyn with async and multiple references, although weirdly if there's only one &str it compiles... idk!

Things that are confusing:

1. There is no "impl Future" - it's hidden, but where? I know it's because async desugares to that, but I think that's confusing.

2. _#14r IDK which lifetime that's referring to. I sort of suspect it's the dyn one.

3. It doesn't tell me what to do

The code to fix this is to add a lifetime:

async fn do_stuff<'a>(boop: Box<dyn std::error::Error + 'a>, beep: &str, borp: &str){
}

So ideally it'd suggest that. An anonymous lifetime works too, actually.

[eholk]

We talked about this in our triage meeting today. This sounds like the same issue as #63033, so we can probably close it as a duplicate.

It looks like what's going on is that the boop parameter would normally be Box<dyn std::error::Error + 'static>, but the compiler finds an opportunity for subtyping with the future and changes it to Box<dyn std::error::Error + 'a>, but then it needs to pick a value for 'a. It doesn't want to introduce a new lifetime, so instead it needs to choose between beep (call that 'b) and borp (call that 'c). Since 'b and 'c have the same constraints, there's no best choice between these two and the compiler gives up.

Fixing #63033 should fix this one too.

@rustbot label AsyncAwait-Triaged

[aliemjay]

Thai no longer an error, fixed along with #63033.