For this sort of problem Rust's safety guarantees don't buy you all that much. If you're generating machine code and implementing a GC that's not the sort of thing that Rust's typesystem can prove correct.
That said, a rust JS runtime would be amazing just because it's easier to integrate into rust projects.
Not sure exactly how much it buys you but I suspect there’s still room for research especially since Mozilla themselves has interest in a Rust JS JIT. I am curious to see if HolyJIT goes anywhere and what kinds of safety improvements it could potentially offer.
That said, a rust JS runtime would be amazing just because it's easier to integrate into rust projects.