Add Style/HashNewKeywordArguments cop

[r7kamura]

I have heard that future versions of Ruby are likely to deprecate the ability to pass default values with keyword arguments to Hash.new.

• https://bugs.ruby-lang.org/issues/19236

So I thought it would be a good idea to prepare a Cop that detects such usage and autocorrects it while it is still available. Since the signature is originally Hash.new(default_value = nil), there should be no problem deprecating the keyword arguments.

• https://ruby-doc.org/3.1.2/Hash.html#method-c-new

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[r7kamura]

I am a little concerned about whether Style is the correct department. Semantically, Lint seems to be more correct, but I chose Style for now because I think Lint department is the place to put the most urgent Cop.

[r7kamura]

Is there any other good way to inspect keyword arguments?

[ydah]

It seems that kwargs is rounded to hash in rubocop-ast.
https://github.com/rubocop/rubocop-ast/blob/57bc6792ed83dfb2890ed1ce1eebc2580fd83aa2/lib/rubocop/ast/builder.rb#L64

I don't know if this is better, but would it be better to define a KwargsNode to replace it?

[r7kamura]

Since the emit_kwargs option is not enabled in RuboCop's parser, I don't think the kwargs type is actually passed there, but changing this option might have too much impact.

• https://github.com/whitequark/parser/blob/8de8b7fa7af471a2159860d6a0a5b615eac9c83c/lib/parser/builders/default.rb#L131-L177

[koic]

Interesting. However, it is too early to judge the direction of this cop force yet. So, it should be decided after https://bugs.ruby-lang.org/issues/19236 is settled. Let's come back to this PR once https://bugs.ruby-lang.org/issues/19236 is closed.

[bbatsov]

Let's mark this PR as WIP in the mean time.