rustc suggests invalid syntax to fix lifetime inferring error

[Bunogi]

I tried this code:

#![feature(async_await)]

use futures::{stream, StreamExt};

struct Foo {}

impl Foo {
    async fn foo(&self) {}

    async fn bar(&self) {
        stream::iter(0..1)
            .then(|_| runtime::spawn(self.foo()))
            .collect::<()>()
            .await;
    }
}

fn main() {}

The code above does not compile, throwing an error with inferring lifetimes. rustc produces the following output:

error: cannot infer an appropriate lifetime
  --> src/main.rs:10:18
   |
10 |     async fn bar(&self) {
   |                  ^^^^^ ...but this borrow...
11 |         stream::iter(0..1)
12 |             .then(|_| runtime::spawn(self.foo()))
   |                       -------------- this return type evaluates to the `'static` lifetime...
   |
note: ...can't outlive the lifetime '_ as defined on the method body at 10:18
  --> src/main.rs:10:18
   |
10 |     async fn bar(&self) {
   |                  ^
help: you can add a constraint to the return type to make it last less than `'static` and match the lifetime '_ as defined on the method body at 10:18
   |
12 |             .then(|_| runtime::spawn + '_(self.foo()))
   |                       ^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: Could not compile `report`.

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

This suggestion rustc makes however, is invalid syntax. Trying to apply it's suggestion causes a syntax error as seen below.

error: expected `:`, found `(`
  --> src/main.rs:12:42
   |
12 |             .then(|_| runtime::spawn + '_(self.foo()))
   |                                          ^ expected `:`

Meta

rustc --version --verbose:

rustc 1.37.0-nightly (de7c4e423 2019-06-23)
binary: rustc
commit-hash: de7c4e42314c56528640e3b663aa10e0caa6bd9b
commit-date: 2019-06-23
host: x86_64-unknown-linux-gnu
release: 1.37.0-nightly
LLVM version: 8.0

[DarkKirb]

Can confirm with rust 1.40.0-nightly

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=1d8e97383618ff6ff61ea2b83156c6f7

this only happens with async fns, the same code without async does not trigger the issue

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=2fab081c235b071b096b5f6c877f22d8

[nikomatsakis]

@rustbot assign @csmoe