Missing warning for an unused const in async fn

[stepantubanov]

I tried this code:

pub async fn foo() {
    const A: usize = 1;
}

I expected to see the warning:

warning: constant A is never used

Instead no warning was reported by the compiler.

Observed in nightly, 1.67.
Seems like the warning was shown in 1.63 and stopped showing in 1.64.

[clubby789]

Bisected to

  commit[0] 2022-06-30UTC: Auto merge of 98377 - davidv1992:add-lifetimes-to-argument-temporaries, r=oli-obk
  commit[1] 2022-07-01UTC: Auto merge of 98706 - flip1995:clippyup, r=Dylan-DPC
  commit[2] 2022-07-01UTC: Auto merge of 98752 - matthiaskrgr:rollup-uwimznc, r=matthiaskrgr
  commit[3] 2022-07-01UTC: Auto merge of 98730 - matthiaskrgr:rollup-2c4d4x5, r=matthiaskrgr
  commit[4] 2022-07-01UTC: Auto merge of 98402 - cjgillot:undead, r=michaelwoerister
  commit[5] 2022-07-01UTC: Auto merge of 98767 - Dylan-DPC:rollup-j1gq5sr, r=Dylan-DPC
  commit[6] 2022-07-01UTC: Auto merge of 93967 - cjgillot:short-struct-span, r=petrochenkov

CI has expired so I couldn't automatically bisect further, but #98402 looks like a likely candidate

[clubby789]

It looks like the issue is that dead code detection doesn't work inside closures and generators (which async functions use in their desugaring)

#[deny(dead_code)]
#[allow(unused_must_use)]
pub fn foo() {
    || {
        fn a() {}
    };
}

This is denied by 1.63 but allowed on 1.64+.
@rustbot label +regression-from-stable-to-stable

---

rust/compiler/rustc_passes/src/dead.rs

Lines 833 to 836 in 3200982

	if parent != module && !live_symbols.contains(&parent) {
	// We already have diagnosed something.
	continue;
	}

Here, the parent of the closure is foo::{closure#0} so checking dead code is skipped.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium