Add new Rails/RootPathnameMethods cop

[leoarnold]

Rails.root is an instance of Pathname. So instead of

File.open(Rails.root.join('db', 'schema.rb'))
File.open(Rails.root.join('db', 'schema.rb'), 'w')
File.read(Rails.root.join('db', 'schema.rb'))
File.binread(Rails.root.join('db', 'schema.rb'))
File.write(Rails.root.join('db', 'schema.rb'), content)
File.binwrite(Rails.root.join('db', 'schema.rb'), content)

we can simply write

Rails.root.join('db', 'schema.rb').open
Rails.root.join('db', 'schema.rb').open('w')
Rails.root.join('db', 'schema.rb').read
Rails.root.join('db', 'schema.rb').binread
Rails.root.join('db', 'schema.rb').write(content)
Rails.root.join('db', 'schema.rb').binwrite(content)

This cop works best when used together with
Style/FileRead, Style/FileWrite, and Rails/RootJoinChain.

---

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).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• 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.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

Nice, thank you!
A few cosmetic notes. Wondering if those different node patterns can be collapsed into one.

[koic]

@leoarnold ping :-)

[leoarnold]

@koic Thanks for the ping. I resolved all comments.

Yet there is one very peculiar problem: codespell wants file test instead of FileTest (literal name of Ruby class from standard library)

• putting it in the ignore file codespell.txt won't help
• the problem has already been reported to codespell project with precisely that word:
  Do not try to propose a fix when there is an uppercase in the middle of a word codespell-project/codespell#314 (comment)
• the team suggest this should be solved using multi-dictionary support:
  Words containing minus signs break variable names codespell-project/codespell#1224 (comment)
• Multi-dictionary support is not yet implemented:
  https://github.com/codespell-project/codespell/projects/1

[pirj]

File.methods - File.superclass.methods also returns e.g. :absolute_path which is not on those lists.
Which makes me think does it make sense to fill in those constants with those calls?
On the other hand, if the Ruby version with which rubocop is run is different from the target Ruby version, there might be differences.
However, does it matter much?

[leoarnold]

... but Pathname does not have a #absolute_path method. Not sure I can follow here.

I generated this list using

File.methods.sort.select do |m|
  Pathname.instance_methods.include?(m) && !Object.instance_methods.include?(m)
end

[pirj]

Understood.
So basically (File.public_methods(false) & Pathname.instance_methods(false)).sort.
Does it make sense to set the constant to this statement?
What are the consequences? New methods being added on File and to Pathname in future Ruby versions change the behaviour of the cop?

[leoarnold]

Well, just because there is File.foo and Pathname#foo, it is not guaranteed that File.foo(p, ...) and p.foo(...) is the same, so I'd rather maintain a list of safe methods.

Also, there may be cases where the Ruby that runs Rubocop is not the Ruby which runs production code (e.g. you develop a gem on Ruby 2, but someone uses it on JRuby 9). Are those standard libraries necessarily equivalent?

[pirj]

It's my personal preference, but I don't feel too confident when some collections from the code are used in tests directly. It may sound far-fetched, but what if someone accidentally removes a line from that array? Specs won't fail.
I'll leave this to @koic to decide on.

[koic]

It covers possible use cases and is maintainable. It will take a while to ship, so we can adjust it later if necessary. Thank you.