bug#67171: 30.0.50; (At least) some VC commands fail with project-prefix-or-any-command

[Juri Linkov]

>>>>> As an aside, it doesn't seem good that whether or not step four prompts
>>>>> you for a directory depends on whether the current buffer happens to be
>>>>> visiting a file or not.  Can we improve that, or is it inherent to the
>>>>> design of the new feature?
>>>> This was improved recently in bug#67145, so now you can set
>>>> 'vc-deduce-backend-nonvc-modes' to nil to avoid this prompt.
>>>> I'm not sure if nil should be the default value.
>>> I wonder if the "deduce in all modes" value should be t rather than nil.
>> Good idea since nil could be used to disable deduction completely.
>> Then maybe it should be defcustom?
>
> Why not.

Maybe something like this:

diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 1bd9ecb2193..bde22536d1f 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1071,19 +1071,23 @@ log-edit-vc-backend
 (defvar diff-vc-backend)
 (defvar diff-vc-revisions)
 
-;; Maybe we could even use comint-mode rather than shell-mode?
-(defvar vc-deduce-backend-nonvc-modes
+(defcustom vc-deduce-backend-nonvc-modes
+  ;; Maybe we could even use comint-mode rather than shell-mode?
   '(dired-mode shell-mode eshell-mode compilation-mode)
   "List of modes not supported by VC where backend should be deduced.
 In these modes the backend is deduced based on `default-directory'.
-When nil, the backend is deduced in all modes.")
+When t, the backend is deduced in all modes."
+  :type '(choice (const :tag "None" nil)
+                hook
+                (const :tag "All" t))
+  :version "30.1")
 
 (defun vc-deduce-backend ()
   (cond ((derived-mode-p 'vc-dir-mode)   vc-dir-backend)
        ((derived-mode-p 'log-view-mode) log-view-vc-backend)
        ((derived-mode-p 'log-edit-mode) log-edit-vc-backend)
        ((derived-mode-p 'diff-mode)     diff-vc-backend)
-       ((or (null vc-deduce-backend-nonvc-modes)
+       ((or (eq vc-deduce-backend-nonvc-modes t)
             (derived-mode-p vc-deduce-backend-nonvc-modes))
         (ignore-errors (vc-responsible-backend default-directory)))
        (vc-mode (vc-backend buffer-file-name))))