Add stderr output from rustfmt on exit code 3

Author: Christian Howe

rust-mode.el

@@ -1195,13 +1195,25 @@ This is written mainly to be used as `end-of-defun-function' for Rust."
   (with-current-buffer (get-buffer-create "*rustfmt*")
     (erase-buffer)
     (insert-buffer-substring buf)
-    (let ((ret (call-process-region (point-min) (point-max) rust-rustfmt-bin t '(t nil) nil)))
-      (if (or (zerop ret) (= ret 3))
-          (progn
-            (if (not (string= (buffer-string) (with-current-buffer buf (buffer-string))))
-                (copy-to-buffer buf (point-min) (point-max)))
-            (kill-buffer))
-        (error "Rustfmt failed, see *rustfmt* buffer for details")))))
+    (let ((tmpf (make-temp-file "rustfmt")))
+      (let ((ret (call-process-region (point-min) (point-max) rust-rustfmt-bin
+                                      t `(t ,tmpf) nil)))
+        (format-message "%d" ret)
+        (cond
+         ((zerop ret)
+          (error "Rustfmt failed, see *rustfmt* buffer for details"))
+         ((= ret 3)
+          (if (not (string= (buffer-string)
+                            (with-current-buffer buf (buffer-string))))
+              (copy-to-buffer buf (point-min) (point-max)))
+          (erase-buffer)
+          (insert-file-contents tmpf)
+          (error "Rustfmt could not format some lines, see *rustfmt* buffer for details"))
+         (t
+          (if (not (string= (buffer-string)
+                            (with-current-buffer buf (buffer-string))))
+              (copy-to-buffer buf (point-min) (point-max)))
+          (kill-buffer)))))))
 
 (defconst rust--format-word "\\b\\(else\\|enum\\|fn\\|for\\|if\\|let\\|loop\\|match\\|struct\\|unsafe\\|while\\)\\b")
 (defconst rust--format-line "\\([\n]\\)")