stuball_spec.rb

require 'rspec'

RSpec.configure do |config|
  def stub_mod(mod, excluded)
    mod.constants.each do |klass_name|
      klass = mod.const_get(klass_name)
      if klass.is_a?(Class)
        unless klass == described_class
          unless excluded.include?(klass)
            class_double(klass.to_s).as_stubbed_const(transfer_nested_constants: true)
          end
        end
      elsif klass.is_a?(Module)
        stub_mod(klass)
      end
    end
  end

  config.before(:each) do
    stub_mod(MyMod, [MyMod::MySpecialClass])
  end
end

module MyMod
  class MyClass
    def foo
      SomethingElse.new.bar if MySpecialClass.new.baz
    end
  end

  class SomethingElse
    def bar
      "If you see this, you forgot to stub me!"
    end
  end

  class MySpecialClass
    def baz
      true
    end
  end
end

RSpec.describe MyMod::MyClass do
  it "should fail because I forgot to stub SomethingElse" do
    expect(subject.foo).to eq("hello")
  end
end