[jolmg]

> The trick is to patch systemd and refer to your patched version separately

I mean, generally when you patch things, you want all dependents to use the patched version. It's just irksome that you know the change you made strictly speaking doesn't need a rebuild of dependents, but nix will still require it in the interest of ensuring reproducibility. Just wish there were an established expert-mode I-know-this-may-break-things way to have a modified derivation act as a drop-in replacement of a particular package/digest.

For example, it would be neat to have derivation foo.nix, from which the digest is calculated. Then, have the system check for a mod/foo.nix or similar path which would be a function that can modify the derivation without having the digest be recalculated on the result. Or maybe this could be done by having a function calcDigest, which sets the digest of a derivation and the system can simply not set the field when it's already been set. This to allow further changes while being treated as if changes haven't been made.