Cannot require ActionDispatch SelectorAssertions

I tried to load and mix in ActionDispatch::Assertions::SelectorAssertions in my specs:

require 'action_dispatch/testing/assertions/selector'

This fails with "ActionDispatch::Assertions::SelectorAssertions is an undefined constant". Of course it's undefined; I'm trying to require a file that defines it! This weird error seemed to originate from the very second line of code in that file:

module ActionDispatch
  module Assertions # <-- this one
    ...
  end
end

Turns out, ActionDisplatch has set "Assertions" constant to autoload 'action_dispatch/testing/assertions', which in turn tries to load all its submodules, which tries to mix in ActionDispatch::Assertions::SelectorAssertions before it's even defined. The solution was to simply load the complete assertions module:

require 'action_dispatch/testing/assertions'

I have no idea how to solve this without getting rid of the autoload, but Rails 3 should make every independent component of it loadable and I think this should definitely fall under that category.