json-rust VS differential-datalog

You can use my crate jss which is essentially just json with field and values of a css properties. I used the json crate here since it allows omitting double quotes on fields that are valid rust identifier. The jss crate has a generated css properties which are used as lookup, transforming to and fro kebab-case properties into snake_case in order make the css code a bit cleaner, while still allowing the use of double quoted fields.

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.