[Fix rubocop#2968] Added DocumentationMethod Cop.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## master (unreleased)
 
+* [#2968](https://github.com/bbatsov/rubocop/issues/2968): Add new `Style/DocumentationMethod` cop. ([@sooyang][])
+
 ### Changes
 
 * [#3341](https://github.com/bbatsov/rubocop/issues/3341): Exclude RSpec tests from inspection by `Style/NumericPredicate` cop. ([@drenmi][])
@@ -2299,3 +2301,4 @@
 [@haziqhafizuddin]: https://github.com/haziqhafizuddin
 [@dvandersluis]: https://github.com/dvandersluis
 [@QuinnHarris]: https://github.com/QuinnHarris
+[@sooyang]: https://github.com/sooyang

config/disabled.yml

@@ -23,6 +23,13 @@ Style/Copyright:
   Description: 'Include a copyright notice in each file before any code.'
   Enabled: false
 
+Style/DocumentationMethod:
+  Description: 'Public methods.'
+  Enabled: false
+  Exclude:
+    - 'spec/**/*'
+    - 'test/**/*'
+
 Style/Encoding:
   Description: 'Use UTF-8 as the source file encoding.'
   StyleGuide: 'https://github.com/bbatsov/ruby-style-guide#utf-8'

lib/rubocop.rb

@@ -212,6 +212,7 @@
 require 'rubocop/cop/style/copyright'
 require 'rubocop/cop/style/def_with_parentheses'
 require 'rubocop/cop/style/preferred_hash_methods'
+require 'rubocop/cop/style/documentation_method'
 require 'rubocop/cop/style/documentation'
 require 'rubocop/cop/style/dot_position'
 require 'rubocop/cop/style/double_negation'

lib/rubocop/cop/style/documentation_method.rb

@@ -0,0 +1,173 @@
+# encoding: utf-8
+# frozen_string_literal: true
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for missing documentation comment for public method.
+      #
+      # @example
+      #   # declaring methods outside of a class
+      #
+      #   # bad
+      #
+      #   def method
+      #     puts "method"
+      #   end
+      #
+      #   class MyClass
+      #   end
+      #
+      #   # good
+      #
+      #   # Method Comment
+      #   def method
+      #     puts "method"
+      #   end
+      #
+      #   class MyClass
+      #   end
+      #
+      # @example
+      #   # declaring methods inside a class
+      #
+      #   # bad
+      #
+      #   class MyClass
+      #     def method
+      #       puts 'method'
+      #     end
+      #   end
+      #
+      #   # good
+      #
+      #   class MyClass
+      #     # Method Comment
+      #     def method
+      #       puts 'method'
+      #     end
+      #   end
+      #
+      # @example
+      #   # declaring methods inside a module
+      #
+      #   # bad
+      #
+      #   module MyModule
+      #     def method
+      #       puts 'method'
+      #     end
+      #   end
+      #
+      #   #good
+      #
+      #   module MyModule
+      #     # Method Comment
+      #     def method
+      #       puts 'method'
+      #     end
+      #   end
+      #
+      #   # singleton methods
+      #
+      #   # bad
+      #
+      #   class MyClass
+      #   end
+      #
+      #   my_class = MyClass.new
+      #
+      #   def my_class.method
+      #     puts 'method'
+      #   end
+      #
+      #   # good
+      #
+      #   class MyClass
+      #   end
+      #
+      #   my_class = MyClass.new
+      #
+      #   # Method Comment
+      #   def my_class.method
+      #     puts 'method'
+      #   end
+      #
+      class DocumentationMethod < Cop
+        include AnnotationComment
+        include OnMethodDef
+        MSG = 'Missing top-level %s documentation method comment.'.freeze
+        METHOD_TYPE = %w(private protected).freeze
+
+        def_node_matcher :constant_definition?, '{def casgn}'
+
+        def on_def(node)
+          check_offenses(node)
+        end
+
+        def on_method_def(node, _method_name, _args, _body)
+          check_offenses(node)
+        end
+
+        private
+
+        def check_offenses(node)
+          _name, _body = *node
+
+          line = node.loc.line
+          return if node.ancestors.first.to_a.include? method_type
+          return if (processed_source[0..line].map(&:strip) &
+            METHOD_TYPE).any?
+          ast_with_comments = processed_source.ast_with_comments
+          return if associated_comment?(node, ast_with_comments)
+          add_offense(node, :keyword, format(MSG, :module))
+        end
+
+        def namespace?(body_node)
+          return false unless body_node
+
+          case body_node.type
+          when :begin
+            body_node.children.all? { |node| constant_definition?(node) }
+          else
+            constant_definition?(body_node)
+          end
+        end
+
+        # Returns true if the node has a comment on the line above it that
+        # isn't an annotation.
+        def associated_comment?(node, ast_with_comments)
+          preceding_comments = preceding_comments(node, ast_with_comments)
+          return false if preceding_comments.empty?
+
+          distance = node.loc.keyword.line - preceding_comments.last.loc.line
+          return false if distance > 1
+          return false unless comment_line_only?(preceding_comments.last)
+
+          # As long as there's at least one comment line that isn't an
+          # annotation, it's OK.
+          preceding_comments.any? do |comment|
+            !annotation?(comment) && !interpreter_directive_comment?(comment)
+          end
+        end
+
+        def preceding_comments(node, ast_with_comments)
+          ast_with_comments[node].select { |c| c.loc.line < node.loc.line }
+        end
+
+        def comment_line_only?(comment)
+          source_buffer = comment.loc.expression.source_buffer
+          comment_line = source_buffer.source_line(comment.loc.line)
+          comment_line =~ /^\s*#/
+        end
+
+        def interpreter_directive_comment?(comment)
+          comment.text =~ /^#\s*(frozen_string_literal|encoding):/
+        end
+
+        def method_type
+          :private || :protected
+        end
+      end
+    end
+  end
+end