let_and_return FP on ugly async Send workaround

Summary

let_and_return trips up on an awful construction I invented to avoid a future-not-Send problem.

The workaround code I have here is terrible, but I'm not aware of a better way to write it. (NB that in this example I use mpc::channel::Receiver as a thing that is Send but not Sync; the example program doesn't make much sense semantically.)

I bet the underlying cause is the same as #8114.

Lint Name

let_and_return

Reproducer

I tried this code:

use std::sync::mpsc;
use std::future::Future;
use std::pin::Pin;

async fn awaitpoint(_: ()) { }

async fn failure() {
    let (_, rx) = mpsc::channel::<()>();

    while let Ok(event) = { let k= &rx; k.recv()} {
        awaitpoint(event).await;
    }
}

fn main() {
    let _: Pin<Box<dyn Future<Output=()> + Send>> = Box::pin(
        async {
            failure().await;
        }
    );
}

I saw this happen:

    Checking foo v0.1.0 (/volatile/rustcargo/Rustup/Arti/experiments)
warning: returning the result of a `let` binding from a block
  --> src/main.rs:10:59
   |
10 |     while let Ok(event) = { let k= ℞ let y = k.recv(); y } {
   |                                         ----------------- ^
   |                                         |
   |                                         unnecessary `let` binding
   |
   = note: `#[warn(clippy::let_and_return)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
help: return the expression directly
   |
10 -     while let Ok(event) = { let k= ℞ let y = k.recv(); y } {
10 +     while let Ok(event) = { let k= ℞  k.recv() } {
   | 

warning: `foo` (bin "foo") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.21s

I expected to see this happen:

Well, ideally, Rust would have accepted

    while let Ok(event) = rx.recv() {

but failing that, it wouldl be nice if clippy didn't suggest changes that cause compiler errors.

Version

rustc 1.61.0-nightly (1bfe40d11 2022-03-18)
binary: rustc
commit-hash: 1bfe40d11c3630254504fb73eeccfca28d50df52
commit-date: 2022-03-18
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

Additional Labels

@rustbot label +I-suggestion-causes-error

Start by reproducing the supplied async Send workaround and its let_and_return warning with the shown Rust code. Trace the let_and_return lint logic and verify that its suggested simplification does not produce a compiler error for this pattern; the issue is resolved when this case is handled without an invalid suggestion.