collection_is_never_read: Handle unit type

[schubart]

changelog: [collection_is_never_read]: Fix false negative
fixes: #10488

[rustbot]

r? @Manishearth

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[llogiq]

What about methods that use &mutably borrowed arguments to get the read values back to the caller?

[schubart]

Are there methods like that in the collection classes? (Ignoring methods from extension traits, we already excluded those in the original PR.)

I could inspect the argument types of a method and look for &mut ones, but I'd prefer to do that in a separate PR since it would be fixing a potential false positive, while this one here fixes a false negative.

[Manishearth]

r=me, though there are conflicts

(sorry about missing this PR)

[bors]

☔ The latest upstream changes (presumably #10614) made this pull request unmergeable. Please resolve the merge conflicts.

[schubart]

Thanks, merge conflicts fixed.

[Manishearth]

@bors r+

[bors]

📌 Commit 3d71145 has been approved by Manishearth

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3d71145 with merge 3b5b2ed...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 3b5b2ed to master...