Check whether fileSystems defines the root FS

nixos/modules/system/boot/stage-1.nix

@@ -330,6 +330,11 @@ in
 
   config = {
 
+    assertions = singleton
+      { assertion = any (fs: fs.mountPoint == "/") (attrValues config.fileSystems);
+        message = "The ‘fileSystems’ option does not specify your root file system.";
+      };
+
     system.build.bootStage1 = bootStage1;
     system.build.initialRamdisk = initialRamdisk;
     system.build.extraUtils = extraUtils;

nixos/modules/tasks/filesystems.nix

@@ -81,6 +81,7 @@ in
   options = {
 
     fileSystems = mkOption {
+      default = {};
       example = {
         "/".device = "/dev/hda1";
         "/data" = {