Split detecting unconstructible pub structs into a new lint from dead_code

[mu001999]

#125572 detects never constructed pub structs, but introduces some regressions (#126169 and #128272).

This does not fix the regressions but just split the detection into a new lint, #127104 and #128329 try to fix them.

I don't know if it is late or not for this PR. But this could at least provide a way to help silence the lint separately for current or potential future regressions.

[rustbot]

r? @nnethercote

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[mu001999]

wired failure:

   3 warnings found (use docker --debug to expand):
   - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 81)
   - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 86)
   - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 101)
  Dockerfile:76
  --------------------
    74 |     
    75 |     COPY scripts/nodejs.sh /scripts/
    76 | >>> RUN sh /scripts/nodejs.sh /node
    77 |     ENV PATH="/node/bin:${PATH}"
    78 |     
  --------------------
  ERROR: failed to solve: process "/bin/sh -c sh /scripts/nodejs.sh /node" did not complete successfully: exit code: 2
  The command has failed after 5 attempts.
  Error: Process completed with exit code 1.

🤔is there any other way to manually restart gh jobs other than push new?

[compiler-errors]

It seems very sketchy that this depends on first_item. Doesn't this suppress dead_code if a pub ADT comes first in the list?

[mu001999]

It seems very sketchy that this depends on first_item. Doesn't this suppress dead_code if a pub ADT comes first in the list?

No, we will only have one pub struct there. Because this function will only be called by warn_dead_code for struct.

[nnethercote]

I will assign to @compiler-errors because he has already started reviewing.

r? @compiler-errors

[compiler-errors]

This PR has the side-effect of now suppressing dead_code warnings if we have unreachable-pub structs, such as mod x { pub Unreachable(i32); }. I think the new lint should only trigger for the new dead_code behavior introduced recently.

[mu001999]

@compiler-errors updated and use tcx.effective_visibilities(()).is_reachable now, and add a test for mod x { pub Unreachable(i32); } in tests/ui/lint/dead-code/unconstructible-pub-struct.rs

[bors]

☔ The latest upstream changes (presumably #128404) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

Let's reintroduce this lint later. I will leave it up to you, but this is probably best closed.

@rustbot author