//随机数生成要用到的const letterBytes = "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"const (   letterIdxBits = 6   letterIdxMask = 1<
     
      = 0; {      if remain == 0 {         cache, remain = src.Int63(), letterIdxMax      }      if idx := int(cache & letterIdxMask); idx < len(letterBytes) {         b[i] = letterBytes[idx]         i--      }      cache >>= letterIdxBits      remain--   }   return string(b)}