The rustport blog post says they lost support for Cygwin because it's not a supported Rust target. Looks like there is a fork for this:

https://github.com/ookiineko-cygport/rust

There have been discussions about adding the target to Rust, but they have not been pursued:

https://github.com/rust-lang/rust/issues/79854

https://github.com/rust-lang/rust/issues/5526