Implement helpers proxy in controller instance level

It is a common pattern in the Rails community that when people want to
:xa
use any kind of helper that is defined inside app/helpers they includes
the helper module inside the controller like:

    module UserHelper
      def my_user_helper
        # ...
      end
    end

    class UsersController < ApplicationController
      include UserHelper

      def index
        render inline: my_user_helper
      end
    end

This has problem because the helper can't access anything that is
defined in the view level context class.

Also all public methods of the helper become available in the controller
what can lead to undesirable methods being routed and behaving as
actions.

Also if you helper depends on other helpers or even Action View helpers
you need to include each one of these dependencies in your controller
otherwise your helper is not going to work.

We already have a helpers proxy at controller class level but that proxy
doesn't have access to the instance variables defined in the
controller.

With this new instance level helper proxy users can reuse helpers in the
controller without having to include the modules and with access to
instance variables defined in the controller.

    class UsersController < ApplicationController
      def index
        render inline: helpers.my_user_helper
      end
    end

actionpack/CHANGELOG.md

+*   Add `ActionController#helpers` to get access to the view context in the controller
+    level.
+
+    *Rafael Mendonça França*
+
+
 ## Rails 5.0.0.beta4 (April 27, 2016) ##
 
 *   Routing: Refactor `:action` default handling to ensure that path

actionpack/lib/action_controller/base.rb

@@ -253,7 +253,7 @@ def self.without_modules(*modules)
     # Define some internal variables that should not be propagated to the view.
     PROTECTED_IVARS = AbstractController::Rendering::DEFAULT_PROTECTED_INSTANCE_VARIABLES + %i(
       @_params @_response @_request @_config @_url_options @_action_has_layout @_view_context_class
-      @_view_renderer @_lookup_context @_routes @_view_runtime @_db_runtime
+      @_view_renderer @_lookup_context @_routes @_view_runtime @_db_runtime @_helper_proxy
     )
 
     def _protected_ivars # :nodoc:

actionpack/lib/action_controller/metal/helpers.rb

@@ -5,7 +5,7 @@ module ActionController
   #
   # In addition to using the standard template helpers provided, creating custom helpers to
   # extract complicated logic or reusable functionality is strongly encouraged. By default, each controller
-  # will include all helpers. These helpers are only accessible on the controller through <tt>.helpers</tt>
+  # will include all helpers. These helpers are only accessible on the controller through <tt>#helpers</tt>
   #
   # In previous versions of \Rails the controller will include a helper which
   # matches the name of the controller, e.g., <tt>MyController</tt> will automatically
@@ -113,5 +113,10 @@ def all_application_helpers
         all_helpers_from_path(helpers_path)
       end
     end
+
+    # Provides a proxy to access helpers methods from outside the view.
+    def helpers
+      @_helper_proxy ||= view_context
+    end
   end
 end

actionpack/test/controller/helper_test.rb

@@ -207,6 +207,22 @@ def test_helper_proxy
     assert methods.include?(:foobar)
   end
 
+  def test_helper_proxy_in_instance
+    methods = AllHelpersController.new.helpers.methods
+
+    # Action View
+    assert_includes methods, :pluralize
+
+    # abc_helper.rb
+    assert_includes methods, :bare_a
+
+    # fun/games_helper.rb
+    assert_includes methods, :stratego
+
+    # fun/pdf_helper.rb
+    assert_includes methods, :foobar
+  end
+
   def test_helper_proxy_config
     AllHelpersController.config.my_var = 'smth'