grass VS differential-datalog

Grass (https://github.com/connorskees/grass) and rsass (https://github.com/kaj/rsass) are both SASS implementation in Rust, which would be super nice to get to a feature-complete state.

There is also grass it's a sass/scss compiler. It's doesn't pass all of the sass specification test but is usable enough that eww uses eww uses it

If the css syntax is not required, but a question is about a flexible and performant rule engine, I would suggest Differential Datalog - I had a great experience, when I tried to implement a system with entities, with multiple inheritance, where the inherited fields - can be specified runtime - even that can be changed that which entity extend what other entities. It has a great documentation and integration with Rust (and Java, and others...). I had to define how inheritance works in my entities, and ddlog could generate Rust code, which continuously evaluates the rules, and updates my program about what fields do my entities have.