Setting aside the fact that using Rust for a JIT won't help that much - a new JS engine is a huge undertaking, especially if the goal is to compete with V8. They could eventually switch to Boa[1] if they wanted more safety, but it's still ways off and doesn't seem necessary enough just yet

Also, isn't the goal of Servo to effectively compete with CEF first and foremost? Because you don't need memory safety for that really