Make rust-mode derive from prog-mode if available

Author: Roy Crihfield

src/etc/emacs/rust-mode.el

@@ -306,8 +306,12 @@
             ((eq (rust-context-align cx) t) (+ (rust-context-column cx) (if closing -1 0)))
             (t (+ base (if closing 0 unit)))))))
 
+;; For compatibility with Emacs < 24, derive conditionally
+(defalias 'rust-parent-mode
+  (if (fboundp 'prog-mode) 'prog-mode 'fundamental-mode))
+
 ;;;###autoload
-(define-derived-mode rust-mode fundamental-mode "Rust"
+(define-derived-mode rust-mode rust-parent-mode "Rust"
   "Major mode for editing Rust source files."
   (set-syntax-table rust-syntax-table)
   (setq major-mode 'rust-mode mode-name "Rust")