ruby - 突变体没有找到测试示例

Question

在“hexx-null”gem 中，我有一个文件：

# lib/hexx/null.rb
module Hexx
  class Null
    # ...
  end
end

RSpec 规范文件是：

# spec/tests/null_spec.rb
describe Hexx::Null do

  describe ".new" do
    # ...
  end

  # ... etc.
end

然后我从 gem root运行突变体：

$ mutant --include lib --require hexx-null --use rspec Hexx::Null

结果是：

Mutant configuration:
Matcher:         #<Mutant::Matcher::Config match_expressions=[<Mutant::Expression: Hexx::Null>] subject_ignores=[] subject_selects=[]>
Integration:     rspec
Expect Coverage: 100.00%
Jobs:            8
Includes:        ["lib"]
Requires:        ["hexx-null"]
Subjects:        0
Mutations:       0
Kills:           0
Alive:           0
Runtime:         0.05s
Killtime:        0.00s
Overhead:        Inf%
Coverage:        0.00%
Expected:        100.00%
Active subjects: 0

似乎突变体没有找到任何例子，但为什么呢？