See: https://github.com/NixOS/nix/commit/7becb1bf1c2ec1544a5374580a97b36273506baf#r33450554
For text files it is possible to do it like so: `builtins.hashString "sha256" (builtins.readFile /tmp/a)` but that doesn't work for binary files. With builtins.hashFile any kind of file can be conveniently hashed.