Rust doesn’t need to copy the data. It’s trivial to pass e.g. Numpy arrays to Rust as slices via Cython (let alone originating in Cython!), modify them, and return them, or use them as input for a new returned struct.
Each of those repos has links to the corresponding Rust “shim” libraries that provide FFIs for dealing with the incoming data, constructing Rust data structures from it, and then transforming it back on the way out.
As a more general comment, using a GC language as the FFI target from a GC language is begging for difficult-if-not-impossible-to-debug crashes down the line.
https://github.com/urschrei/simplification
https://github.com/urschrei/lonlat_bng
https://github.com/urschrei/pypolyline
Each of those repos has links to the corresponding Rust “shim” libraries that provide FFIs for dealing with the incoming data, constructing Rust data structures from it, and then transforming it back on the way out.
As a more general comment, using a GC language as the FFI target from a GC language is begging for difficult-if-not-impossible-to-debug crashes down the line.