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

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.