Allow routing concerns to accept a callable

This allows us to make alterations to the generated routes based on the
scope of the current mapper, and otherwise allows us to move larger
blocks of concerns out of the routes file, altogether.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1613,8 +1613,8 @@ module Concerns
         #   end
         #
         # Any routing helpers can be used inside a concern.
-        def concern(name, &block)
-          @concerns[name] = block
+        def concern(name, callable = nil, &block)
+          @concerns[name] = callable || block
         end
 
         # Use the named concerns
@@ -1631,7 +1631,7 @@ def concern(name, &block)
         def concerns(*names)
           names.flatten.each do |name|
             if concern = @concerns[name]
-              instance_eval(&concern)
+              concern.call(self)
             else
               raise ArgumentError, "No concern named #{name} was found!"
             end
@@ -1645,6 +1645,10 @@ def initialize(set) #:nodoc:
         @concerns = {}
       end
 
+      def current_scope
+        @scope
+      end
+
       include Base
       include HttpHelpers
       include Redirection

actionpack/test/abstract_unit.rb

@@ -358,6 +358,7 @@ def index() render :nothing => true end
 class ThreadsController  < ResourcesController; end
 class MessagesController < ResourcesController; end
 class CommentsController < ResourcesController; end
+class ReviewsController < ResourcesController; end
 class AuthorsController < ResourcesController; end
 class LogosController < ResourcesController; end
 

actionpack/test/dispatch/routing/concerns_test.rb

@@ -1,6 +1,16 @@
 require 'abstract_unit'
 
 class RoutingConcernsTest < ActionDispatch::IntegrationTest
+  class Reviewable
+    def self.call(mapper)
+      if mapper.current_scope[:controller] == 'posts'
+        mapper.resources :reviews
+      elsif mapper.current_scope[:controller] == 'videos'
+        mapper.resources :reviews, as: :video_reviews
+      end
+    end
+  end
+
   Routes = ActionDispatch::Routing::RouteSet.new.tap do |app|
     app.draw do
       concern :commentable do
@@ -11,8 +21,10 @@ class RoutingConcernsTest < ActionDispatch::IntegrationTest
         resources :images, only: :index
       end
 
-      resources :posts, concerns: [:commentable, :image_attachable] do
-        resource :video, concerns: :commentable
+      concern :reviewable, Reviewable
+
+      resources :posts, concerns: [:commentable, :image_attachable, :reviewable] do
+        resource :video, concerns: [:commentable, :reviewable]
       end
 
       resource :picture, concerns: :commentable do
@@ -63,6 +75,18 @@ def test_accessing_concern_from_resources_using_only_option
     assert_equal "404", @response.code
   end
 
+  def test_accessing_callable_concern_from_resources
+    get "/posts/1/reviews/1"
+    assert_equal "200", @response.code
+    assert_equal "/posts/1/reviews/1", post_review_path(post_id: 1, id: 1)
+  end
+
+  def test_callable_concern_can_adapt_to_mapper
+    get "/posts/1/video/reviews/1"
+    assert_equal "200", @response.code
+    assert_equal "/posts/1/video/reviews/1", post_video_video_review_path(post_id: 1, id: 1)
+  end
+
   def test_accessing_concern_from_a_scope
     get "/videos/comments"
     assert_equal "200", @response.code