Future compat warning with `.await`ing a `BoxFuture`

[Centril]

The code is the same as in #62284:

#![feature(async_await)]

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

type BoxFuture = Pin<Box<dyn Future<Output = ()>>>;

async fn foo() -> BoxFuture {
    Box::pin(bar()) as _
}

async fn bar() {
    let _ = foo().await;
}

This results in:

warning[E0381]: use of possibly uninitialized variable: `_`
  --> src/lib.rs:17:13
   |
17 |     let _ = foo().await;
   |             ^^^^^^^^^^^ use of possibly uninitialized value
   |
   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
   = note: for more information, try `rustc --explain E0729`

[matthewjasper]

minimized

fn main() {
    let _ = loop {
        break Box::new(()) as Box<dyn Send>;
    };
}

The MIR we generate is broken, so the borrow checker is correct to complain:

        StorageDead(_2); // _2 is uninit after this.
        _1 = move _2 as std::boxed::Box<dyn std::marker::Send> (Pointer(Unsize));
        drop(_2) -> [return: bb7, unwind: bb6]; // What?

[matthewjasper]

Not an issue on beta.

[pnkfelix]

triage: P-high, removing nomination.