How to test a ruby method that does nothing

test_nothing.rb

require "rubygems"
require "rspec"

class TestNothing
  def do_nothing
  end

  def return_nil
    nil
  end

  def do_something
    1 + 1
  end
end

describe "TestNothing" do
  def method_should_do_nothing(klassname, methodname)
    has_called = false
    set_trace_func proc { |event, file, line, id, binding, classname|
      if classname.to_s == klassname
        has_called = true
        event.should_not eq("line"), "Err! #{methodname} is supposed to do nothing"
      end
    }

    instance = Object.const_get(klassname).new
    instance.send(methodname)
    has_called.should be_true
  end

  context "Method should do nothing" do
    it "should pass" do
      method_should_do_nothing("TestNothing", "do_nothing")
    end

    it "should return nil and still pass" do
      method_should_do_nothing("TestNothing", "return_nil")
    end

    it "should raise an exception when it does something" do
      expect{ method_should_do_nothing("TestNothing", "do_something") }.to raise_error
    end
  end
end