building Nix...
building the system configuration...
error: A definition for option `users.users.us.packages."[definition 1-entry 586]"' is not of type `package'. Definition values:
- In `/etc/nixos/configuration.nix':
is there a way to find that element “[definition 1-entry 586]” without binary search and co.
It is very likely that you either use an override(Attrs) “inline” and forgot about the surrounding parenthesis, or you added some font"package" to the list that has been restructured years ago to be a set of individual fonts rather than a single package containing all of them years ago, but still appears in blog posts the old way, or which would be the most annoying thing: there changed a package recently the same way as that font package did.
In that case it would of course not help to look at the most recent changes in your configuration.
In my opinion, it would have been the best to not even have that list grow this much. Any packages list that has more ~10 entries is a red flag for me, and should be looked and and decided if splitting in individual modules would make sense.
If you were linking your config here, we might be able to help to debug the problem. (Or see it with just looking at the list)