[refset]

> It would be interesting if someone purpose-built a relation and rules database for compilers

While not quite in rustc proper, along these lines: https://github.com/rust-lang/chalk + https://github.com/rust-lang/polonius

[Permik]

Also related, not a DB but an incremental computation engine: https://github.com/salsa-rs/salsa

This can be found in things like the rust-analyzer and other actual compilers.