I’m not sure what you mean by this. When you bridge an NSString to String, its representation in memory is unchanged, and all operations are handled through resilient function calls. When bridging in the other direction, String storage is an NSString subclass, as I’ve already cited previously. That there are optimizations possible to improve string performance in no way changes the semantics or syntax of the language, where as is a coercion operator that does not convert between types.