The other main (safe) alternative to Rc<RefCell<T>> for mutable aliasing is Vec + generational indices, which also involves an expect/unwrap assertion whenever

The other main (safe) alternative to Rc<RefCell<T>> for mutable aliasing is Vec + generational indices, which also involves an expect/unwrap assertion whenever you expect something to be alive and it isn't. If you need mutable aliasing, you're going to incur run-time costs, whether in Rust or any other language. The other approach that I like for mitigating the cost of mutable aliasing (in Rust and Vale and some other languages) is to rearchitect the program such that the world (or some subset of it) is frozen, we do a lot of heavy calculation and make an "effect", and then unfreeze the world to apply it. Vale's region borrow checker [1] will make that zero-cost, like Rust's borrow checker does. If we must have mutable aliasing, I like the balance you speak of, where one can use a blend of runtime-checked aliasing at the high level and borrow checking at the low level. I hope that borrow checker zen moves in that direction. Cone [2] emphasizes that exact balance, and I think there's a lot of promise there. [1] https://vale.dev/ref/regions https://vale.dev/ref/regions [2] http://cone.jondgoodwin.com/ http://cone.jondgoodwin.com/