如何直接在空接口上调用String方法？

The only solution I know is using fmt.Sprint or similar functions. I already look into builtin package but it has only error interface, string is just a normal type, not interface.

As @volker mentioned; you cannot as the empty interface has no methods.

Please note: fmt.Sprint, fmt.Sprintf etc. does call Stringer interface first if its exists. It is elegant way.

Example of calling stringer interface after type assertion.

var a SomeType
if v, ok := a.(fmt.Stringer); ok {
    fmt.Println(v.String())
}