Add new UnifiedInteger cop

[pocke]

Feature

This cop detects using Fixnum and Bignum class, e.g.

# test.rb
a.is_a? Fixnum
a.is_a? ::Bignum
a.is_a? MyNamespace::Bignum

$ rubocop test.rb
Inspecting 1 file
W

Offenses:

test.rb:2:9: W: Use Integer instead of Fixnum
a.is_a? Fixnum
        ^^^^^^
test.rb:3:9: W: Use Integer instead of Bignum
a.is_a? ::Bignum
        ^^^^^^^^

1 file inspected, 2 offenses detected

Target Problem

From Ruby 2.4, Fixnum and Bignum will be unified to Integer. https://bugs.ruby-lang.org/issues/12005

Fixnum and Bignum will become a reference of Integer from Ruby 2.4, e.g.

# ruby 2.4.0-preview2
Fixnum == Integer # => true
Bignum == Integer # => true

So, Using Fixnum or Bignum is confusing from Ruby 2.4.
And until Ruby 2.4, the cop is useful for migrating to Ruby 2.4.

See also #3491

---

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 are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[bbatsov]

Wrap the class names in backticks. The message should end with a ..

[bbatsov]

Ingeter -> Integer

[bbatsov]

Same here.

[bbatsov]

And here.

[bbatsov]

I think using shared examples would be better than generating the tests like this.

[bbatsov]

You'll also have to rebase this on top of the current master.

[bbatsov]

I wonder if the cop should be enabled for all Ruby versions or just for 2.4+.

[backus]

The semantics slightly change starting in 2.4 so it probably makes most sense to only enable for 2.4 https://blog.blockscore.com/new-features-in-ruby-2-4/#simplified-integers

[pocke]

I don't think so.

In many cases, using Fixnum is a mistake in Ruby 2.3 or older too. Then we can use Integer instead of Fixnum.
I've found the mistake in some projects.

e.g.

# This statement does nothing when x is a Bignum.
case x
when Fixnum # it's should be `Integer`
  do_something
when Float
  do_something2
end

So, I think this cop shoud be enabled for all Ruby versions. It is useful to detect the mistake, and migration to Ruby 2.4.

[pocke]

I've fixed the message, spec, and typo. and rebased.

[bbatsov]

👍