[Fix #3936] Add new Style/MixinGrouping cop

This cop checks for grouping of `mixins` in class and `module` bodies.
By default, it enforces a separated style, i.e.:

```
class Foo
  include Bar
  include Qux
end
```

but it can be configured to enforce a grouped style as well.

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#3893](https://github.com/bbatsov/rubocop/issues/3893): Add a new configuration, `IncludeActiveSupportAliases`, to `Performance/DoublStartEndWith`. This configuration will check for ActiveSupport's `starts_with?` and `ends_with?`. ([@rrosenblum][])
 * [#3889](https://github.com/bbatsov/rubocop/pull/3889): Add new `Style/EmptyLineAfterMagicComment` cop. ([@backus][])
 * [#3800](https://github.com/bbatsov/rubocop/issues/3800): Make `Style/EndOfLine` configurable with `lf`, `crlf`, and `native` (default) styles. ([@jonas054][])
+* [#3936](https://github.com/bbatsov/rubocop/issues/3936): Add new `Style/MixinGrouping` cop. ([@drenmi][])
 
 ### Changes
 

config/default.yml

@@ -728,6 +728,16 @@ Style/MethodName:
     - snake_case
     - camelCase
 
+# Checks the grouping of mixins (`include`, `extend`, `prepend`) in `class` and
+# `module` bodies.
+Style/MixinGrouping:
+  EnforcedStyle: separated
+  SupportedStyles:
+    # separated: each mixed in module goes in a separate statement.
+    # grouped: mixed in modules are grouped into a single statement.
+    - separated
+    - grouped
+
 Style/ModuleFunction:
   EnforcedStyle: module_function
   SupportedStyles:

config/enabled.yml

@@ -426,6 +426,11 @@ Style/MethodMissing:
   StyleGuide: '#no-method-missing'
   Enabled: true
 
+Style/MixinGrouping:
+  Description: 'Checks for grouping of mixins in `class` and `module` bodies.'
+  StyleGuide: '#mixin-grouping'
+  Enabled: true
+
 Style/ModuleFunction:
   Description: 'Checks for usage of `extend self` in modules.'
   StyleGuide: '#module-function'

lib/rubocop.rb

@@ -311,6 +311,7 @@
 require 'rubocop/cop/style/method_name'
 require 'rubocop/cop/style/method_missing'
 require 'rubocop/cop/style/missing_else'
+require 'rubocop/cop/style/mixin_grouping'
 require 'rubocop/cop/style/module_function'
 require 'rubocop/cop/style/multiline_array_brace_layout'
 require 'rubocop/cop/style/multiline_assignment_layout'

lib/rubocop/cop/style/mixin_grouping.rb

@@ -0,0 +1,101 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for grouping of mixins in `class` and `module` bodies.
+      # By default it enforces mixins to be placed in separate declarations,
+      # but it can be configured to enforce grouping them in one declaration.
+      #
+      # @example
+      #
+      #   EnforcedStyle: separated (default)
+      #
+      #   @bad
+      #   class Foo
+      #     include Bar, Qox
+      #   end
+      #
+      #   @good
+      #   class Foo
+      #     include Bar
+      #     include Qox
+      #   end
+      #
+      #   EnforcedStyle: grouped
+      #
+      #   @bad
+      #   class Foo
+      #     extend Bar
+      #     extend Qox
+      #   end
+      #
+      #   @good
+      #   class Foo
+      #     extend Bar, Qox
+      #   end
+      class MixinGrouping < Cop
+        include ConfigurableEnforcedStyle
+
+        MIXIN_METHODS = [:extend, :include, :prepend].freeze
+        MSG = 'Put `%s` mixins in %s.'.freeze
+
+        def on_send(node)
+          _reciever, method_name, *_args = *node
+
+          return unless MIXIN_METHODS.include?(method_name)
+
+          check(node)
+        end
+
+        private
+
+        def check(send_node)
+          if separated_style?
+            check_separated_style(send_node)
+          else
+            check_grouped_style(send_node)
+          end
+        end
+
+        def check_grouped_style(send_node)
+          return unless sibling_mixins?(send_node)
+
+          add_offense(send_node, :expression)
+        end
+
+        def check_separated_style(send_node)
+          _reciever, _method_name, *args = *send_node
+
+          return if args.one?
+
+          add_offense(send_node, :expression)
+        end
+
+        def sibling_mixins?(send_node)
+          siblings = send_node.parent.each_child_node(:send)
+                              .reject { |sibling| sibling == send_node }
+
+          siblings.any? do |sibling_node|
+            sibling_node.method_name == send_node.method_name
+          end
+        end
+
+        def message(send_node)
+          suffix =
+            separated_style? ? 'separate statements' : 'a single statement'
+
+          format(MSG, send_node.method_name, suffix)
+        end
+
+        def grouped_style?
+          style == :grouped
+        end
+
+        def separated_style?
+          style == :separated
+        end
+      end
+    end
+  end
+end

