Merge pull request #226 from hercules-ci/nixpkgs-lib

flake-update: Get the nixpkgs/lib subtree only

dev/flake-module.nix

@@ -11,6 +11,36 @@
     enable = true;
     autoMergeMethod = "merge";
     when.dayOfMonth = 1;
+    flakes = {
+      "." = { };
+      "dev" = { };
+    };
+    effect.settings = {
+      # Only fetch the `lib` subtree.
+      # NOTE: Users don't have to do this. They are recommended to use follows
+      #       and just use the `nixpkgs` they're already fetching anyway.
+      #       It doesn't have to be `lib/` only!
+      git.update.script = lib.mkBefore ''
+        echo 'Fetching nixpkgs-lib tree'
+        branch="nixos-unstable"
+        mkdir ~/nixpkgs
+        git -C ~/nixpkgs init
+        git -C ~/nixpkgs remote add origin https://github.com/NixOS/nixpkgs.git
+        git -C ~/nixpkgs fetch origin --filter=blob:none --depth=1 "$branch"
+        commit="$(git -C ~/nixpkgs rev-parse FETCH_HEAD)"
+        tree="$(git -C ~/nixpkgs rev-parse FETCH_HEAD:lib)"
+
+        echo 'Adjusting nixpkgs-lib.url'
+        sed -i flake.nix -e \
+          's^    nixpkgs-lib\.url = ".*^    nixpkgs-lib\.url = "https://github.com/NixOS/nixpkgs/archive/'$tree'.tar.gz"; # '$commit' /lib from '$branch'^'
+        git diff
+        grep -F "$tree" flake.nix >/dev/null || {
+          echo 'failed to write new tree to flake.nix'
+          exit 1
+        }
+        git commit flake.nix -m 'flake.nix: Update nixpkgs-lib tree'
+      '';
+    };
   };
 
   perSystem = { config, pkgs, ... }: {