Implement files under /etc

nix/modules/default.nix

@@ -1,5 +1,6 @@
 { lib
 , pkgs
+, config
 , ...
 }:
 let
@@ -30,16 +31,42 @@ let
             '';
           })
       );
-in
-{
-  options = {
-    system-manager.services = lib.mkOption {
-      type = with lib.types; listOf str;
+
+  etcFiles = {
+    foo = {
+      text = ''
+        This is just a test!
+      '';
+      target = "foo_test";
+    };
+
+    "baz/bar/foo2" = {
+      text = ''
+        Another test!
+      '';
+      mode = "symlink";
+    };
+
+    foo3 = {
+      text = "boo!";
+      mode = "0700";
+      user = "root";
+      group = "root";
+    };
+
+    out-of-store = {
+      source = "/run/systemd/system/";
     };
   };
-
+in
+{
   config = {
-    system-manager.services = lib.attrNames services;
+    system.stateVersion = lib.trivial.release;
+    system-manager = {
+      etcFiles = lib.attrNames etcFiles;
+      services = lib.attrNames services;
+    };
+    environment.etc = etcFiles;
     systemd = { inherit services; };
   };
 }

nix/modules/system-manager.nix

@@ -0,0 +1,34 @@
+{ lib
+, pkgs
+, config
+, ...
+}:
+{
+  options.system-manager = {
+    services = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+
+    etcFiles = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+  };
+
+  config = {
+    # Avoid some standard NixOS assertions
+    boot = {
+      loader.grub.enable = false;
+      initrd.enable = false;
+    };
+
+    assertions = lib.flip map config.system-manager.etcFiles (entry:
+      {
+        assertion = lib.hasAttr entry config.environment.etc;
+        message = lib.concatStringsSep " " [
+          "The entry ${entry} that was passed to system-manager.etcFiles"
+          "is not present in environment.etc"
+        ];
+      }
+    );
+  };
+}