gomock优化diff展示

使用gomock时可以调用EXPECT()为你的mock对象设置各种期望和返回值，用于验证期望值和运行时的值是否一致。通常情况下可以通过gomock来清晰的看出期望值和真实值的区别，但是当数据结构为指针切片时就只能看到下面这种提示，不能清晰的看出二者的区别是什么。

Got: [0xc000060610] ([]*demo.Item)
Want: is equal to [0xc0000605b0] ([]*demo.Item)

 可以通过设置 gomock Modifying Failure Messages的能力来优化这一点，举例如下。

// 优化前错误日志如下, 此时只能看到数据有 diff， 但不知道是什么
//--- FAIL: TestSaveToFile (1.00s)
//    demo/demo.go:12: Unexpected call to *demo.MockSaver.Save([[0xc000060610]]) at demo/demo.go:12 because: 
//        expected call at demo/demo_test.go:14 doesn't match the argument at index 0.
//        Got: [0xc000060610] ([]*demo.Item)
//       Want: is equal to [0xc0000605b0] ([]*demo.Item)
//    demo/controller.go:269: missing call(s) to *demo.MockSaver.Save(is equal to [0xc0000605b0] ([]*demo.Item)) demo/demo_test.go:14
//    demo/controller.go:269: aborting test due to missing call(s)
func TestSaveToFile(t *testing.T) {
	ctrl := gomock.NewController(t)
	defer ctrl.Finish()
	mockSaver := NewMockSaver(ctrl)
	mockSaver.EXPECT().Save([]*Item{{Name: "bar"}})

	Save(mockSaver, []*Item{{Name: "foo"}})
}

// 优化后错误日志如下，此时不仅能看到diff，也能看到差异是什么
//--- FAIL: TestSaveToFileV2 (0.00s)
//    demo/demo.go:12: Unexpected call to *demo.MockSaver.Save([[0xc00009e5f0]]) at demo/demo.go:12 because: 
//        expected call at demo/demo_test.go:23 doesn't match the argument at index 0.
//        Got: [{"Name":"foo"}]
//        Want: [{"Name":"bar"}]
//    demo/controller.go:269: missing call(s) to *demo.MockSaver.Save([{"Name":"bar"}]) demo/demo_test.go:23
//    demo/controller.go:269: aborting test due to missing call(s)
func TestSaveToFileV2(t *testing.T) {
	ctrl := gomock.NewController(t)
	defer ctrl.Finish()
	mockSaver := NewMockSaver(ctrl)
	mockSaver.EXPECT().Save(eq([]*Item{{Name: "bar"}}))  // 注意这里调用了 eq 方法

	Save(mockSaver, []*Item{{Name: "foo"}})
}

func eq(want interface{}) gomock.Matcher {
	return gomock.GotFormatterAdapter(
		gomock.GotFormatterFunc(
			func(got interface{}) string {
				b, _ := json.Marshal(got)
				return string(b)
			},
		),
		gomock.WantFormatter(
			gomock.StringerFunc(func() string {
				b, _ := json.Marshal(want)
				return string(b)
			}),
			gomock.Eq(want),
		),
	)
}