bug#63988: 30.0.50; Recent header line format changes cause spin/seg fault with format-mode-line

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: Eshel Yaron <me@eshelyaron.com>,  63988@debbugs.gnu.org,
>    aaronjensen@gmail.com,  monnier@iro.umontreal.ca
> Date: Wed, 05 Jul 2023 11:30:41 -0400
> 
> > It sounds like the consensus here is that indeed this feature is not
> > worth the complications, and so, unless I hear some good reasons not
> > to do so, I intend to delete it in a week's time.
> 
> Ugh, I am only just now seeing this... I would have appreciated a CC
> since I'm the one who requested this feature, or maybe a mail sent to
> the original bug report...
> 
> Anyway, the reason for this feature remains what is described in
> bug#63825: displaying a header line only when there is information to be
> displayed in the header line.

The discussion indicated that the reason for the feature, while it is
valid, is not serious enough to justify the complications described in
the thread.

> Concretely, for which-function-mode: it is nice to have
> which-func-format displayed in the header line, but in some kinds of
> buffers it does not have any information to display.  When a window is
> displaying such a buffer, I'd prefer to not have a header line, because
> the header line will just be empty.
> 
> This use case has no need for the full-fledged :eval mode of this
> feature.  It works fine with the non-nil car case.

That the car is nil doesn't necessarily mean that the header will not
show something.

> I could add this functionality to which-func-mode directly: it could
> learn to set header-line-format buffer-locally only in buffers that
> support which-func-mode.  Would that be acceptable?  If so I'll send a
> patch for that.

If you mean that some Lisp will set header-line-format nil when some
condition is true, I think it should be fine, assuming you can arrange
for this Lisp to run whenever the condition could change.