Fix scrolling after rustfmt. (#351)

Co-authored-by: apiraino <apiraino@users.noreply.github.com>

Author: mookid

rust-mode.el

@@ -1569,7 +1569,12 @@ Return the created process."
 			  (rust--format-get-loc buffer point))
 		    window-loc))))))
     (unwind-protect
-        (rust--format-call (current-buffer))
+        ;; save and restore window start position
+        ;; after reformatting
+        ;; to avoid the disturbing scrolling
+        (let ((w-start (window-start)))
+          (rust--format-call (current-buffer))
+          (set-window-start (selected-window) w-start))
       (dolist (loc buffer-loc)
         (let* ((buffer (pop loc))
                (pos (rust--format-get-pos buffer (pop loc))))