[FIX #4563] Style/TrailingUnderscoreVariable cop leaves an unclosed parentheses

[smakagon]

This fixes autocorrect issue with Style/TrailingUnderscoreVariable.
Without this fix cop would autocorrect this code:

(foo, _) = [1, 2, 3]

To this:

(foo, = [1, 2, 3]

With the fix it autocorrects this code:

(foo, _) = [1, 2, 3]

To the following:

(foo,) = [1, 2, 3]

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[Drenmi]

I would change the description of this test case and the next to mention that it removes the entire assignment, not just the parentheses. 🙂

[smakagon]

Yes, makes sense, I'll change that.

[Drenmi]

Method name should be pluralized as #unused_variables_only?? 🙂

[smakagon]

Thanks, will rename.

[Drenmi]

Changes look good! I was a bit surprised by this cop's reliance on ranges, though, instead of working directly with the child nodes of (mlhs). WDYT @smakagon?

[smakagon]

@Drenmi I was surprised too. This cop looked pretty simple to me, but when I started to look into implementation I figured out that it's not that easy.

I think it was implemented basing on ranges because we have to deal with commas and spaces (and parentheses now), but in general I feel like there is a room for improvement.
Code I added is not the best one I ever wrote too, though.

I was focused mostly on the fix for autocorrect. Do you think we should refactor it?

[Drenmi]

@smakagon: I think the PR is great. Short and focused. I'd say no need to refactor. Was just curious why the original implementer used this approach. 🙂

[bbatsov]

It might be nice to still refactor this cop down the road.