Add new lint: used_underscore_items

[WeiTheShinobi]

Closes #13260

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changelog: new [used_underscore_items] lint against using items with a single leading underscore

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[y21]

FWIW, pretty much almost all of the x86 platform intrinsics are prefixed with an underscore: https://doc.rust-lang.org/stable/core/arch/x86_64/index.html

Seems unfortunate if we start linting on any use of them... 🤔

[llogiq]

I think a good balance here would be to check only the use of intra-crate items (that is items from the current crate), as the user cannot control how others name their items.

[llogiq]

I'm not sure why we compare names instead of crate indices. Otherwise this looks good.

[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit d40e04a has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit d40e04a with merge 603d5a1...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 603d5a1 to master...