bug#46374: Regression: erronous calls to PRED switch major-mode of unrelated modified buffers

[Juri Linkov]

> This means we need to invent some ad-hoc format to distinguish between
> these cases.  For example, to create a lexically-bound predicate
> at the beginning, it could be called with e.g.
>
>   (save-some-buffers t '(eval . save-some-buffers-root))
>
> and defcustom will look like:
>
> (defcustom save-some-buffers-default-predicate nil
>   :type '(choice (const :tag "Default" nil)
>                  (function :tag "Only in subdirs of root"
>                            (eval . save-some-buffers-root))
>                  (function :tag "Custom function"))

Or maybe simply '(save-some-buffers-root):

  (defcustom save-some-buffers-default-predicate nil
    :type '(choice (const :tag "Default" nil)
                   (function :tag "Only in subdirs of root"
                             (save-some-buffers-root))

Then the following two calls both will have exactly the same effect:

    (save-some-buffers t '(save-some-buffers-root))
    (save-some-buffers t (save-some-buffers-root))