Deprecated implicit local assignments when rendering partials

actionpack/CHANGELOG

@@ -1,5 +1,7 @@
 *Edge*
 
+* Deprecated implicit local assignments when rendering partials [Josh Peek]
+
 * Introduce current_cycle helper method to return the current value without bumping the cycle.  #417 [Ken Collins]
 
 * Allow polymorphic_url helper to take url options. #880 [Tarmo Tänav]

actionpack/lib/action_view/renderable_partial.rb

@@ -27,8 +27,14 @@ module ActionView
 
     def render_partial(view, object = nil, local_assigns = {}, as = nil)
       object ||= local_assigns[:object] ||
-        local_assigns[variable_name] ||
-        view.controller.instance_variable_get("@#{variable_name}") if view.respond_to?(:controller)
+        local_assigns[variable_name]
+
+      if view.respond_to?(:controller)
+        object ||= ActiveSupport::Deprecation::DeprecatedObjectProxy.new(
+          view.controller.instance_variable_get("@#{variable_name}"),
+          "@#{variable_name} will no longer be implicitly assigned to #{variable_name}"
+        )
+      end
 
       # Ensure correct object is reassigned to other accessors
       local_assigns[:object] = local_assigns[variable_name] = object

actionpack/test/controller/new_render_test.rb

@@ -832,8 +832,10 @@ EOS
   end
 
   def test_partial_with_implicit_local_assignment
-    get :partial_with_implicit_local_assignment
-    assert_equal "Hello: Marcel", @response.body
+    assert_deprecated do
+      get :partial_with_implicit_local_assignment
+      assert_equal "Hello: Marcel", @response.body
+    end
   end
 
   def test_render_missing_partial_template

activesupport/lib/active_support/deprecation.rb

@@ -162,6 +162,22 @@ module ActiveSupport
         end
     end
 
+    class DeprecatedObjectProxy < DeprecationProxy
+      def initialize(object, message)
+        @object = object
+        @message = message
+      end
+
+      private
+        def target
+          @object
+        end
+
+        def warn(callstack, called, args)
+          ActiveSupport::Deprecation.warn(@message, callstack)
+        end
+    end
+
     # Stand-in for <tt>@request</tt>, <tt>@attributes</tt>, <tt>@params</tt>, etc.
     # which emits deprecation warnings on any method call (except +inspect+).
     class DeprecatedInstanceVariableProxy < DeprecationProxy #:nodoc: