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

[Eshel Yaron via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi,

With both Emacs 29.2 and with the current master:

1. emacs -Q
2. (setq completion-styles '(substring))
3. C-u M-x customize-mode TAB
4. "Assertion failed: (stringp suffix)"

I not sure exactly why this error occurs, but IIUC it has to do with the
fact that customize-mode let-binds completion-regexp-list.  I suspect
that the substring completion style doesn't handle that correctly.

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


Best,

Eshel