bug#64048: 30.0.50; global-display-line-numbers-mode has no easy way to set modes where it is enabled

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 64048@debbugs.gnu.org
> Date: Wed, 14 Jun 2023 12:48:54 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Let me correct my initial suggestion:
> >> 
> >> (define-globalized-minor-mode global-display-line-numbers-mode
> >>   display-line-numbers-mode display-line-numbers--turn-on
> >>   :predicate t)
> >
> > This change is probably harmless, but OTOH what does it give you that
> > the current code doesn't?  AFAIU, you still cannot customize the modes
> > where line numbers will be off.
> >
> > IOW, :predicate is not a means for user customizations of the modes
> > where the minor mode will be turned on.  Or am I missing something?
> 
> >> :predicate defines the default value of
> >> `global-display-line-numbers-modes' variable.
> >
> > It does?  Where is this documented?
> 
> Yup. It is not documented on API level, but when :predicate is non-nil,
> `define-globalized-minor-mode' does the following:
> 
> (setq turn-on-function
>       `(lambda ()
>          (require 'easy-mmode)
>          (when (easy-mmode--globalized-predicate-p ,MODE-predicate)
>            (funcall ,turn-on-function))))
> 
> ;; Minor mode docstring:
> (if predicate
>     (concat
>      "\n\n"
>      (internal--format-docstring-line
>       "`%s' is used to control which modes this minor mode is used in."
>       MODE-predicate)) ;; <global-mode-name>-modes
>   "")
> 
> ,(when predicate
>           `(defcustom ,MODE-predicate ,(car predicate)
>              ,(format "Which major modes `%s' is switched on in.
> This variable can be either t (all major modes), nil (no major modes),
> or a list of modes and (not modes) to switch use this minor mode or
> not.  For instance
> 
>   (c-mode (not message-mode mail-mode) text-mode)
> 
> means \"use this mode in all modes derived from `c-mode', don't use in
> modes derived from `message-mode' or `mail-mode', but do use in other
> modes derived from `text-mode'\".  An element with value t means \"use\"
> and nil means \"don't use\".  There's an implicit nil at the end of the
> list."
>                       mode)
>              :type '(repeat sexp)
>              ,@group))

Stefan, is our current documentation of :predicate incomplete?