Enzyme.jl

iMLSensitivity (https://sensitivity.sciml.ai/dev/, as part of the new https://docs.sciml.ai/dev/) and setup a system which amounts to 15 hours of direct unit tests doing a combinatoric check of arguments with 4 hours of downstream testing (https://github.com/SciML/SciMLSensitivity.jl/actions/runs/25...). What that identified is that any remaining issues that can arise are due to the implicit parameters mechanism in Zygote (Zygote.params). To counteract this upstream issue, we (a) try to default to never default to Zygote VJPs whenever we can avoid it (hence defaulting to Enzyme and ReverseDiff first as previously mentioned), and (b) put in a mechanism for early error throwing if Zygote hits any not implemented derivative case with an explicit error message (https://github.com/SciML/SciMLSensitivity.jl/blob/v7.0.1/src...). We have alerted the devs of the machine learning libraries, and from this there has been a lot of movement. In particular, a globals-free machine learning library, Lux.jl, was created with fully explicit parameters https://lux.csail.mit.edu/dev/, and thus by design it cannot have this issue. In addition, the Flux.jl library itself is looking to do a redesign that eliminates implicit parameters (https://github.com/FluxML/Flux.jl/issues/1986). Which design will be the one in the end, that's uncertain right now, but it's clear that no matter what the future designs of the deep learning libraries will fully cut out that part of Zygote.jl. And additionally, the other AD libraries (Enzyme and Diffractor for example) do not have this "feature", so it's an issue that can only arise from a specific (not recommended) way of using Zygote (which now throws explicit error messages early and often if used anywhere near SciML because I don't tolerate it). tl;dr: don't use Zygote.params, the ML AD devs know this its usage is being removed ASAP. So from this, SciML should be rather safe and if not, please share some details and I'd be happy to dig in.

, but the custom rules system is rather close and that's the piece that's needed to make the full transition.

it would be hard to enforce rules and get performance. So some language that has a front end over an optimizing compiler (LLVM) would probably make the most sense. Zygote and Diffractor uses Julia's IR, but there are other ways to do this as well. Enzyme (https://github.com/wsmoses/Enzyme.jl) uses the LLVM IR directly for doing source-to-source translations. Using some dialect of LLVM (provided by MLIR) might be an interesting place to write a more ML-focused flexible AD system. Swift for Tensorflow used the Swift IR. This mindset starts to show why those tools were chosen.

her in Julia, there's really no need for that.