feat(system-defaults): warn once when Customize tries to save

custom-file points at a throwaway temp file so Customize edits never persist — deliberate, since config lives in Elisp, but silent. A user who clicks "Save for Future Sessions" loses the edit on exit with no hint. I advise custom-save-all (the chokepoint both customize-save-variable and the Customize button funnel through) with a one-shot :before warning that explains the discard and points at the Elisp init files. The advice removes itself after firing, so it warns once per session, and the body never runs at load, so startup stays quiet.

2 files changed, 36 insertions, 0 deletions

diff --git a/modules/system-defaults.el b/modules/system-defaults.el
index 8ef6181a..eccc6c35 100644
--- a/modules/system-defaults.el
+++ b/modules/system-defaults.el
@@ -104,6 +104,22 @@ Used to disable functionality with defalias \='somefunc \='cj/disabled)."
 (setq custom-file (make-temp-file
                    "emacs-customizations-trashbin-"))
 
+(defun cj/--warn-customize-discarded (&rest _)
+  "Warn once that Customize edits land in a throwaway `custom-file'.
+`custom-file' is a temp file that is never read back, so anything saved
+through the Customize interface is lost on exit.  This advice fires the
+first time `custom-save-all' writes, then removes itself so the warning
+appears only once per session."
+  (advice-remove 'custom-save-all #'cj/--warn-customize-discarded)
+  (display-warning
+   'cj/system-defaults
+   (concat "Customize edits are discarded: `custom-file' is a throwaway "
+           "temp file that is never loaded back. Edit configuration in the "
+           "Elisp init files instead.")
+   :warning))
+
+(advice-add 'custom-save-all :before #'cj/--warn-customize-discarded)
+
 ;; ------------------------- Re-Enabling Functionality -------------------------
 
 (put 'narrow-to-region 'disabled nil)               ;; narrow-to-region is extremely useful!
diff --git a/tests/test-system-defaults.el b/tests/test-system-defaults.el
index bc3694db..3c5e5977 100644
--- a/tests/test-system-defaults.el
+++ b/tests/test-system-defaults.el
@@ -64,5 +64,25 @@ they are actually wired onto the minibuffer hooks."
       (should (memq 'cj/minibuffer-setup-hook minibuffer-setup-hook))
       (should (memq 'cj/minibuffer-exit-hook minibuffer-exit-hook)))))
 
+;;; Customize-save warning
+
+(ert-deftest test-system-defaults-customize-save-warns-once ()
+  "Normal: the first custom-save-all warns; the second does not (one-shot).
+Customize writes land in a throwaway custom-file, so the user must be told
+their edit will not persist.  The advice removes itself after warning once."
+  (test-system-defaults--with-load-environment
+    (let ((warnings '()))
+      (cl-letf (((symbol-function 'display-warning)
+                 (lambda (group &rest _) (push group warnings)))
+                ((symbol-function 'custom-save-all) #'ignore))
+        (test-system-defaults--load)
+        ;; No warning merely from loading the module.
+        (should (null warnings))
+        (custom-save-all)
+        (should (equal warnings '(cj/system-defaults)))
+        ;; Second save must not warn again.
+        (custom-save-all)
+        (should (equal warnings '(cj/system-defaults)))))))
+
 (provide 'test-system-defaults)
 ;;; test-system-defaults.el ends here