In implementing teqp (https://github.com/usnistgov/teqp), I found that automatic differentiation was MUCH faster than complex step derivatives and multicomplex numbers. As you wrote the library in Rust, have you benchmarked your hyper dual derivatives with autodiff? I used this package in C++: https://github.com/autodiff/autodiff. I think you could also use this in Rust as Rust is C++-derived?