to OC

  func _bridgeToObjectiveCImpl() -> AnyObject {
    if _guts.isSmall {
      return _guts.asSmall.withUTF8 { bufPtr in
        // TODO(String bridging): worth isASCII check for different encoding?
        return _swift_stdlib_CFStringCreateWithBytes(
            nil, bufPtr.baseAddress._unsafelyUnwrappedUnchecked,
            bufPtr.count,
            kCFStringEncodingUTF8, 0)
        as AnyObject
      }
    }
    if _guts._object.isImmortal {
      // TODO: We'd rather emit a valid ObjC object statically than create a
      // shared string class instance.
      let gutsCountAndFlags = _guts._object._countAndFlags
      return __SharedStringStorage(
        immortal: _guts._object.fastUTF8.baseAddress!,
        countAndFlags: _StringObject.CountAndFlags(
          sharedCount: _guts.count, isASCII: gutsCountAndFlags.isASCII))
    }

    _internalInvariant(_guts._object.hasObjCBridgeableObject,
      "Unknown non-bridgeable object case")
    return _guts._object.objCBridgeableObject
  }
}

可以看到，如果 String 是不可变的，那么转到 NSString 会有一次内存 copy 操作。

Because NSString is immutable, it is just as though the storage was shared by a copy. The first in any sequence of mutating operations causes elements to be copied into unique, contiguous storage which may cost O(n) time and space, where n is the length of the string’s encoded representation (or more, if the underlying NSString has unusual performance characteristics).