Add let_underscore_drop

[smoelius]

This line generalizes let_underscore_lock (#5101) to warn about any initializer expression that implements Drop.

So, for example, the following would generate a warning:

struct Droppable;
impl Drop for Droppable {
    fn drop(&mut self) {}
}
let _ = Droppable;

I tried to preserve the original let_underscore_lock functionality in the sense that the warning generated for

let _ = mutex.lock();

should be unchanged.

Please keep the line below
changelog: Add lint [let_underscore_drop]

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @flip1995 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[flip1995]

Don't forget to update the reference files and run cargo dev update_lints after applying the review.

[smoelius]

@flip1995 Thanks for the review.

Could you please sanity check that this was the right way to address the map_clone and map_flatten tests?
06e81bb#diff-ef362d1c15b3b4ea77af80401c4b546728b49f799e29ff11f722c4690515ea8b
06e81bb#diff-154c4677379d20cf922a34a0a42cbce62c1a36d8852b28e7d5ada6aaa980cd03

[flip1995]

Could you please sanity check that this was the right way to address the map_clone and map_flatten tests?

LGTM

[flip1995]

Please also allow the lint in tests/ui/filter_methods.rs. Or better, swap out

rust-clippy/tests/ui/filter_methods.rs

Lines 1 to 2 in 06e81bb

	#![warn(clippy::all, clippy::pedantic)]
	#![allow(clippy::missing_docs_in_private_items)]

With just

#![warn(clippy::filter_map)]

[smoelius]

Or better, ...

As an outsider, a change like that looks to me like it would go against an established convention. Maybe it would be better left for a separate PR?

[flip1995]

It is an ancient convention to warn on all lints in test files. Now one test file should only test one lint. But we don't care enough about this to fix that.

So both's fine.

[flip1995]

@bors r+

Thanks!

[bors]

📌 Commit 4852cca has been approved by flip1995

[bors]

⌛ Testing commit 4852cca with merge dd826b4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing dd826b4 to master...