emacs: use use-package for rust-mode

- Add note pertaining to weird behavior with rustc+direnv

config/emacs/modules/rust.nix

@@ -23,9 +23,15 @@ with lib;
     ];
 
     init.rust = ''
-      (require 'rust-mode)
+      (use-package rust-mode)
       (use-package rustic)
 
+      ;; TODO direnv keeps complaining about the various temp folders that direnv+cargo seem to create not existing
+      ;; 1) What creates these?
+      ;; 2a) Can we ignore the errors?
+      ;; 2b) Can we ensure these folders are either created normally when editing Rust files in emacs?
+      ;; 2c) Can we prevent these folders from being created, if it doesn't affect my current workflow? (direnv+cargo)
+
       (setq rustic-lsp-client 'eglot)
       (setq rustic-lsp-server 'rust-analyzer)
       (setq rustic-compile-backtrace 1)