Add setter for 'xref-marker-ring-length'

* lisp/progmodes/xref.el (xref-marker-ring-length): Add setter.
* etc/NEWS: Document last change.  (Bug#32849)

etc/NEWS

@@ -460,6 +460,11 @@ for example.
 This command finds definitions of the identifier at the place of a
 mouse click event, and is intended to be bound to a mouse event.
 
++++
+*** Changing 'xref-marker-ring-length' works after 'xref.el' is loaded.
+Previously, setting 'xref-marker-ring-length' would only take effect
+if set before 'xref.el' was loaded.
+
 ** Ecomplete
 
 *** The ecomplete sorting has changed to a decay-based algorithm.

lisp/progmodes/xref.el

@@ -317,8 +317,12 @@ backward."
 ;;; Marker stack  (M-. pushes, M-, pops)
 
 (defcustom xref-marker-ring-length 16
-  "Length of the xref marker ring."
-  :type 'integer)
+  "Length of the xref marker ring.
+If this variable is not set through Customize, you must call
+`xref-set-marker-ring-length' for changes to take effect."
+  :type 'integer
+  :initialize #'custom-initialize-default
+  :set #'xref-set-marker-ring-length)
 
 (defcustom xref-prompt-for-identifier '(not xref-find-definitions
                                             xref-find-definitions-other-window
@@ -354,6 +358,14 @@ elements is negated: these commands will NOT prompt."
 (defvar xref--marker-ring (make-ring xref-marker-ring-length)
   "Ring of markers to implement the marker stack.")
 
+(defun xref-set-marker-ring-length (var val)
+  "Set `xref-marker-ring-length'.
+VAR is the symbol `xref-marker-ring-length' and VAL is the new
+value."
+  (set-default var val)
+  (if (ring-p xref--marker-ring)
+      (ring-resize xref--marker-ring val)))
+
 (defun xref-push-marker-stack (&optional m)
   "Add point M (defaults to `point-marker') to the marker stack."
   (ring-insert xref--marker-ring (or m (point-marker))))