This project looks like a very nice lightweight way to implement policy in a Rust application; I really like the ergonomics of the builder. Despite being very different systems, the core permissions check being the same signature as a call to SpiceDB[0] (e.g. the subject, action, resource, and context) shows the beauty of the authorization problem-domain regardless of the implementation.
I would like to add some color that a policy engine is not all you need to implement authorization for your applications. Without data, there's nothing for a policy engine to execute a policy against and not all data is going to be conveniently in the request context to pass along. I'd like to see more policy engines take stances on how their users should get that data to their applications to improve the DX. Without doing so, you get the OPA[1] ecosystem where there are bunch of implementations filling the gap as an afterthought, which is great, but doesn't give a first-class experience.