The borrow checker is a good tool (and it makes Rust objectively better than Zig to me), but it unfortunately doesn't prevent writing bad code

An issue with Bun is that it interfaces with a C++ JS engine and it needs unsafe. In this case, the best practice is to write a safe binding to encapsulate this external dependency (that's why in Rust we have -sys crates with raw unsafe bindings, and other crates with a safe interface on top), and then write your business logic entirely in safe Rust

However, the Rust port of Bun didn't follow such best practices (perhaps with good SDD practices it could, not sure about that). The resulting code has literally thousands of unsafe blocks. It also contains plenty of UB. The port already costed hundreds of thousands of dollars. It's unclear if Mythos/Fable is able to refactor it further to remove unsafe usage without introducing further UB, and how much it will cost

(here It's important to note something. Rust UB is in some abstract sense harder to deal with than C or Zig UB, because it also needs to uphold the guarantees of safe Rust. If you get to write your business logic in safe Rust that's a good deal, but the price of that is that your unsafe code has extra responsibilities)