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;