The rust core is indeed called from the ruby library (as it is with all of our 5 other host libraries). The core itself is pretty complex (there's a whole parser/interpreter in there), so maintaining it in a bunch of languages would be a bit hectic.
There are some files inside `lib/oso/polar/ffi` that define the C bindings used by the rest of the library. Here's an example: https://github.com/osohq/oso/blob/main/languages/ruby/lib/os...
We use the ffi gem to make that work: https://github.com/ffi/ffi
EDIT: hobofan beat me to it! :)
- It triggers the rust build in the Makefile: https://github.com/osohq/oso/blob/1d3bf5a4a997a574c2b19084a0...
- There is a bunch of FFI code (also in the ffi directory): https://github.com/osohq/oso/blob/1d3bf5a4a997a574c2b19084a0...