読み込みも JSONSerialization だった。 https://github.com/apple/swift/pull/9005/files#diff-97ae7c2e808a30701d6e28271b8b6073R818

Foundation Encoders by itaiferber · Pull Request #9005 · apple/swift

This implements the JSONEncoder, JSONDecoder, PropertyListEncoder, and PropertyListDecoder types proposed by SE-0167. This pulls all Foundation-specific work out of #8124 for separate integration. ...