descriptive assert_template error when partial wasn't rendered

When `assert_template` is used with the :locals option, and the
partial was not rendered, a method_missing error was raised.
This changes first checks, if the partial actually was rendered
and raises a descriptive error.

actionpack/lib/action_controller/test_case.rb

@@ -127,6 +127,9 @@ def assert_template(options = {}, message = nil)
           if expected_locals = options[:locals]
             if defined?(@_rendered_views)
               view = expected_partial.to_s.sub(/^_/,'')
+              partial_was_not_rendered_msg = "expected %s to be rendered but it was not." % view
+              assert_includes @_rendered_views.rendered_views, view, partial_was_not_rendered_msg
+
               msg = 'expecting %s to be rendered with %s but was with %s' % [expected_partial,
                                                                              expected_locals,
                                                                              @_rendered_views.locals_for(view)]

actionpack/lib/action_view/test_case.rb

@@ -134,6 +134,10 @@ def locals_for(view)
           @rendered_views[view]
         end
 
+        def rendered_views
+          @rendered_views.keys
+        end
+
         def view_rendered?(view, expected_locals)
           locals_for(view).any? do |actual_locals|
             expected_locals.all? {|key, value| value == actual_locals[key] }

actionpack/test/template/test_case_test.rb

@@ -329,6 +329,15 @@ class RenderTemplateTest < ActionView::TestCase
       assert_template partial: '_partial', locals: { 'second' => '2' }
     end
 
+    test 'raises descriptive error message when template was not rendered' do
+      controller.controller_path = "test"
+      render(template: "test/hello_world_with_partial")
+      e = assert_raise ActiveSupport::TestCase::Assertion do
+        assert_template partial: 'i_was_never_rendered', locals: { 'did_not' => 'happen' }
+      end
+      assert_match "i_was_never_rendered to be rendered but it was not.", e.message
+      assert_match 'Expected ["/test/partial"] to include "i_was_never_rendered"', e.message
+    end
   end
 
   module AHelperWithInitialize