Dont check `must_use` on nested `impl Future` from fn Fixes (but does not close, per beta policy) #111484 Also fixes a `FIXME` left in the code about (presumably) false-positives on non-async `#[must_use] fn() -> impl Future` cases, though if that's not desirable to include in the beta backport then I can certainly revert it. Beta nominating as it fixes a beta ICE.