Enable read-only-mode for built-in snippets

Because they shouldn't be edited out of ~/.emacs.d/.local -- if a
snippet in ~/.doom.d/snippets has the same name, built-in snippets will
be ignored.

modules/editor/snippets/autoload/snippets.el

@@ -102,6 +102,13 @@ buggy behavior when <delete> is pressed in an empty field."
       (yas-activate-extra-mode mode)
     (yas-deactivate-extra-mode mode)))
 
+;;;###autoload
+(defun +snippets|read-only-maybe ()
+  "Enable `read-only-mode' if snippet is built-in."
+  (when (file-in-directory-p default-directory doom-local-dir)
+    (read-only-mode 1)
+    (message "This is a built-in snippet, enabling read only mode. Use `yas-new-snippet' to redefine snippets")))
+
 
 ;;
 ;; Commands

modules/editor/snippets/config.el

@@ -45,6 +45,9 @@
     ;; tell smartparens overlays not to interfere with yasnippet keybinds
     (advice-add #'yas-expand :before #'sp-remove-active-pair-overlay))
 
+  ;; Enable `read-only-mode' for built-in snippets (in `doom-local-dir')
+  (add-hook 'snippet-mode-hook #'+snippets|read-only-maybe)
+
   (when (featurep! :editor evil)
     ;; evil visual-mode integration for `yas-insert-snippet'
     (define-key yas-minor-mode-map [remap yas-insert-snippet] #'+snippets/expand-on-region)))