bug#31558: 27.0; `custom-file' settings messed up by Emacs 27

[Michael Albinus <michael.albinus@gmx.de>]

Drew Adams <drew.adams@oracle.com> writes:

Hi Drew,

> Admittedly, this is probably not a problem that others will run into
> often.  But it is quite annoying for me.
>
> I think the problem was introduced in Emacs 27, but I'm not positive.

I suspect it is also in Emacs 26.1, see below.

> This is the problem:
>
> If I update a user option and then save the new value, it is saved to my
> `custom-file', as usual.  But thereafter my `custom-file' cannot be
> loaded by Emacs 20, because of these two entries that have been updated:
>
>  '(tramp-default-method "ftp" nil (tramp))
>  '(tramp-verbose 9 nil (tramp))

In Tramp 2.3 (integrated in Emacs 26.1), the defcustoms have changed:

--8<---------------cut here---------------start------------->8---
(defcustom tramp-default-method
  ...
  :group 'tramp
  :type 'string
  :require 'tramp) ;; new in Tramp 2.3.
--8<---------------cut here---------------end--------------->8---

The additional arguments are due to the ":require 'tramp", which was
added that time.

> But why must Emacs now add that "nil (tramp)" to my custom settings?
> Must defining the option value require the library?

IIRC, there was a reason for this. Don't remember the details :-(

> Should Emacs (e.g. 27) be doing that systematically?  If it should, then
> what's the best way for me to prevent it from doing that, for my use
> case?

Maybe you put at the beginning of your custom file

--8<---------------cut here---------------start------------->8---
;; This is for Emacs < 22
(or (locate-library "tramp")
    (provide 'tramp))
--8<---------------cut here---------------end--------------->8---

Best regards, Michael.