Configuration values should be on AV::Base.

actionpack/lib/action_view/helpers/form_helper.rb

@@ -528,7 +528,7 @@ def fields_for(record_or_name_or_array, *args, &block)
           object_name = ActionController::RecordIdentifier.singular_class_name(object)
         end
 
-        builder = options[:builder] || ActionView.default_form_builder
+        builder = options[:builder] || ActionView::Base.default_form_builder
         yield builder.new(object_name, object, self, options, block)
       end
 
@@ -1208,21 +1208,8 @@ def nested_child_index(name)
     end
   end
 
-  class << ActionView
-    attr_accessor :default_form_builder
+  class Base
+    cattr_accessor :default_form_builder
+    @@default_form_builder = ::ActionView::Helpers::FormBuilder
   end
-
-  self.default_form_builder = ::ActionView::Helpers::FormBuilder
-
-  # 2.3 compatibility
-  class << Base
-    def default_form_builder=(builder)
-      ActionView.default_form_builder = builder
-    end
-
-    def default_form_builder
-      ActionView.default_form_builder
-    end
-  end
-
 end

actionpack/test/template/form_helper_test.rb

@@ -1195,8 +1195,8 @@ def test_form_for_with_labelled_builder
   end
 
   def test_default_form_builder
-    old_default_form_builder, ActionView.default_form_builder =
-      ActionView.default_form_builder, LabelledFormBuilder
+    old_default_form_builder, ActionView::Base.default_form_builder =
+      ActionView::Base.default_form_builder, LabelledFormBuilder
 
     form_for(:post, @post) do |f|
       concat f.text_field(:title)
@@ -1213,7 +1213,7 @@ def test_default_form_builder
 
     assert_dom_equal expected, output_buffer
   ensure
-    ActionView.default_form_builder = old_default_form_builder
+    ActionView::Base.default_form_builder = old_default_form_builder
   end
 
   def test_default_form_builder_with_active_record_helpers