doomemacs is a configuration framework for GNU Emacs. I quite like it for its declarative config format, good defaults and overall user-friendliness.

• Doomemacs recommends re-generating or 'sync'-ing its declarative config with active Emacs config state every time system is updated. I believe this is recommended because quite a few Emacs packages use APIs provided by some other native code, like mu4e, vterm or jinx. I use NixOS as my OS of choice, which is also a declarative in nature. Now the problem was, NixOS encourages playing with your system by providing fearless upgrades and rollbacks. It means I tend to update my system often. Emacs also encourages tinkering, by being so damn introspectable and moldable piece of software. The result is they are both being updated often and hence I'm often running 'doom sync' after every nixos update. This is not fun.
• Problem #2 is keeping track of 2 separate repositories, which also isn't fun. Because both NixOS and Emacs have a tendency of taking over your entire digital existence, I've been wanting to merge them for a while now, but was stuck running 'doom sync' and tracking two repos¹. Again, not fun.

• 'doom sync' must run after every NixOS update automatically
• Bonus: Track both NixOS and Doomemacs config in single repo.

• While merging my doomemacs config with pre-existing NixOS config repo, I wanted to preserve commit history of both, and merge them with preferred file relocation.
• Thankfully, I had already done this before :). I made minor changes and the repos were merged just how I wanted.
• As for running doom sync, I found that home-manager has an option to automatically link any file to any location, and even run any command after it is run. The total config change was tiny:

 1home = {
 2  file = {
 3    doom = {
 4      enable = true;
 5      executable = false;
 6      recursive = true;
 7      source = ./doom;
 8      target = "/home/bhankas/.doom.d";
 9    };
10  };
11  activation.doom = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
12    if [ -x "/home/bhankas/.config/emacs/bin/doom" ]
13          /home/bhankas/.config/emacs/bin/doom sync
14    fi
15  '';
16};