bug#69501: 30.0.50; customize-mode error with substring completion style

[Stefan Monnier]

> In general, let-binding completion-regexp-list around completing-read is
> not a great idea IMO

Definitely: `completion-regexp-list` should be let-bound only right
around calls to `try/all-completions`.

> because it affects recursive minibuffers as well.
> So perhaps the best solution is to avoid doing that in customize-mode,
> and to rely only on the predicate argument of completing-read instead?

Indeed (see patch below).

We should probably also change the completion code so it defends against
such let-bindings of `completion-regexp-list`.


        Stefan


diff --git a/lisp/cus-edit.el b/lisp/cus-edit.el
index 8fad51dc116..f004002333b 100644
--- a/lisp/cus-edit.el
+++ b/lisp/cus-edit.el
@@ -1159,14 +1159,15 @@ customize-mode
 for the MODE to customize."
   (interactive
    (list
-    (let ((completion-regexp-list '("-mode\\'"))
-         (group (custom-group-of-mode major-mode)))
+    (let ((group (custom-group-of-mode major-mode)))
       (if (and group (not current-prefix-arg))
          major-mode
        (intern
         (completing-read (format-prompt "Mode" (and group major-mode))
                          obarray
-                         'custom-group-of-mode
+                         (lambda (s)
+                           (and (string-match "-mode\\'" (symbol-name s))
+                                (custom-group-of-mode s)))
                          t nil nil (if group (symbol-name major-mode))))))))
   (customize-group (custom-group-of-mode mode)))