Add impure links module

flake.nix

@@ -16,12 +16,15 @@
         specialArgs = { inherit system inputs; };
 
         modules = [
+          (import ./modules/impurity.nix self)
           ./modules/preserve-system.nix
           ./modules/core.nix
           ./modules/user
           ./systems/lenovo.nix
         ];
       };
+
+      lenovo-impure = self.nixosConfigurations.lenovo.extendModules { modules = [ { impurity.enable = true; } ]; };
     };
   };
 }

modules/impurity.nix

@@ -0,0 +1,39 @@
+self: { lib, config, pkgs, ... }: with lib; let
+  relativePath = path: assert types.path.check path;
+    with builtins; strings.removePrefix ("${toString self}") (toString path);
+
+  impurityGroupEnabled = group: let
+    impurityGroups = strings.splitString " " (builtins.getEnv "IMPURITY_GROUPS");
+  in group == "" || builtins.elem group impurityGroups;
+
+  impurePath = let
+    impurePathEnv = builtins.getEnv "IMPURITY_PATH";
+  in if impurePathEnv == ""
+     then throw "impurity.enable is true but IMPURITY_PATH is not set"
+     else /. + impurePathEnv;
+
+  createImpurePath = path: impurePath + (relativePath path);
+in {
+  options.impurity.enable = mkOption {
+    type = types.bool;
+    default = false;
+    description = "Enable impure symlinks";
+  };
+
+  config._module.args.impurity = rec {
+    groupedLink = groupspec: path:
+      assert types.path.check path; let
+        groups =
+          if groupspec == null
+          then [ "" ]
+          else if (types.listOf types.string).check groupspec
+               then groupspec
+               else assert types.string.check groupspec;
+                 strings.splitString " " groupspec;
+      in if config.impurity.enable && lists.any (group: impurityGroupEnabled group) groups
+         then createImpurePath path
+         else path;
+
+    link = path: groupedLink null path;
+  };
+}