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

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Fri, 09 Jun 2023 21:09:45 -0400
> 
> 
> (setq header-line-format '(:eval (format-mode-line "")))
> 
> This causes Emacs to spin as of commit:
> 4f66cbbfe520ee31ef26676e09a926217d9736fe
> 
> After some time, it will segfault.

It causes infinite recursion, since format-mode-line also calls
window_wants_header_line (indirectly).

But what is the purpose of such a strange (to use a civilized word)
setting of header-line-format?  Why do you need :eval at all in this
case?

IOW, why not say "don't do that" and be done?