Swift 2到3迁移String.fromCString

关于fromCString函数存在大量错误,这些错误以前在Swift 2中的String上可用

到处都是错误大致相同：

‘fromCString’ is unavailable: Please use String.init?(validatingUTF8:) instead. Note that it no longer accepts NULL as a valid input. Also consider using String(cString:),that will attempt to repair ill-formed code units.

因为我有两个选择可供选择我不确定给出原作者意图的正确选择.

例如,这是一个带错误的片段.

extension JSON : CustomStringConvertible {
    /// stringifies self.
    /// if pretty:true it pretty prints
    public func toString(pretty:Bool=false)->String {
        switch _value {
        case is NSError: return "(_value)"
        case is NSNull: return "null"
        case let o as NSNumber:
            switch String.fromCString(o.objCType)! {
            case "c","C":
                return o.boolValue.description
            case "q","l","i","s":
                return o.int64Value.description
            case "Q","L","I","S":
                return o.uint64Value.description
            default:
                switch o.doubleValue {
                case 0.0/0.0:   return "0.0/0.0"    // NaN
                case -1.0/0.0:  return "-1.0/0.0"   // -infinity
                case +1.0/0.0:  return "+1.0/0.0"   //  infinity
                default:
                    return o.doubleValue.description
                }
            }
        case let o as NSString:
            return o.debugDescription
        default:
            let opts = pretty ? JSONSerialization.WritingOptions.prettyPrinted : JSONSerialization.WritingOptions()
            if let data = (try? JSONSerialization.data(
                withJSONObject: _value,options:opts)) as NSData? {
                    if let result = NSString(
                        data:data as Data,encoding:String.Encoding.utf8.rawValue
                        ) as? String {
                            return result
                    }
            }
            return "YOU ARE NOT SUPPOSED TO SEE THIS!"
        }
    }
    public var description:String { return toString() }
}

请注意上面代码中的fromCString.正确模拟Swift 2行为的正确方法是什么？