I'm guessing language purity is important when it comes to this. Translating the "unison way" to an imperative world of statefulness and random side effects could be rough.
You don't need it down to each function. Just having hashes this on the #include/import level of the source code would be a win for optimizing build and test of big code bases. Most build systems today just operate on file time stamps which has a lot of tricky cases where the build system gets out of sync and requires a make clean to get back into decent shape.
