Add a rake task for generation a new cop

Rakefile

@@ -16,6 +16,8 @@ require 'rspec/core'
 require 'rspec/core/rake_task'
 require 'rubocop/rake_task'
 
+Dir['tasks/**/*.rake'].each { |t| load t }
+
 RSpec::Core::RakeTask.new(:spec)
 
 RSpec::Core::RakeTask.new(:ascii_spec) do |t|

manual/contributing.md

@@ -16,6 +16,8 @@ Before submitting a patch or a pull request make sure all tests are
 passing and that your patch is in line with the [contribution
 guidelines](https://github.com/bbatsov/rubocop/blob/master/.github/CONTRIBUTING.md).
 
+See also. [development.md](development.md)
+
 ## Documentation
 
 Good documentation is just as important as good code.

manual/development.md

@@ -0,0 +1,17 @@
+## Add a new cop
+
+Use a rake task to generate a cop template.
+
+```sh
+$ bundle exec rake new_cop[Category/Name]
+created
+- lib/rubocop/cop/category/name.rb
+- spec/rubocop/cop/category/name_spec.rb
+
+Do 4 steps
+- Add an entry to `New feature` section in CHANGELOG.md
+  - e.g. Add new `Category/Name` cop. ([@your_id][])
+- Add `require 'lib/rubocop/cop/category/name'` into lib/rubocop.rb
+- Add an entry into config/enabled.yml or config/disabled.yml
+- Implement a new cop to the generated file!
+```

mkdocs.yml

@@ -15,6 +15,7 @@ pages:
 - Integration with other tools: integration_with_other_tools.md
 - Support: support.md
 - Contributing: contributing.md
+- Development: development.md
 extra_css:
   - css/extra.css
 theme: readthedocs

tasks/new_cop.rake

@@ -0,0 +1,110 @@
+# encoding: utf-8
+# frozen_string_literal: true
+
+desc 'Generate a new cop template'
+task :new_cop, [:cop] do |_task, args|
+  def to_snake(str)
+    str
+      .gsub(/([A-Z]+)([A-Z][a-z])/, '\1_\2')
+      .gsub(/([a-z\d])([A-Z])/, '\1_\2')
+      .tr('-', '_')
+      .downcase
+  end
+
+  cop_name = args[:cop] # Category/Name
+  raise ArgumentError, 'One argument is required' unless cop_name
+  category, name = *cop_name.split('/', 2)
+  unless category && name
+    raise ArgumentError, 'Specify a cop name with Category/Name style'
+  end
+
+  cop_code = <<-END
+# encoding: utf-8
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module #{category}
+      # TODO: Write cop description and example of bad / good code.
+      #
+      # @example
+      #   # bad
+      #   bad_method()
+      #
+      #   # bad
+      #   bad_method(args)
+      #
+      #   # good
+      #   good_method()
+      #
+      #   # good
+      #   good_method(args)
+      class #{name} < Cop
+        # TODO: Implement the cop into here.
+        #
+        # In many cases, you can use a node matcher for matching node pattern.
+        # See. https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/node_pattern.rb
+        #
+        # For example
+        MSG = 'Message of #{cop_name}'.freeze
+
+        def_node_matcher :bad_method?, <<-PATTERN
+          (:send nil :bad_method ...)
+        PATTERN
+
+        def on_send(node)
+          return unless bad_method?(node)
+          add_offense(node, :expression)
+        end
+      end
+    end
+  end
+end
+  END
+
+  cop_path = "lib/rubocop/cop/#{to_snake(cop_name)}.rb"
+  File.write(cop_path, cop_code)
+
+  spec_code = <<-END
+# encoding: utf-8
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe RuboCop::Cop::#{category}::#{name} do
+  let(:config) { RuboCop::Config.new }
+  subject(:cop) { described_class.new(config) }
+
+  # TODO: Write test code
+  #
+  # For example
+  it 'registers an offense for offending code' do
+    inspect_source(cop, 'bad_method')
+    expect(cop.offenses.size).to eq(1)
+    expect(cop.messages)
+      .to eq(['Message of #{cop_name}'])
+  end
+
+  it 'accepts' do
+    inspect_source(cop, 'good_method')
+    expect(cop.offenses).to be_empty
+  end
+end
+  END
+
+  spec_path = "spec/rubocop/cop/#{to_snake(cop_name)}_spec.rb"
+  File.write(spec_path, spec_code)
+
+  puts <<-END
+created
+- #{cop_path}
+- #{spec_path}
+
+Do 4 steps
+- Add an entry to `New feature` section in CHANGELOG.md
+  - e.g. Add new `#{cop_name}` cop. ([@your_id][])
+- Add `require '#{cop_path.gsub(/\.rb$/, '')}'` into lib/rubocop.rb
+- Add an entry into config/enabled.yml or config/disabled.yml
+- Implement a new cop to the generated file!
+  END
+end