Go back to the current line and column after formatting.

The previous approach simply moved to the same character offset. This is
unlikely to preserve the position of point, as rustfmt often changes
whitespace, changing the number of characters before point.

Instead, we go back to the line number and column number we were on
before. Provided that rustfmt has not radically changed the number of
lines, this will typically put point back to its previous position, or
at least close.

Improves, but doesn't completely solve, issue #162.

Author: Wilfred

rust-mode.el

@@ -1270,10 +1270,16 @@ This is written mainly to be used as `end-of-defun-function' for Rust."
   (unless (executable-find rust-rustfmt-bin)
     (error "Could not locate executable \"%s\"" rust-rustfmt-bin))
 
-  (let ((cur-point (point))
+  (let ((cur-line (line-number-at-pos))
+        (cur-column (current-column))
         (cur-win-start (window-start)))
     (rust--format-call (current-buffer))
-    (goto-char cur-point)
+    ;; Move to the same line and column as before.  This is best
+    ;; effort: if rustfmt inserted lines before point, we end up in
+    ;; the wrong place. See issue #162.
+    (goto-char (point-min))
+    (forward-line (1- cur-line))
+    (forward-char cur-column)
     (set-window-start (selected-window) cur-win-start))
 
   ;; Issue #127: Running this on a buffer acts like a revert, and could cause