the request class is never changed, so just use it directly in the method body

actionpack/lib/action_controller/metal.rb

@@ -232,13 +232,13 @@ module ActionController
     end
 
     # Returns a Rack endpoint for the given action name.
-    def self.action(name, klass = ActionDispatch::Request)
+    def self.action(name)
       if middleware_stack.any?
         middleware_stack.build(name) do |env|
-          new.dispatch(name, klass.new(env))
+          new.dispatch(name, ActionDispatch::Request.new(env))
         end
       else
-        lambda { |env| new.dispatch(name, klass.new(env)) }
+        lambda { |env| new.dispatch(name, ActionDispatch::Request.new(env)) }
       end
     end
   end

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -39,7 +39,7 @@ module ActionDispatch
             return [404, {'X-Cascade' => 'pass'}, []]
           end
 
-          dispatch(controller, params[:action], req.env)
+          dispatch(controller, params[:action], req)
         end
 
         def prepare_params!(params)
@@ -69,8 +69,8 @@ module ActionDispatch
           ActiveSupport::Dependencies.constantize(const_name)
         end
 
-        def dispatch(controller, action, env)
+        def dispatch(controller, action, req)
-          controller.action(action).call(env)
+          controller.action(action).call(req.env)
         end
 
         def normalize_controller!(params)