WIP

railties/test/generators/app_generator_test.rb

@@ -318,7 +318,7 @@ def test_app_update_preserves_sprockets
     FileUtils.cd(destination_root) do
       config = "config/environments/production.rb"
       assert_no_changes -> { File.readlines(config).grep(/config\.assets/) } do
-        run_app_update
+        run_app_update_debug
       end
     end
   end
@@ -1489,6 +1489,16 @@ def run_app_update(app_root = destination_root, flags: "--force")
       end
     end
 
+    def run_app_update_debug(app_root = destination_root, flags: "--force")
+      Dir.chdir(app_root) do
+        gemfile_contents = File.read("Gemfile")
+        gemfile_contents.sub!(/^(gem "rails").*/, "\\1, path: #{File.expand_path("../../..", __dir__).inspect}")
+        File.write("Gemfile", gemfile_contents)
+
+        system({ "BUNDLE_GEMFILE" => "Gemfile" }, "bin/rails app:update #{flags} --trace", exception: true)
+      end
+    end
+
     def action(*args, &block)
       capture(:stdout) { generator.send(*args, &block) }
     end