never patterns: `!` argument not detected as diverging on async fn

[Nadrieril]

With #120104, a function with a ! pattern in a binding is detected as diverging:

fn never_arg(!: Void) -> ! {}
fn never_let(x: Void) -> ! {
  let ! = x;
}

This doesn't work the arguments of an async fn however:

async fn never_let(x: Void) -> ! {
  let ! = x;
} // ok
async fn never_arg(!: Void) -> ! {} // ERROR mistmatched types

[traviscross]

@rustbot labels +AsyncAwait-Triaged +WG-async

We reviewed this today in WG-async triage. We agree this is a bug.

[compiler-errors]

I looked into this and it has to do with the fact that we desugar the latter (never_arg) into something like:

fn never_arg(x: Void) -> ! {
    async move {
        let ! = x;
        let body = {
        };
        body
    }
}

I don't believe this is going to be easy or perhaps even possible to fix given the way that we don't/can't(?) make the inner body block (which corresponds to the body of the original async fn) to be diverging based off of a pattern in the outer block.

[Nadrieril]

And I presume the desugaring is this way because of drop order subtleties?

[compiler-errors]

yep