Adding Fil-C-like runtime checks to Rust is definitely an interesting direction. As I mentioned upthread. It's not just the availability of the safe API that's interesting, though, but also the prohibition on using the unsafe API in the entire program and all dependencies. Which Rust could also do in theory but not yet in practice AFAIK.

Rust is less safe because it has a feature for turning off safety, called `unsafe`. Fil-C does not have a way to turn off safety, and it enforces safety checks at runtime. That's the reasoning anyways. If you define another language Rust-Without-Any-Unsafe, then maybe that one is safer than Fil-C.