Comment by estebank

I would assume that it would opportunistically try to run the borrow checker, and if it fails on the access of a specific field, turn that access into an Arc/Rc everywhere, leaving any other access as references. This leaves you with invisible performance cliffs, where accessing a field in a new place suddenly increases the cost of accessing it everywhere else, but it does give you the "just do what I want, damn it!" development experience. I doubt Rust itself could do that without alienating its current userbase, but a RustScript could.