Impls and impl items inherit dead_code lint level of the corresponding traits and trait items

[mu001999]

https://github.com/rust-lang/rust/blob/master/compiler/rustc_passes/src/dead.rs#L360-L361 won't insert assoc items into the live set, so that impl items cannot be marked live.

This PR lets impls and impl items can inherit lint levels of the corresponding traits and trait items.

Fixes #144060

r? @petrochenkov

[compiler-errors]

This behavior seems really non-local and pretty broad. #[allow(dead_code)] on a trait definition suppresses all dead code warnings in all impls with this change?

[mu001999]

#[allow(dead_code)] on a trait definition suppresses all dead code warnings in all impls with this change?

@compiler-errors yes, and I think this makes sense. We would have only one trait definition but many impls, letting #[allow(dead_code)] on traits propagate to impls is convenient. Otherwise we need to add #[allow(dead_code)] also on each impls which use unused items.

[petrochenkov]

r? @compiler-errors

[petrochenkov]

Hmm, why did rustbot assign it to me again?
I'll just block this on #144863 now.
@rustbot blocked

[mu001999]

Hmm, why did rustbot assign it to me again?

I tried reassigning, but rustbot didn’t respond after waiting a while, so I deleted the comment. 😂

[bors]

☔ The latest upstream changes (presumably #144863) made this pull request unmergeable. Please resolve the merge conflicts.

[mu001999]

@rustbot ready

[tgross35]

@cjgillot gentle ping for a review here

[wesleywiser]

Nominated the issue for lang team discussion, let's see what they think before merging.

[mu001999]

oh, I missed the non-local in the note from @compiler-errors

and if the trait is non-local, the dead-code lint level won't spread to the local impls. cc @wesleywiser

we only care about local traits, see https://github.com/rust-lang/rust/pull/144113/files#diff-719b4f36bbbf3b3bf48683429408f5d27e5633f70c1144a93554bd536d5bf655R786

[joshtriplett]

Temporarily removing T-compiler so we can do a lang FCP on this. (Note that this is fixing something perceived as a stable-to-stable regression.)

[joshtriplett]

@rfcbot merge

[rust-rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[tmandry]

@rfcbot reviewed