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

[Eli Zaretskii]

> From: Eshel Yaron <me@eshelyaron.com>
> Cc: aaronjensen@gmail.com,  63988@debbugs.gnu.org,  Stefan Monnier
>  <monnier@iro.umontreal.ca>
> Date: Sat, 10 Jun 2023 12:07:22 +0300
> 
> > So maybe we should declare this feature a failed experiment and remove
> > it?
> 
> FWIW I think that make sense, but IMO it'd be best to only remove the
> treatment of `:eval` in `window_wants_header_line`, and keep the new
> treatment of `header-line-format` being a cons cell with a void or
> nil-valued variable car.  That's still useful because it works well with
> minor mode variables, and it's less risky as it doesn't involve
> evaluating arbitrary lisp, just inspecting a variable.

Why would it make sense to leave the non-nil car case?

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.