modules/nix: init activation-script (nushell#848)

Author: Schweber

modules/nix/README.md

@@ -1,6 +1,6 @@
 # Nix
 
-Commands for working with [nix](https://nixos.org/).
+Commands for working with [nix/nixos](https://nixos.org/).
 
 ### ns
 Shorthand search (`nix search nixpkgs ...`) with much nicer output.
@@ -24,3 +24,31 @@ Shorthand search (`nix search nixpkgs ...`) with much nicer output.
 ╰───┴────────────────┴──────────────────────────────────────────────────────────────┴─────────╯
 ```
 
+### activation-script
+
+Shows changed packages after running `nixos-rebuild switch/boot` and change in total size of installed packages. Changes that don't affect the installed size are filtered out. Intended to be added to `configuration.nix` of `NixOS`.
+
+```
+╭─────────────────┬─────────────┬─────────────┬───────────╮
+│     Package     │ Old_Version │ New_Version │   Diff    │
+├─────────────────┼─────────────┼─────────────┼───────────┤
+│ xwayland        │ ∅           │ 24.1.0      │    5.3 MB │
+│ lsp-plugins     │ 1.2.15      │ 1.2.16      │    1.1 MB │
+│ libei           │ ∅           │ 1.2.1       │  547.6 KB │
+│ libXfont2       │ ∅           │ 2.0.6       │  270.0 KB │
+│ xkbcomp         │ ∅           │ 1.4.7       │  235.7 KB │
+│ font-util       │ ∅           │ 1.4.1       │  234.1 KB │
+│ libxkbfile      │ ∅           │ 1.1.3       │  174.6 KB │
+│ xfsprogs        │ 6.6.0       │ 6.8.0       │   61.0 KB │
+│ xcb-util-errors │ ∅           │ 1.0.1       │   48.5 KB │
+│ libieee1284     │ 0.2.11      │ ∅           │ -123.1 KB │
+│ libgnomekbd     │ 3.28.1      │ ∅           │ -702.0 KB │
+│ xwayland        │ 24.1.0      │ ∅           │   -5.3 MB │
+│ libgweather     │ 4.4.0       │ ∅           │  -23.8 MB │
+╰─────────────────┴─────────────┴─────────────┴───────────╯
+
+╭──────┬──────────╮
+│ Diff │ -22.0 MB │
+╰──────┴──────────╯
+```
+

modules/nix/activation-script

@@ -0,0 +1,11 @@
+### add to configuration.nix
+
+nix.settings.experimental-features = ["nix-command"];
+
+system.activationScripts.diff = ''
+  if [[ -e /run/current-system ]]; then
+    echo
+    ${pkgs.nushell}/bin/nu -c "let diff_closure = ${pkgs.nix}/bin/nix store diff-closures /run/current-system '$systemConfig'; if \$diff_closure != \"\" { let table = \$diff_closure | lines | where \$it =~ KiB | where \$it =~ → | parse -r '^(?<Package>\S+): (?<Old_Version>[^,]+)(?:.*) → (?<New_Version>[^,]+)(?:.*, )(?<DiffBin>.*)$' | insert Diff { get DiffBin | ansi strip | str trim -l -c '+' | into filesize } | reject DiffBin | sort-by -r Diff; print \$table; \$table | math sum }"
+  fi
+'';
+