Why wait for ruby 2.1?

private_def.rb

gem 'minitest', '>= 5'

require 'minitest/autorun'

module Kernel
  def private_def method_name, &block
    define_method method_name, &block
    private method_name
  end
end

class PrivateDefTest < MiniTest::Test
  class Foo
    private_def :foo do
      :foo
    end
  end

  def test_Foo_foo_exists
    assert_equal :foo, Foo.new.send(:foo)
  end

  def test_Foo_foo_is_private
    assert_raises NoMethodError do
      Foo.new.foo
    end
  end
end