No, that is not what this "region borrow checking" solves! Both Rust and your "pure functions" are perfectly capable of enforcing this on new code, that is not

No, that is not what this "region borrow checking" solves! Both Rust and your "pure functions" are perfectly capable of enforcing this on new code, that is not the point. Neither of them can address the line you quoted from the RFC. The existing code they're talking about is not pure. It uses multiple non-const pointers to the same object, at the same time, and temporarily carving out immutable access in between those uses is not the hard part- Rust's scheme handles that just fine already.