bug#65386: [PATCH] ; Refine some 'package-vc' docstrings

[Eshel Yaron via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Philip Kaludercic <philipk@posteo.net> writes:

> Mauro Aranda <maurooaranda@gmail.com> writes:
>
>>
>> I haven't watch this thread closely, but do note that creating dynamic
>> choices is quite possible, if I understand correctly what you're looking
>> for.
>>
>> If you can, take a look at the defcustom of completion-styles, in
>> minibuffer.el.  Its type uses a choice with a specialized
>> :convert-widget function to keep the choices up-to date.
>
> This might be exactly what we are looking for.  It should be possible to
> adapt `completion--update-styles-options' pretty much directly to our
> example.

Mauro, thanks for the great tip.  As Philip said, this seems to be
exactly what we need here.  Accordingly, I'm attaching an updated patch for the
`defcustom` types (the two other patches from my previous message are
not affected and remain applicable).


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From 0550c73148b23135de47229435205914d8080786 Mon Sep 17 00:00:00 2001
From: Eshel Yaron <me@eshelyaron.com>
Date: Sun, 20 Aug 2023 16:20:54 +0200
Subject: [PATCH v2] ; Refine 'defcustom' types in 'package-vc'

Only include VC backends that support cloning in the ':type' of
'package-vc-heuristic-alist' and 'package-vc-default-backend', and
compute the list of relevant on demand to keep it fresh.

* lisp/emacs-lisp/package-vc.el (package-vc--cloning-backend-p)
(package-vc--update-backends): New functions.
(package-vc--backend-type): New 'defconst'.
(package-vc-heuristic-alist, package-vc-default-backend): Use it.
---
 lisp/emacs-lisp/package-vc.el | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/lisp/emacs-lisp/package-vc.el b/lisp/emacs-lisp/package-vc.el
index a3762d252b0..7c60c55d67f 100644
--- a/lisp/emacs-lisp/package-vc.el
+++ b/lisp/emacs-lisp/package-vc.el
@@ -62,6 +62,24 @@ package-vc
 (defconst package-vc--elpa-packages-version 1
   "Version number of the package specification format understood by package-vc.")
 
+(defun package-vc--cloning-backend-p (be)
+  "Return non-nil if the VC backend BE supports cloning repositories."
+  (or (vc-find-backend-function be 'clone)
+      (alist-get 'clone (get be 'vc-functions))))
+
+(defun package-vc--update-backends (widget)
+  "Update WIDGET with VC backends suitable for cloning VCS repositories."
+  (widget-put widget :args
+              (seq-keep (lambda (be)
+                          (when (package-vc--cloning-backend-p be)
+                            (widget-convert (list 'const be))))
+                        vc-handled-backends))
+  widget)
+
+(defconst package-vc--backend-type
+  '(choice :convert-widget package-vc--update-backends)
+  "The type of VC backends that support cloning package VCS repositories.")
+
 (defcustom package-vc-heuristic-alist
   `((,(rx bos "http" (? "s") "://"
           (or (: (? "www.") "github.com"
@@ -103,9 +121,7 @@ package-vc-heuristic-alist
 the URL-REGEXP of the association.  If no match is found,
 `package-vc-install' uses `package-vc-default-backend' instead."
   :type `(alist :key-type (regexp :tag "Regular expression matching URLs")
-                :value-type (choice :tag "VC Backend"
-                                    ,@(mapcar (lambda (b) `(const ,b))
-                                              vc-handled-backends)))
+                :value-type ,package-vc--backend-type)
   :version "29.1")
 
 (defcustom package-vc-default-backend 'Git
@@ -116,8 +132,7 @@ package-vc-default-backend
 
 The value must be a member of `vc-handled-backends' that supports
 the `clone' VC function."
-  :type `(choice ,@(mapcar (lambda (b) (list 'const b))
-                           vc-handled-backends))
+  :type package-vc--backend-type
   :version "29.1")
 
 (defcustom package-vc-register-as-project t
-- 
2.41.0