bug#66716: Reconcile display-fill-column-indicator-character behavior and customization description

[Joseph Turner]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Mon, 23 Oct 2023 14:23:06 -0700
>> From:  Joseph Turner via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>> AFAICT, setting display-fill-column-indicator-character to nil means
>> that no fill indicator is displayed.
>
> That's not true, AFAICT.  The default value is nil, but when you turn
> on the mode, it modifies the value of the character to be either
> U+2502 or '|'.
>
>> This patch changes the customization description to match the
>> current behavior.
>
> I don't think the patch is correct.

Indeed, the patch is incorrect.  I misunderstood the behavior of
display-fill-column-indicator-mode when
display-fill-column-indicator-character is nil.

>> However, I like the behavior that's currently described in the
>> customization option:
>>
>> "If possible, use U+2502 to indicate fill column, otherwise use |"
>>
>> What do folks think about changing the behavior of
>> display-fill-column-indicator-character to match this description?
>
> I don't understand the proposal.  We don't use U+2502 by default
> because not all terminals support it.

When enabling display-fill-column-indicator in .dir-locals.el, is it
possible to dynamically set display-fill-column-indicator-character
according to what characters are displayable without using eval?

Currently in my project, I have:

((emacs-lisp-mode . ((display-fill-column-indicator . t)
                     ;; This won't work on some terminals
                     (display-fill-column-indicator-character . ?\u2502)
                     (fill-column . 80))))

Please let me know if you have suggestions for improvement!

Thank you!!!

Joseph