ruby_constant_scopes.rb

module Foo
  module Bar
  end
end

class Foo::Bar::Quux
end

class Foo::Bar::Buz
  def fails
    p Module.nesting
    Quux.new
  end
end

module Foo::Bar
  class Buz
    def succeeds
      p Module.nesting
      Quux.new
    end
  end
end

buz = Foo::Bar::Buz.new
buz.succeeds
buz.fails