Re: custom-set-variables considered harmful

[Stefan Monnier]

> Here's a sample patch to do that.  Comments?

Based on the feedback, I added a config var to control the use of `setq'.
I also added a backward compatibility hack which makes it so that your
"new style config" will partly work in Emacs<27.

Any objection to installing the patch below into `master`?


        Stefan


diff --git a/lisp/cus-edit.el b/lisp/cus-edit.el
index 4965adfd56..d7166294c2 100644
--- a/lisp/cus-edit.el
+++ b/lisp/cus-edit.el
@@ -4374,6 +4382,23 @@ custom-file
 if only the first line of the docstring is shown."))
   :group 'customize)
 
+(defcustom custom-mimic-plain-elisp nil
+  "If non-nil, save user settings with the new format.
+This new format tries to mimick the code that would be written by hand
+in plain Elisp.  But it relies on `custom-autogenerated-user-settings' which
+is a new macro in Emacs-27, so settings saved with this will not work
+reliably in Emacs<27 (although a backward compatibility trick is used
+which should make them work to some extent)."
+  :type 'boolean
+  :group 'customize)
+
+(defcustom custom-mimic-plain-elisp-use-setq nil
+  "If non-nil, use `setq' when possible in generated code.
+If nil, `custom-mimic-plain-elisp' will only use `customize-set-variable',
+which is the more reliable option."
+  :type 'boolean
+  :group 'customize)
+
 (defun custom-file (&optional no-error)
   "Return the file name for saving customizations."
   (if (or (null user-init-file)
@@ -4501,10 +4526,32 @@ custom-save-delete
 
 (defvar sort-fold-case) ; defined in sort.el
 
+(defun custom--mimic-plain-elisp (symbol exp requests comment)
+  "Print the code corresponding to setting SYMBOL to EXP."
+  ;; Do the inverse conversion of custom-autogenerated-user-settings.
+  (let* ((e (cond
+             ((eq (get symbol 'custom-set) #'custom-set-minor-mode)
+              `(,symbol ,(if (memq exp '(t nil))
+                             (if exp 1 -1)
+                           '(if ,exp 1 -1))))
+             ((or (not custom-mimic-plain-elisp-use-setq)
+                  (get symbol 'custom-set))
+              `(customize-set-variable ',symbol ,exp))
+             ((local-variable-if-set-p
+               symbol (get-buffer-create "*scratch*"))
+              `(setq-default ,symbol ,exp))
+             (t `(setq ,symbol ,exp)))))
+    (dolist (e (nconc (mapcar (lambda (r) `(require ',r)) requests)
+                      (if comment (list comment e) (list e))))
+      (princ " ")
+      (pp e)
+      (unless (bolp) (princ "\n")))))
+
 (defun custom-save-variables ()
   "Save all customized variables in `custom-file'."
   (save-excursion
     (custom-save-delete 'custom-set-variables)
+    (custom-save-delete 'custom-autogenerated-user-settings)
     (let ((standard-output (current-buffer))
          (saved-list (make-list 1 0))
          sort-fold-case)
@@ -4519,8 +4566,21 @@ custom-save-variables
       (setq saved-list (sort (cdr saved-list) 'string<))
       (unless (bolp)
        (princ "\n"))
-      (princ "(custom-set-variables
- ;; custom-set-variables was added by Custom.
+      (when (and custom-mimic-plain-elisp
+                 (not (save-excursion
+                        (re-search-backward
+                         "custom-autogenerated-user-settings" nil t))))
+        (princ ";; Backward compatibility definition for Emacs<27")
+        (princ ";; auto-added by `custom-save-variables'.\n")
+        (pp '(unless (fboundp 'custom-autogenerated-user-settings)
+               (defalias 'custom-autogenerated-user-settings 'progn)))
+        (unless (bolp) (princ "\n")))
+      (princ (if custom-mimic-plain-elisp
+                 "(custom-autogenerated-user-settings
+ ;; This custom-autogenerated-user-settings was added by Custom."
+               "(custom-set-variables
+ ;; This custom-set-variables was added by Custom."))
+      (princ "
  ;; If you edit it by hand, you could mess it up, so be careful.
  ;; Your init file should contain only one such instance.
  ;; If there is more than one, they won't work right.\n")
@@ -4555,6 +4555,8 @@
            ;; with the customize facility.
            (unless (bolp)
              (princ "\n"))
+            (if custom-mimic-plain-elisp
+                (custom--mimic-plain-elisp symbol (car value) requests comment)
            (princ " '(")
            (prin1 symbol)
            (princ " ")
@@ -4576,7 +4578,7 @@
                (when comment
                  (princ " ")
                  (prin1 comment))))
-           (princ ")"))))
+             (princ ")")))))
       (if (bolp)
          (princ " "))
       (princ ")")
diff --git a/lisp/custom.el b/lisp/custom.el
index 352fc6bd53..b641740266 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1,4 +1,4 @@
-;;; custom.el --- tools for declaring and initializing options
+;;; custom.el --- tools for declaring and initializing options  -*- 
lexical-binding:t -*-
 ;;
 ;; Copyright (C) 1996-1997, 1999, 2001-2017 Free Software Foundation,
 ;; Inc.
@@ -926,6 +933,36 @@ custom-fix-face-spec
          (nreverse result))
       spec)))
 
+(defmacro custom-autogenerated-user-settings (&rest body)
+  "Install user customizations of variable values specified in ARGS.
+Expect the format output by `custom-save-variables'."
+  (let* ((settings '())
+         (requests '())
+         (comment nil)
+         (mk (lambda (x e)
+               (push
+                `'(,x ,e
+                      . ,(when (or comment requests)
+                           `(nil ,(prog1 (nreverse requests)
+                                    (setq requests '()))
+                                 . ,(when comment
+                                      (prog1 (list comment)
+                                        (setq comment nil))))))
+                settings))))
+    (dolist (e body)
+      (pcase e
+        (`(,(or 'setq 'setq-default) ,x ,e) (funcall mk x e))
+        (`(require ',p) (push p requests))
+        (`(customize-set-variable ',x ,e) (funcall mk x e))
+        (`(,x ,(and v (or 1 -1))) (funcall mk x (> v 0)))
+        (`(,x (if ,e 1 -1)) (funcall mk x e))
+        ((pred stringp)
+         (and comment (message "Dropping extra comment: %S" comment))
+         (setq comment e))
+        (_
+         (message "Unrecognized expression in 
custom-autogenerated-user-settings: %S" e))))
+    `(custom-set-variables ,@(nreverse settings))))
+
 (defun custom-set-variables (&rest args)
   "Install user customizations of variable values specified in ARGS.
 These settings are registered as theme `user'.