* Generational References involve some pretty deep compiler support. One can implement something like it in Rust with a thread-local side table, but with >3x overhead compared to language-supported generational references.

* Higher RAII requires that a language not treat destructors and panic handlers as the same thing. Unfortunately, Rust uses one function (drop) for both. Additionally, it would be a backwards-incompatible change; Rust generic parameters already assume a zero-arg drop() is available for all parameters, even though there's no trait bound declared for them.

* Regions get most of the borrow checker's optimizing power with a fraction of the user complexity. Having both borrow checking and regions would just be adding complexity though, which defeats the purpose a bit.

Still, there are a few other features of Vale that we're thinking of proposing to Rust, especially around concurrency, determinism, and better memory safety guarantees.