since you already defined a general syntax-level transformation for this, i doubt there is a fundamental reason why this can’t be done. i’ve been implementing a lot of language extensions recently using the new package plugins feature, so i’m pretty confident someone could create a syntax-level language extension to do just that, where the tool in question would “compile” this down to standard swift.
i just think that 9 times out of 10, this kind of problem is better solved with an appropriate in-place mutating method.