[bouk]

Hope this is upstreamed at some point. Good to see there's still a lot of gains to be made for Rust compile time

[jasonjmcghee]

Wouldn't this be quite difficult to upstream to users in the state presented?

At the least you'd want to make caching opt-in per macro unless something special was enabled. Forcing the end developer to blacklist incompatible macros feels like an anti-pattern.