nixos/version: Warn about using the default of system.stateVersion

nixos/modules/misc/version.nix

@@ -146,6 +146,15 @@ in
       "/etc/os-release".source = initrdRelease;
       "/etc/initrd-release".source = initrdRelease;
     };
+
+    # We have to use `warnings` because when warning in the default of the option
+    # the warning would also be shown when building the manual since the manual
+    # has to evaluate the default.
+    #
+    # TODO Remove this and drop the default of the option so people are forced to set it.
+    # Doing this also means fixing the comment in nixos/modules/testing/test-instrumentation.nix
+    warnings = lib.optional (options.system.stateVersion.highestPrio == (lib.mkOptionDefault { }).priority)
+      "system.stateVersion is not set, defaulting to ${config.system.stateVersion}. Read why this matters on https://nixos.org/manual/nixos/stable/options.html#opt-system.stateVersion.";
   };
 
   # uses version info nixpkgs, which requires a full nixpkgs path

nixos/modules/testing/test-instrumentation.nix

@@ -129,6 +129,9 @@ in
     # Make sure we use the Guest Agent from the QEMU package for testing
     # to reduce the closure size required for the tests.
     services.qemuGuest.package = pkgs.qemu_test.ga;
+
+    # Squelch warning about unset system.stateVersion
+    system.stateVersion = lib.mkDefault lib.trivial.release;
   };
 
 }