Rust also has a saner module system that makes incremental builds much quicker. They're still pretty slow, but not as agonizingly slow as a clean build.
Rust is only fast compared to C++ (and then only sometimes). Literally every other language is significantly faster to compile. I'm hoping this can change though. Rust with fast compile times and a more complete library ecosystem (and ideally better IDE support) would be an incredibly productive language.
Compared with C++, Scala projects are not slow to compile when you use Gradle multi-project builds (with granular sub-projects) and Gradle build caching.
I think the most promising thing on this front is Cranelift (https://github.com/CraneStation/cranelift). The idea is that, when cranelift is ready, we might use it for debug builds (but still use LLVM for release builds).