New cop: RUBY_VERSION in gemspec

[pocke]

Is your feature request related to a problem? Please describe.

Gemspec should not contain RUBY_VERSION as a condition to switch dependencies.
For example:

Gem::Specification.new do |s|
  if RUBY_VERSION >= '2.5'
    s.add_runtime_dependency 'gem_a'
  else
    s.add_runtime_dependency 'gem_b'
  end
end

It has unexpected behavior. The RUBY_VERSION is determined by rake release.
If I use Ruby 2.5 to execute rake release, the gem always has gem_a as a dependency even if a user installs the gem with Ruby 2.4 or older.

We have several ways to fix the problem.

• Remove the dependencies and use post_install_message to notice the gems.
• Add both gems to the dependency.
• If the dependencies are for development, move them to Gemfile.

Describe the solution you'd like

Detect reference of RUBY_VERSION in *.gemspec and add an offense to it.
For example: def_node_matcher :ruby_version?, '(const nil :RUBY_VERSION)'

Describe alternatives you've considered

The solution is loose. It checks RUBY_VERSION in anywhere of gemspec.
We can implement stricter detection. For example, detect RUBY_VERSION only if it is placed a conditon.

But I think the looser version is better. Because I guess the cop will have many false negative if we choose the stricter version. And I guess the looser version does not have too many false positives.

Additional context

Today I found using RUBY_VERSION in a gemspec.
rafaelsales/ulid#17
So I think the cop is meaningful.

[bbatsov]

You've got my blessing! :-)

I was also thinking we should add a section on Gemfile/gemspec to https://rubystyle.guide/. Our cops have a lot of good advice that's not part of the guide yet.

[pocke]

I was also thinking we should add a section on Gemfile/gemspec to https://rubystyle.guide/. Our cops have a lot of good advice that's not part of the guide yet.

Definitely! It will be helpful for beginners of gem developer.