Introduce Module#concerning

A natural, low-ceremony way to separate responsibilities within a class.

Imported from https://github.com/37signals/concerning#readme

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diff --git a/activesupport/lib/active_support/core_ext/module/concerning.rb b/activesupport/lib/active_support/core_ext/module/concerning.rb
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+require 'active_support/concern'
+
+class Module
+  # = Bite-sized separation of concerns
+  #
+  # We often find ourselves with a medium-sized chunk of behavior that we'd
+  # like to extract, but only mix in to a single class.
+  #
+  # Extracting a plain old Ruby object to encapsulate it and collaborate or
+  # delegate to the original object is often a good choice, but when there's
+  # no additional state to encapsulate or we're making DSL-style declarations
+  # about the parent class, introducing new collaborators can obfuscate rather
+  # than simplify.
+  #
+  # The typical route is to just dump everything in a monolithic class, perhaps
+  # with a comment, as a least-bad alternative. Using modules in separate files
+  # means tedious sifting to get a big-picture view.
+  #
+  # = Dissatisfying ways to separate small concerns
+  #
+  # == Using comments:
+  #
+  #   class Todo
+  #     # Other todo implementation
+  #     # ...
+  #
+  #     ## Event tracking
+  #     has_many :events
+  #
+  #     before_create :track_creation
+  #     after_destroy :track_deletion
+  #
+  #     private
+  #       def track_creation
+  #         # ...
+  #       end
+  #   end
+  #
+  # == With an inline module:
+  #
+  # Noisy syntax.
+  #
+  #   class Todo
+  #     # Other todo implementation
+  #     # ...
+  #
+  #     module EventTracking
+  #       extend ActiveSupport::Concern
+  #
+  #       included do
+  #         has_many :events
+  #         before_create :track_creation
+  #         after_destroy :track_deletion
+  #       end
+  #
+  #       private
+  #         def track_creation
+  #           # ...
+  #         end
+  #     end
+  #     include EventTracking
+  #   end
+  #
+  # == Mix-in noise exiled to its own file:
+  #
+  # Once our chunk of behavior starts pushing the scroll-to-understand it
+  # boundary, we give in and move it to a separate file. At this size, the
+  # overhead feels in good proportion to the size of our extraction, despite
+  # diluting our at-a-glance sense of how things really work.
+  #
+  #   class Todo
+  #     # Other todo implementation
+  #     # ...
+  #
+  #     include TodoEventTracking
+  #   end
+  #
+  # = Introducing Module#concerning
+  #
+  # By quieting the mix-in noise, we arrive at a natural, low-ceremony way to
+  # separate bite-sized concerns.
+  #
+  #   class Todo
+  #     # Other todo implementation
+  #     # ...
+  #
+  #     concerning :EventTracking do
+  #       included do
+  #         has_many :events
+  #         before_create :track_creation
+  #         after_destroy :track_deletion
+  #       end
+  #
+  #       private
+  #         def track_creation
+  #           # ...
+  #         end
+  #     end
+  #   end
+  #
+  #   Todo.ancestors
+  #   # => Todo, Todo::EventTracking, Object
+  #
+  # This small step has some wonderful ripple effects. We can
+  # * grok the behavior of our class in one glance,
+  # * clean up monolithic junk-drawer classes by separating their concerns, and
+  # * stop leaning on protected/private for crude "this is internal stuff" modularity.
+  module Concerning
+    # Define a new concern and mix it in.
+    def concerning(topic, &block)
+      include concern(topic, &block)
+    end
+
+    # A low-cruft shortcut to define a concern.
+    #
+    #   concern :EventTracking do
+    #     ...
+    #   end
+    #
+    # is equivalent to
+    #
+    #   module EventTracking
+    #     extend ActiveSupport::Concern
+    #
+    #     ...
+    #   end
+    #   include EventTracking
+    def concern(topic, &module_definition)
+      const_set topic, Module.new {
+        extend ::ActiveSupport::Concern
+        module_eval(&module_definition)
+      }
+    end
+  end
+  include Concerning
+end