Do not generate local vars for partials without object or collection

Previously rendering a partial without giving :object or :collection
would generate a local variable with the partial name by default.

This was noticed due to warnings in Ruby 2.0 of not used variables,
which turned out to be the generation of not used variables inside
partials that do not contain objects related to them.

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Do not generate local variables for partials without object or collection.
+    Previously rendering a partial without giving `:object` or `:collection`
+    would generate a local variable with the partial name by default.
+
+    *Carlos Antonio da Silva*
+
 *   Return the last valid, non-private IP address from the X-Forwarded-For,
     Client-IP and Remote-Addr headers, in that order. Document the rationale
     for that decision, and describe the options that can be passed to the

actionpack/lib/action_view/renderer/partial_renderer.rb

@@ -452,7 +452,7 @@ def merge_prefix_into_object_path(prefix, object_path)
 
     def retrieve_template_keys
       keys = @locals.keys
-      keys << @variable
+      keys << @variable         if @object || @collection
       keys << @variable_counter if @collection
       keys
     end

actionpack/test/fixtures/test/_partial_name_local_variable.erb

+<%= partial_name_local_variable %>

actionpack/test/template/render_test.rb

@@ -318,6 +318,13 @@ def test_render_partial_using_collection
       @controller_view.render(customers, :greeting => "Hello")
   end
 
+  def test_render_partial_without_object_or_collection_does_not_generate_partial_name_local_variable
+    exception = assert_raises ActionView::Template::Error do
+      @controller_view.render("partial_name_local_variable")
+    end
+    assert_match "undefined local variable or method `partial_name_local_variable'", exception.message
+  end
+
   # TODO: The reason for this test is unclear, improve documentation
   def test_render_partial_and_fallback_to_layout
     assert_equal "Before (Josh)\n\nAfter", @view.render(:partial => "test/layout_for_partial", :locals => { :name => "Josh" })