build

# File actionpack/lib/action_dispatch/middleware/stack.rb, line 148
    def build(app = nil, &block)
      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