Side note: There's an effort to cache proc macro invocations so that they get executed only once if the item they annotate hasn't changed: https://github.com/rust-lang/rust/pull/129102

There are multiple caveats on providing this to users (we can't assume that macro invocations are idempotent, so the new behavior would have to be opt in, and this only benefits incremental compilation), but it's in our radar.