I had also this issue when using NixOps. I did add the builtins.trace and the message appeared, but not the warning. I am using flakes and the only source is NixOS 22.11. I’ll update here if the warning comes back.
Edit: Nevermind, in my case, it was because I am using a systemd container and didn’t set system.stateVersion inside it.
If you do see it again, you can get a trace with NIX_ABORT_ON_WARN=1 + --impure.
The trace will be rather verbose, but is very likely to tell you which option was getting evaluated when the warning was encountered.
I completely forgot about the container issue. That should have been among the first things I asked about.
Ah, interesting. Could be that the trace is from some systemd container that did not get updated the last time I did a rebuild. Because of that I would not have seen some trace, am I right?
I will observe and report back if something comes up again!
No, updated or not, the nix code would still have been evaluated, generating the trace.