Why isn’t my Stringer interface method getting invoked? When using fmt.Println

When calling fmt.Println, myCar is implicitly converted to a value of type interface{} as you can see from the function signature. The code from the fmt package then does a type switch to figure out how to print this value, looking something like this:

switch v := v.(type) {
case string:
    os.Stdout.WriteString(v)
case fmt.Stringer:
    os.Stdout.WriteString(v.String())
// ...
}

However, the fmt.Stringer case fails because Car doesn’t implement String (as it is defined on *Car). Calling String manually works because the compiler sees that String needs a *Car and thus automatically converts myCar.String() to (&myCar).String(). For anything regarding interfaces, you have to do it manually. So you either have to implement String on Car or always pass a pointer to fmt.Println:

fmt.Println(&myCar)