> making sure the JIT doesn't emit anything broken or naughty is completely outside the scope of Rust.

It's also outside the scope of the project if they're using Webkit's engine for that part. Which means Bun itself isn't a JIT, it's all the stuff built around the Webkit JIT, so whether or not Rust is useful for the JIT is entirely immaterial to the question of if Bun would benefit from Rust.