bug#64347: 30.0.50; Some customize faces shown as edited with -Q

[Mauro Aranda <maurooaranda@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

 >> Date: Sun, 9 Jul 2023 08:44:23 -0300
 >> Cc: 64347@debbugs.gnu.org, stephen.berman@gmx.net
 >> From: Mauro Aranda <maurooaranda@gmail.com>
 >>
 >> Eli Zaretskii <eliz@gnu.org> writes:
 >>
 >>  > I guess I'm missing something here: why do we need those pre-filter
 >>  > and post-filter conversions?  The C code understands both forms of
 >>  > :line-width, so there should be no need for Lisp to do any
 >>  > conversions, right?  So why do we do that? why not simply leave the
 >>  > spec as it was originally?
 >>
 >> Custom needs the pre-filter in order to present a Custom buffer to edit
 >> the face.
 >> Let's say there's a face:
 >> (defface foo
 >>    '((t (:box (:line-width 1 :color "black"))))
 >>    "...")
 >>
 >> And let's say a user wants to customize it via Custom.
 >> M-x customize-face RET foo
 >> should show the user a buffer with all the capabilities to edit it.
 >> Because we have an integer for the :line-width property, the user will
 >> be presented with an integer Widget to edit the value, without giving
 >> the user the opportunity to edit the HWIDTH and VWIDTH separately.
 >>
 >> So the pre-filter takes the (:line-width 1), and converts it into
 >> (:line-width (1 . 1)), and now the Custom buffer will have
 >> a cons Widget.  If we didn't do that conversion, that would be a Bug
 >> report, I'm sure.
 >
 > OK, but why does it have to do that on the original value? It could
 > do that on a copy that serves for the display and editing, in which
 > case the original value could be left intact if the user didn't change
 > it or did change, but didn't click Apply.  (If the user does modify
 > the original value, then any conversions are okay, since the variable
 > is really "edited".)

I think my description was inaccurate, because it seemed to imply that
it is a destructive operation.  It is not, it leaves the original value
intact.

But when deciding to set a state, Custom always consults the spec built
from the data the face Widget has.