I've never looked at it too closely, but wycats has talked about how the Ruby code calls Rust code through the FFI and how Skylight doing some special things to

I've never looked at it too closely, but wycats has talked about how the Ruby code calls Rust code through the FFI and how Skylight doing some special things to make that work especially smoothly by directly calling some internal functions of Rust's failure handling bits, and about how the Rust code crashing shouldn't take down the rails process. That seems to suggest it's not just an IPC thing.