bug#53636: 29.0.50; face-remapping broken on master

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 53636@debbugs.gnu.org,  tsdh@gnu.org
> Date: Sat, 05 Feb 2022 07:20:57 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > No, it isn't.  We had examples with hundreds of faces, back when we
> > decided to store faces in a hash-table.
> 
> Sorry, I don't follow -- examples of what that had hundreds of faces?

People reported they have hundreds of faces defined in their Emacs
sessions.  See this message, for example:

  https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01031.html

It reports to have 600 to 800 faces defined (with 129 in "emacs -Q").

> I'm saying that finding the faces that inherit from a specific face is
> not prohibitively expensive -- iterating over all the symbols and
> finding that info would be pretty fast.

You'd need to loop through all those hundreds of faces for each change
in face-remapping-alist.  Imagine what this will do to "C-x C-=" and
its ilk.

> If it's not, then extending defface to keep track of this
> information would be simple enough, too.

Still a major complication, since inheritance is recursive.

All that just to have one face that mode-line-* and header-line
etc. can inherit from?  Doesn't this look like a tail wagging the dog?