Earlier quoted context omitted.
I have not looked at the implementation but isn't the idea to write a Lispy language in Rust (in other words, Mathematica the language ) and then write the differentiation and other routines in that .
They had to patch the Rust compiler to natively support AutoDiff. Contrast with Julia where it can be a regular Julia library,
I don't know that you can match something speedwise like a JIT or Expression Templates in rust though without using something like Enzyme.