lib/rubocop/formatter/html_formatter.rb

@@ -61,7 +61,8 @@ def render_html
 
       # This class provides helper methods used in the ERB template.
       class ERBContext
-        include PathUtil, TextUtil
+        include PathUtil
+        include TextUtil
 
         SEVERITY_COLORS = {
           refactor:   Color.new(0xED, 0x9C, 0x28, 1.0),

lib/rubocop/formatter/simple_text_formatter.rb

@@ -9,7 +9,8 @@ module Formatter
     # Offenses are displayed at compact form - just the
     # location of the problem and the associated message.
     class SimpleTextFormatter < BaseFormatter
-      include Colorizable, PathUtil
+      include Colorizable
+      include PathUtil
 
       COLOR_FOR_SEVERITY = {
         refactor:   :yellow,
@@ -90,7 +91,8 @@ def message(offense)
 
       # A helper class for building the report summary text.
       class Report
-        include Colorizable, TextUtil
+        include Colorizable
+        include TextUtil
 
         def initialize(file_count, offense_count, correction_count, rainbow)
           @file_count = file_count

manual/cops.md

@@ -308,6 +308,7 @@ In the following section you find all available cops:
 * [Style/MethodMissing](cops_style.md#stylemethodmissing)
 * [Style/MethodName](cops_style.md#stylemethodname)
 * [Style/MissingElse](cops_style.md#stylemissingelse)
+* [Style/MixinGrouping](cops_style.md#stylemixingrouping)
 * [Style/ModuleFunction](cops_style.md#stylemodulefunction)
 * [Style/MultilineArrayBraceLayout](cops_style.md#stylemultilinearraybracelayout)
 * [Style/MultilineAssignmentLayout](cops_style.md#stylemultilineassignmentlayout)

manual/cops_style.md

@@ -2765,6 +2765,54 @@ EnforcedStyle | both
 SupportedStyles | if, case, both
 
 
+## Style/MixinGrouping
+
+Enabled by default | Supports autocorrection
+--- | ---
+Enabled | No
+
+This cop checks for grouping of mixins in `class` and `module` bodies.
+By default it enforces mixins to be placed in separate declarations,
+but it can be configured to enforce grouping them in one declaration.
+
+### Example
+
+```ruby
+EnforcedStyle: separated (default)
+
+# bad
+class Foo
+  include Bar, Qox
+end
+
+# good
+class Foo
+  include Bar
+  include Qox
+end
+
+EnforcedStyle: grouped
+
+# bad
+class Foo
+  extend Bar
+  extend Qox
+end
+
+# good
+class Foo
+  extend Bar, Qox
+end
+```
+
+### Important attributes
+
+Attribute | Value
+--- | ---
+EnforcedStyle | separated
+SupportedStyles | separated, grouped
+
+
 ## Style/ModuleFunction
 
 Enabled by default | Supports autocorrection