I made an effort to make github workflows to bootstrap rust, and in the process llvm/clang, for Cygwin.
https://github.com/jeremyd2019/cygwin-rust-bootstrap/blob/main/.github/workflows/bootstrap_llvm.yml https://github.com/jeremyd2019/cygwin-rust-bootstrap/actions/runs/15260152503 The LLVM packages are input to https://github.com/jeremyd2019/cygwin-rust-bootstrap/blob/main/.github/workflows/bootstrap_rust.yml https://github.com/jeremyd2019/cygwin-rust-bootstrap/actions/runs/15292314203 which builds a patched curl and updated libgit2, cross-compiles rust x86_64-pc-windows-gnu -> x86_64-pc-cygwin, and then uses the cross-compiled rust to build native x86_64-pc-cygwin
