It definitely changes how a developer works in a language if they know they can depend on this kind of implementation. In Clojure, knowing up front that all dat

It definitely changes how a developer works in a language if they know they can depend on this kind of implementation. In Clojure, knowing up front that all data structures have shared persistence allows you to do things that you'd never consider in a language where this wasn't guaranteed. I think Swift could greatly enhance its power in the functional paradigm if indeed it did what you describe, but also making this clearly known to everyone who uses it.