6 files changed, 68 insertions, 12 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index f0c869fba0..57e4adb457 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -34,7 +34,11 @@ module ActionDispatch
       end
 
       def build(app)
-        InstrumentationProxy.new(klass.new(app, *args, &block), inspect)
+        klass.new(app, *args, &block)
+      end
+
+      def build_instrumented(app)
+        InstrumentationProxy.new(build(app), inspect)
       end
     end
 
@@ -119,7 +123,14 @@ module ActionDispatch
     end
 
     def build(app = nil, &block)
-      middlewares.freeze.reverse.inject(app || block) { |a, e| e.build(a) }
+      instrumenting = ActiveSupport::Notifications.notifier.listening?(InstrumentationProxy::EVENT_NAME)
+      middlewares.freeze.reverse.inject(app || block) do |a, e|
+        if instrumenting
+          e.build_instrumented(a)
+        else
+          e.build(a)
+        end
+      end
     end
 
     private
diff --git a/actionpack/test/controller/show_exceptions_test.rb b/actionpack/test/controller/show_exceptions_test.rb
index 8724f9bcdb..1d68a359dc 100644
--- a/actionpack/test/controller/show_exceptions_test.rb
+++ b/actionpack/test/controller/show_exceptions_test.rb
@@ -99,7 +99,7 @@ module ShowExceptions
   class ShowFailsafeExceptionsTest < ActionDispatch::IntegrationTest
     def test_render_failsafe_exception
       @app = ShowExceptionsOverriddenController.action(:boom)
-      middleware = @app.instance_variable_get(:@middleware)
+      middleware = @app
       @exceptions_app = middleware.instance_variable_get(:@exceptions_app)
       middleware.instance_variable_set(:@exceptions_app, nil)
       $stderr = StringIO.new
diff --git a/actionpack/test/dispatch/middleware_stack_test.rb b/actionpack/test/dispatch/middleware_stack_test.rb
index 90f2eccd19..c534e60c74 100644
--- a/actionpack/test/dispatch/middleware_stack_test.rb
+++ b/actionpack/test/dispatch/middleware_stack_test.rb
@@ -121,9 +121,6 @@ class MiddlewareStackTest < ActiveSupport::TestCase
   end
 
   test "instruments the execution of middlewares" do
-    app = @stack.build(proc { |env| [200, {}, []] })
-    env = {}
-
     events = []
 
     subscriber = proc do |*args|
@@ -131,6 +128,9 @@ class MiddlewareStackTest < ActiveSupport::TestCase
     end
 
     ActiveSupport::Notifications.subscribed(subscriber, "process_middleware.action_dispatch") do
+      app = @stack.build(proc { |env| [200, {}, []] })
+
+      env = {}
       app.call(env)
     end
 
diff --git a/guides/source/testing.md b/guides/source/testing.md
index 18eecf49fa..9540bb2af5 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -781,7 +781,7 @@ This can be helpful for viewing the browser at the point a test failed, or
 to view screenshots later for debugging.
 
 Two methods are provided: `take_screenshot` and `take_failed_screenshot`.
-`take_failed_screenshot` is automatically included in `after_teardown` inside
+`take_failed_screenshot` is automatically included in `before_teardown` inside
 Rails.
 
 The `take_screenshot` helper method can be included anywhere in your tests to
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 65274f0946..5bd4b06274 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -247,6 +247,40 @@ In that case, `app/models/concerns` is assumed to be a root directory (because i
 
 The `Concerns::` namespace worked with the classic autoloader as a side-effect of the implementation, but it was not really an intended behavior. An application using `Concerns::` needs to rename those classes and modules to be able to run in `zeitwerk` mode.
 
+#### Autoloaded Constants and Explicit Namespaces
+
+If a namespace is defined in a file, as `Hotel` is here:
+
+```
+app/models/hotel.rb         # Defines Hotel.
+app/models/hotel/pricing.rb # Defines Hotel::Pricing.
+```
+
+the `Hotel` constant has to be set using the `class` or `module` keywords. For example:
+
+```ruby
+class Hotel
+end
+```
+
+is good.
+
+Alternatives like
+
+```ruby
+Hotel = Class.new
+```
+
+or
+
+```ruby
+Hotel = Struct.new
+```
+
+won't work, child objects like `Hotel::Pricing` won't be found.
+
+This restriction only applies to explicit namespaces. Classes and modules not defining a namespace can be defined using those idioms.
+
 #### Spring and the `test` Environment
 
 Spring reloads the application code if something changes. In the `test` environment you need to enable reloading for that to work:
@@ -305,6 +339,17 @@ To fix this, just remove the wildcards:
 config.autoload_paths << "#{config.root}/lib"
 ```
 
+#### How to Use the Classic Autoloader in Rails 6
+
+Applications can load Rails 6 defaults and still use the classic autoloader by setting `config.autoloader` this way:
+
+```ruby
+# config/application.rb
+
+load_defaults "6.0"
+config.autoloader = :classic
+```
+
 Upgrading from Rails 5.1 to Rails 5.2
 -------------------------------------
 
diff --git a/railties/lib/rails/application/bootstrap.rb b/railties/lib/rails/application/bootstrap.rb
index e3c0759f95..a20f86dd93 100644
--- a/railties/lib/rails/application/bootstrap.rb
+++ b/railties/lib/rails/application/bootstrap.rb
@@ -20,13 +20,13 @@ module Rails
       initializer :set_eager_load, group: :all do
         if config.eager_load.nil?
           warn <<-INFO
-config.eager_load is set to nil. Please update your config/environments/*.rb files accordingly:
+            config.eager_load is set to nil. Please update your config/environments/*.rb files accordingly:
 
-  * development - set it to false
-  * test - set it to false (unless you use a tool that preloads your test environment)
-  * production - set it to true
+              * development - set it to false
+              * test - set it to false (unless you use a tool that preloads your test environment)
+              * production - set it to true
 
-INFO
+          INFO
           config.eager_load = config.cache_classes
         end
       end