Missing warning for an unused const in async fn #108296

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1. stepantubanov

   added

   C-bugCategory: This is a bug.Category: This is a bug.

   on Feb 21, 2023
2. clubby789 commented on Feb 21, 2023

   

   clubby789

   on Feb 21, 2023

   Last edited by clubby789

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   More actions

   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
3. clubby789 commented on Feb 21, 2023

   

   clubby789

   on Feb 21, 2023

   Last edited by clubby789

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   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.
4. rustbot

   added

   regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.

   I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}Issue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}

   on Feb 21, 2023
5. apiraino commented on Feb 21, 2023

   

   apiraino

   on Feb 21, 2023

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   WG-prioritization assigning priority (Zulip discussion).

   @rustbot label -I-prioritize +P-medium
6. rustbot

   added

   P-mediumMedium priorityMedium priority

   and removed

   I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}Issue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}

   on Feb 21, 2023
7. apiraino

   added

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   on Feb 21, 2023
8. matthiaskrgr

   added a commit that references this issue on Feb 22, 2023

   Rollup merge of rust-lang#108315 - clubby789:dead-code-in-closure, r=…

   ...

   This issue will close once commit 783617b is merged into the 'master' branch.

   783617b
9. bors

   closed this as completedin 783617bon Feb 23, 2023