unused_unit: don't lint on closure return types

[ada4a]

As discussed in #clippy > #15035

Fixes #15035

changelog: [unused_unit] don't lint on closure return types

[rustbot]

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[samueltardieu]

Could you submit the first two commits (with the proposed change) as a separate PR assigned to me? The third commit (removing -> ()) might need discussion, while the first two are non-controversial.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[samueltardieu]

Discussion on https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/.2315035/with/535865672, thanks for starting the thread there @ada4a.