Add a machine-applicable suggestion to "unreachable pattern"

[Nadrieril]

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[rustbot]

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[y21]

IMO more often than not this lint suggests that there's some other kind of bug and removing the unreachable pattern isn't what the user actually wants, so I'm not sure this should be machine applicable and autofixed by rustfix.

E.g. when using a constant as a pattern but that constant is suddenly no longer in scope (can happen after a refactor):

mod ext {
  pub const BAR: i32 = 1;
}

match 1 {
  BAR => {},
  _ => {}
}

The last arm has an unreachable arm warning, but removing the last arm here is not what the user wants. It's more an issue with the binding pattern that is causing this arm to be unreachable

[lcnr]

r? compiler-errors

[compiler-errors]

@rustbot author

[Nadrieril]

IMO more often than not this lint suggests that there's some other kind of bug and removing the unreachable pattern isn't what the user actually wants

That's fair. I wonder if I should restrict it to the "empty pattern" cases, so it can function as a migration lint but not more. I doubt there'd be false positives there.

@rustbot ready

[Nadrieril]

I changed the suggestion to only apply to empty pattern. That way the lint makes the transition to min_exhaustive_patterns easier, without the risk of removing an arm unexpectedly.

[bors]

☔ The latest upstream changes (presumably #129359) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

r=me when rebased

@rustbot author

[Nadrieril]

I'll wait for #129103 to get in first

[Nadrieril]

@bors r=@compiler-errors

[bors]

📌 Commit 1f69638 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1f69638 with merge f9567d0...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing f9567d0 to master...

[rust-timer]

Finished benchmarking commit (f9567d0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary 3.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.2%	[3.0%, 3.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 758.501s -> 757.31s (-0.16%)
Artifact size: 341.18 MiB -> 341.20 MiB (0.01%)