Add the possibility to define assertions to be checked before activation.

nix/modules/system-manager.nix

@@ -14,6 +14,24 @@
       type = with lib.types; listOf str;
       default = [ ];
     };
+
+    preActivationAssertions = lib.mkOption {
+      type = with lib.types; attrsOf (submodule ({ name, ... }: {
+        options = {
+          enable = lib.mkEnableOption "the assertion";
+
+          name = lib.mkOption {
+            type = lib.types.str;
+            default = name;
+          };
+
+          script = lib.mkOption {
+            type = lib.types.str;
+          };
+        };
+      }));
+      default = { };
+    };
   };
 
   config = {
@@ -46,6 +64,27 @@
           }
         );
 
+    system-manager.preActivationAssertions = {
+      osVersion =
+        let
+          supportedIds = [ "nixos" "ubuntu" ];
+        in
+        {
+          enable = true;
+          script = ''
+            source /etc/os-release
+            ${lib.concatStringsSep "\n" (lib.flip map supportedIds (supportedId: ''
+              if [ $ID = "${supportedId}" ]; then
+                exit 0
+              fi
+            ''))}
+            echo "This OS is not currently supported."
+            echo "Supported OSs are: ${lib.concatStringsSep ", " supportedIds}"
+            exit 1
+          '';
+        };
+    };
+
     # Add the system directory for systemd
     system-manager.etcFiles = [ "systemd/system" ];