Add new Layout/EmptyLineAroundAttributeAccessor cop

[koic]

This PR adds new Layout/EmptyLineAroundAttributeAccessor cop that checks for a newline after attribute accessor.

# bad
attr_accessor :foo
def do_something
end

# good
attr_accessor :foo

def do_something
end

# good
attr_accessor :foo
attr_reader :bar
attr_writer :baz
attr :qux

def do_something
end

This PR first checks an empty line after an attribute accessor to implement the example shown in The Ruby Style Guide's rule.
rubocop/ruby-style-guide#817

In the future this cop will provide an EnforcedStyle: around that checks empty lines before and after an attribute accessor, and this EnforcedStyle: only_after option could be users selectable option.

---

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 to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• 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.

[bbatsov]

Looks good, but you should also add some coverage of attr - the ancient macro that probably nobody uses, but still exists. :-)

[koic]

A legacy method attr was not included in the documentation. However, this cop's actual behavior also detects attr, so I updated the documentation to mention attr as well.

[bbatsov]

I guess we're in business then! :-) Thanks!

[aried3r]

Hey! I just updated to rubocop 0.83 and tried this new cop. I get the intention behind it when reading the examples, but how does the style guide suggest handling situations like these:

attr_accessor :validate_vat
alias validate_vat? validate_vat

# potentially more code

validates :vat_number, presence: true, if: :validate_vat?

# more code 

def do_something
  do_the_thing if validate_vat?
end

Personally I like to keep aliasing to add ? close to attr_accessor instead of separating it.

I know I can just disable this cop, but I was wondering if that's a common pattern the cop should be able to handle or if that's very specific to our code base at work.

[marcandre]

I agree with @aried3r ; ideally the cop should be able to ignore lines like the alias, as well as any send call with an argument matching any of the argument of attr_accessor (e.g. alias_method :anything, :validate_vat, protected :validate_vat, etc.)

[koic]

I see. Perhaps customizable allow list such as IgnoredMethods option can be created. I think it can have some allowed method names (e.g. alias) by default.