Autodiff Upstreaming - enzyme backend Tracking issue: https://github.com/rust-lang/rust/issues/124509 Part of https://github.com/rust-lang/rust/pull/129175 This PR should allow building Enzyme from source on Tier 1 targets (when also building LLVM), except MSVC. It's only a small fraction (~200 lines) of the whole upstream PR, but due to bootstrapping and the number of configurations in which rustc can be build I assume that this will be the hardest to merge, so I'm starting with it. Happy to hear what changes are required to be able to upstream this code. **Content:** It contains a new configure flag `--enable-llvm-enzyme`, and will build the new Enzyme submodule when it is set. **Discussion:** Apparently Rust CI isn't able to clone repositories outside the rust-lang org? At least I'm seeing this error in CI: ``` [email protected]: Permission denied (publickey). fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists. ``` Does that mean we would need to mirror github.com/EnzymeAD/Enzyme in rust-lang, until LLVM upgrades Enzyme from an Incubator project to something that ships as part of the monorepo? Tracking: - https://github.com/rust-lang/rust/issues/124509