Require rust-ts-mode carefully

Fix rust-lang#524

Author: condy0919

rust-mode-treesitter.el

@@ -6,7 +6,8 @@
 ;;; Code:
 
 ;;;###autoload
-(when (version<= "29.1" emacs-version)
+(when (and (version<= "29.1" emacs-version)
+           (treesit-language-available-p 'rust))
   ;; We have the when macro because of
   ;; https://github.com/rust-lang/rust-mode/issues/520
   (require 'treesit)