Prove control was yielded to a block via a particular mechanism

example_spec.rb

class C
  def x(&block)
    instance_eval &block if block_given?
  end
end

describe C do
  describe "#x" do
    it "executes the block" do
      expect{ |p| described_class.x &p }.to yield_control
    end
    
    it "executes the block with instance_eval" do
      described_class.should_receive :target
      described_class.x { target }
    end
  end
end