`unreachable_code` warning when invoke `.await` on async diverging function

[nextzhou]

I tried this code:

async fn foo() {
    endless().await;
}

async fn endless() -> ! {
    loop {}
}

I expected to see this happen: no build warn

Instead, this happened:

warning: unreachable expression
 --> src/main.rs:8:5
  |
8 |     endless().await;
  |     ^^^^^^^^^^^^^^^
  |
  = note: #[warn(unreachable_code)] on by default

Meta

rustc --version --verbose:

rustc 1.37.0-nightly (5f3656ce9 2019-06-11)
binary: rustc
commit-hash: 5f3656ce9a2212fad872605b7a4ee103a155e9f3
commit-date: 2019-06-11
host: x86_64-unknown-linux-gnu
release: 1.37.0-nightly
LLVM version: 8.0

[nikomatsakis]

We discussed this a bit on Zulip. The likely cause is that parts of the await desugaring are being linted against. We can likely check the span to see if the code that is being linted against is part of the .await desugaring, but we do want to be careful to ensure that

async fn foo() {
    return; bar().await;
}

async fn bar() {
}

still gets a lint warning.

[nikomatsakis]

Marking as deferred as this doesn't seem like a problem that needs to block stabilization.

[Centril]

cc @varkor who has done some work on the inhabited logic stuff.

[ChrisGreenaway]

I'm now getting this warning even for functions that loop but eventually return. It worked fine with nightly-2019-08-21, but the latest nightly (2019-09-17) is broken.

For example, this code generates a warning:

async fn run(&mut self) {
    loop {
        match self.target_status {
            Some(_) => self.update_status().await,
            None => match &self.current_status {
                OperationStatus::PENDING => self.set_target_status(OperationStatus::EXECUTING),
                OperationStatus::EXECUTING => match self.operation.run().await {
                    Ok(()) => self.set_target_status(OperationStatus::SUCCESSFUL),
                    Err(e) => self.set_target_status(OperationStatus::FAILED(format!("{:?}", e))),
                },
                _ => return,
            },
        }
    }
}

with the warning repeated twice:

  --> src\operations.rs:82:29
   |
82 |       async fn run(&mut self) {
   |  _____________________________^
83 | |         loop {
84 | |             match self.target_status {
85 | |                 Some(_) => self.update_status().await,
...  |
95 | |         }
96 | |     }
   | |_____^
error: unreachable expression
  --> src\operations.rs:82:29
   |
82 |       async fn run(&mut self) {
   |  _____________________________^
83 | |         loop {
84 | |             match self.target_status {
85 | |                 Some(_) => self.update_status().await,
...  |
95 | |         }
96 | |     }
   | |_____^

[PvdBerg1998]

Can confirm. I believe this regressed somewhere after nightly 09-09-2019.