源码如下：

$ cat spec/nested_subject_spec.rb 
class ArrayWithOneElement < Array
  def initialize(*)
    super
    unshift "first element"
  end
end

describe Array do
  describe ArrayWithOneElement do
    context "referenced as subject" do
      it "contains one element" do
        expect(subject).to include("first element")
      end
    end
  end
end

下面是执行结果：

$ rspec spec/nested_subject_spec.rb 
F

Failures:

  1) Array ArrayWithOneElement referenced as subject contains one element
     Failure/Error: expect(subject).to include("first element")
       expected [] to include "first element"
     # ./spec/nested_subject_spec.rb:12:in `block (4 levels) in <top (required)>'

Finished in 0.00099 seconds
1 example, 1 failure

Failed examples:

rspec ./spec/nested_subject_spec.rb:11 # Array ArrayWithOneElement referenced as subject contains one element

官方网站上也说应该是 pass 的，但我执行就 fail 了，不得其解