Fix assert_template(file:) with Rails 6

lib/rails/controller/testing.rb

@@ -2,6 +2,7 @@
 require 'rails/controller/testing/test_process'
 require 'rails/controller/testing/integration'
 require 'rails/controller/testing/template_assertions'
+require 'rails/controller/testing/instrument_determine_template'
 
 module Rails
   module Controller
@@ -18,6 +19,12 @@ def self.install
           include Rails::Controller::Testing::TestProcess
         end
 
+        if ActiveSupport::VERSION::MAJOR >= 6
+          ActiveSupport.on_load(:action_view) do
+            ActionView::TemplateRenderer.prepend(InstrumentDetermineTemplate)
+          end
+        end
+
         ActiveSupport.on_load(:action_view_test_case) do
           include Rails::Controller::Testing::TemplateAssertions
         end

lib/rails/controller/testing/instrument_determine_template.rb

@@ -0,0 +1,18 @@
+module Rails
+  module Controller
+    module Testing
+      module InstrumentDetermineTemplate
+        def determine_template(options)
+          super.tap do |template|
+            if template.is_a?(ActionView::Template::RawFile)
+              ActiveSupport::Notifications.instrument(
+                "!render_template.action_view",
+                { virtual_path: nil, identifier: template.identifier }
+              )
+            end
+          end
+        end
+      end
+    end
+  end
+end