Fix useless_attribute FP on exported_private_dependencies lint attributes

[kpreid]

The exported_private_dependencies lint can fire on any mention of an item from a private dependency, including mentions inside of use. Therefore, useless_attribute should not fire on lint attributes mentioning exported_private_dependencies attached to a use.

changelog: [useless_attribute]: fix false positive on exported_private_dependencies lint attributes

PR #15645 was used as an example for preparing this PR.

[rustbot]

r? @samueltardieu

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[samueltardieu]

If you don't link to an issue, please describe what the problem is.

[samueltardieu]

What bothers me a bit here is that this line wouldn't raise exported_private_dependencies (an expect instead of allow would fail), so the attribute would be useless here. Could you come up with an example where this attribute is useful and should not be warned about by Clippy?

[kpreid]

This will require modifying the UI test suite so that at least one of its --extern dependencies is marked priv, which is an unstable feature. Is that acceptable? If so, how should that be done? I took a look at compile-test.rs but I got lost and could not determine where the cargo invocation that pulls in the contents of clippy_test_deps actually occurs so that I could add -Zpublic-dependency to that cargo invocation (as opposed to the clippy-driver invocation).

[samueltardieu]

Indeed, this is overkill. No need to change. Thanks for looking it up!

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[kpreid]

If you don't link to an issue, please describe what the problem is.

I have updated the PR description.