manual_async_fn false positive

[Seeker14491]

The following code triggers the manual_async_fn lint:

use std::future::Future;

struct A;

impl A {
    fn f(&self) -> impl Future<Output = ()> {
        async {}
    }
}

fn main() {
    let _future = {
        let a = A;
        a.f()
    };
}

However, applying the suggestion of using an async fn causes the code to stop compiling:

error[E0597]: `a` does not live long enough
  --> src\main.rs:14:9
   |
12 |     let _future = {
   |         ------- borrow later stored here
13 |         let a = A;
14 |         a.f()
   |         ^ borrowed value does not live long enough
15 |     };
   |     - `a` dropped here while still borrowed

This is due to async fn implicitly capturing the lifetime of &self in the returned future, while the impl Future syntax does not.

Meta

• cargo clippy -V: clippy 0.0.212 (50fc24d 2020-06-25)
• rustc -Vv:

  rustc 1.46.0-nightly (50fc24d8a 2020-06-25)
  binary: rustc
  commit-hash: 50fc24d8a172a853b5dfe40702d6550e3b8562ba
  commit-date: 2020-06-25
  host: x86_64-pc-windows-msvc
  release: 1.46.0-nightly
  LLVM version: 10.0
  

[ebroto]

To be consistent with async fn, we should check that the returned future captures all the input lifetimes, or that there are no input lifetimes. See here.

I will work on fixing this in the coming days.

[ebroto]

So holidays and other stuff came and I could not dedicate a lot of time to clippy until now. Sorry for the delay!