指针接收器混淆

Question

我对以下代码的行为感到困惑。操场

var foo json.RawMessage

_ = json.Unmarshal([]byte(`{ "zoo": 123 }`), &foo)

enc := json.NewEncoder(os.Stdout)

// Works as expected

_ = enc.Encode(struct{ Foo *json.RawMessage }{&foo})

// MarshalJSON has a pointer reciever, so it doesn't get invoked here

_ = enc.Encode(struct{ Foo json.RawMessage }{foo})

// How is MarshalJSON being invoked if .Foo is not a pointer?

_ = enc.Encode(&struct{ Foo json.RawMessage }{foo})

输出：

{"Foo":{"zoo":123}}

{"Foo":"eyAiem9vIjogMTIzIH0="}

{"Foo":{"zoo":123}}

我不明白为什么第三个调用json.Encoder.Encode能够访问，json.RawMessage.MarshalJSON即使它不是指针。