* lisp/vc/diff-mode.el (diff-setup-buffer-type): Move to diff-setup-whitespace

* lisp/vc/vc.el: Add declare-function for diff-setup-buffer-type (bug#51016).

lisp/vc/diff-mode.el

@@ -1475,21 +1475,6 @@ See `after-change-functions' for the meaning of BEG, END and LEN."
     ;; Added when diff--font-lock-prettify is non-nil!
     (cl-pushnew 'display font-lock-extra-managed-props)))
 
-(defun diff-setup-buffer-type ()
-  "Try to guess the `diff-buffer-type' from content of current Diff mode buffer.
-`outline-regexp' is updated accordingly."
-  (save-excursion
-    (goto-char (point-min))
-    (setq-local diff-buffer-type
-                (if (re-search-forward "^diff --git" nil t)
-                    'git
-                  nil)))
-  (when (eq diff-buffer-type 'git)
-    (setq diff-outline-regexp
-          (concat "\\(^diff --git.*\n\\|" diff-hunk-header-re "\\)"))
-    (setq-local outline-level #'diff--outline-level))
-  (setq-local outline-regexp diff-outline-regexp))
-
 (defvar whitespace-style)
 (defvar whitespace-trailing-regexp)
 
@@ -1591,6 +1576,21 @@ modified lines of the diff."
                     "^[-+!] .*?\\([\t ]+\\)$"
                   "^[-+!<>].*?\\([\t ]+\\)$"))))
 
+(defun diff-setup-buffer-type ()
+  "Try to guess the `diff-buffer-type' from content of current Diff mode buffer.
+`outline-regexp' is updated accordingly."
+  (save-excursion
+    (goto-char (point-min))
+    (setq-local diff-buffer-type
+                (if (re-search-forward "^diff --git" nil t)
+                    'git
+                  nil)))
+  (when (eq diff-buffer-type 'git)
+    (setq diff-outline-regexp
+          (concat "\\(^diff --git.*\n\\|" diff-hunk-header-re "\\)"))
+    (setq-local outline-level #'diff--outline-level))
+  (setq-local outline-regexp diff-outline-regexp))
+
 (defun diff-delete-if-empty ()
   ;; An empty diff file means there's no more diffs to integrate, so we
   ;; can just remove the file altogether.  Very handy for .rej files if we

lisp/vc/vc.el

@@ -739,6 +739,7 @@
 (require 'cl-lib)
 
 (declare-function diff-setup-whitespace "diff-mode" ())
+(declare-function diff-setup-buffer-type "diff-mode" ())
 
 (eval-when-compile
   (require 'dired))
@@ -1739,7 +1740,7 @@ to override the value of `vc-diff-switches' and `diff-switches'."
 	       (insert (cdr messages) ".\n")
 	       (message "%s" (cdr messages))))
 	(diff-setup-whitespace)
-        (diff-setup-buffer-type)
+	(diff-setup-buffer-type)
 	(goto-char (point-min))
 	(when window
 	  (shrink-window-if-larger-than-buffer window)))