False positive for `let_underscore_future`

Summary

I just came across a case where I think the new let_underscore_future #9721 produces a false positive:

#[tokio::main]
async fn main() {
    let _ = tokio::spawn(async {
        println!("spawned")
    });
}

Playground

In this case the returned JoinHandle is a future, but it doesn't have to be awaited to make the provided future start execution.

Reproducer

I tried this code:

#[tokio::main]
async fn main() {
    let _ = tokio::spawn(async {
        println!("spawned")
    });
}

I expected to see this happen:

There should be no clippy warnings.

Instead, this happened:

warning: non-binding `let` on a future
 --> src/main.rs:3:5
  |
3 | /     let _ = tokio::spawn(async {
4 | |         println!("spawned")
5 | |     });
  | |_______^

Version

1.67.0-nightly

(2022-11-22 ff8c8dfbe66701531e3e)

Additional Labels

No response

Start with the implementation and tests for the let_underscore_future lint, then run the Tokio spawn reproducer from the issue. Done means this case produces no warning while the lint continues to identify non-binding lets on futures that should be awaited.