Curious if semantics of this subdomain of "allowed/legal" programs will be easy to understand, or will it be "try and see on case-by-case basis if your code compiles". Still, this way or the other, probably user will have to gradually learn and build intuition, as with learning any other aspect of any programming language.

Also: will be/is the lifetime-checking done in the same way in the interpreter, even though it uses GC? Otherwise, those are in effect two, I think significantly different dialects of the language.