bug#44611: Prefix arg for xref-goto-xref

[Juri Linkov]

>>    (defun next-error-quit-window ()
>>      (when (and (eq current-prefix-arg 0)
>>                 (not (eq next-error-last-buffer (current-buffer))))
>>        (let ((window (get-buffer-window next-error-last-buffer)))
>>          (when (window-live-p window)
>>            (with-selected-window window
>>              (quit-restore-window window))))))
>>    (add-hook 'next-error-hook 'next-error-quit-window)
>
> Since next-error-hook is a defcustom, perhaps we can/should include this
> function as one of the advertised values in :type?
>
> Even if we keep the list nil by default.

I completely forgot that we have 'next-error-found-function',
so implemented it with 'next-error-hook'.  I don't remember
why we have both.

Anyway, here is a patch with corrections from Martin:

diff --git a/lisp/simple.el b/lisp/simple.el
index 94d79c2bf1..73020c2c8e 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -377,11 +377,22 @@ next-error-internal
                 (not (eq prev next-error-last-buffer)))
         (message "Current locus from %s" next-error-last-buffer)))))
 
+(defun next-error-quit-window (from-buffer to-buffer)
+  "Quit window of FROM-BUFFER when the prefix arg is 0.
+Intended to be used in `next-error-found-function'."
+  (when (and (eq current-prefix-arg 0) from-buffer
+             (not (eq from-buffer to-buffer)))
+    (let ((window (get-buffer-window from-buffer)))
+      (when (window-live-p window)
+        (quit-restore-window window)))))
+
 (defcustom next-error-found-function #'ignore
   "Function called when a next locus is found and displayed.
 Function is called with two arguments: a FROM-BUFFER buffer
 from which next-error navigated, and a target buffer TO-BUFFER."
   :type '(choice (const :tag "No default" ignore)
+                 (const :tag "Quit previous window with M-0"
+                        next-error-quit-window)
                  (function :tag "Other function"))
   :group 'next-error
   :version "27.1")