bug#51469: 27.2; Mismatch: `set-face-attribute' and `face-all-attributes'

[Drew Adams <drew.adams@oracle.com>]

Why is the &rest argument ARGUMENTS of `set-face-attribute' a plist of
attributes, but `face-all-attributes' returns an alist of attributes?

Just doing something like the following isn't possible directly:

(apply #'set-face-attribute 'some-face
                            nil
                            (face-all-attributes 'other-face))

To accomplish that hand-off, you need to convert the alist returned by
`face-all-attributes' to a plist, and pass that to `set-face-attribute'.
And the conversion function needs to check that the attributes are
acceptable for `set-face-attribute' (else an error is raised).

Why should we need to do that each time, instead of just being able to
pass the result of one to the other?

E.g., something like `foo', applied to (face-all-attributes FACE1
FRAME1), should give a PLIST1 as expected by `set-frame-attribute',
using (apply #'set-face-attribute FACE2 FRAME2 PLIST1):

(defun foo (alist)
  "Return a plist of valid face attributes from ALIST.
ALIST is a list of conses with car a face attribute and with cdr
its value.  The returned PLIST is acceptable as an argument to
`set-face-attribute'."
  (let ((plist  ()))
    (dolist (pair  alist)
      (when (member (car pair) 
		    '(:family :foundry :width :height :weight
			      :slant :foreground :background
			      :underline :color :overline
			      :strike-through :box
			      :inverse-video :stipple :font
			      :inherit))
	(push (car pair) plist)
	(push (cdr pair) plist)))
    (setq plist  (nreverse plist))))

What other uses of these two functions would suggest that they should,
as they do, use different ways to express the list of face attributes?
Is there a good reason for this status quo?  If not, can we change it to
get a better fit and not require conversion?

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19042
System Description: Microsoft Windows 10 Pro (v10.0.2009.19042.1288)