Display changed derivations on activation

hm/nix/default.nix

@@ -2,6 +2,7 @@
 {
   config = {
     home.packages = with pkgs; [
+      nvd
       nix-diff
       nix-tree
       nix-output-monitor

os/default.nix

@@ -8,6 +8,7 @@
     ./ccc
     ./common.nix
     ./desktop.nix
+    ./diff
     ./gaming
     ./geoffrey.nix
     ./printing

os/diff/default.nix

@@ -0,0 +1,14 @@
+{ pkgs, lib, config, ... }:
+{
+  config = {
+    system.activationScripts.diff = {
+      supportsDryActivation = true;
+      text = ''
+        if [ -h /run/current-system ]
+        then
+            ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff /run/current-system "$systemConfig"
+        fi
+      '';
+    };
+  };
+}