New variable 'comment-setup-function' for multi-language modes.

* lisp/newcomment.el (comment-setup-function): New variable.
(comment-normalize-vars): Call non-nil 'comment-setup-function'.

* lisp/textmodes/mhtml-ts-mode.el
(mhtml-ts-mode--comment-current-lang): New internal variable.
(mhtml-ts-mode--comment-setup): New function.
(mhtml-ts-mode): Set 'comment-setup-function' to
'mhtml-ts-mode--comment-setup' instead of using
'c-ts-common-comment-setup' only for JavaScript.

https://lists.gnu.org/archive/html/emacs-devel/2025-05/msg00025.html

2 files changed, 26 insertions, 2 deletions

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 4da3b55d2f0..a6a1b7340a6 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -344,6 +344,9 @@ terminated by the end of line (i.e., `comment-end' is empty)."
   "Return the mirror image of string S, without any trailing space."
   (comment-string-strip (concat (nreverse (string-to-list s))) nil t))
 
+(defvar comment-setup-function nil
+  "Function to set up variables needed by commenting functions.")
+
 ;;;###autoload
 (defun comment-normalize-vars (&optional noerror)
   "Check and set up variables needed by other commenting functions.
@@ -351,6 +354,8 @@ All the `comment-*' commands call this function to set up various
 variables, like `comment-start', to ensure that the commenting
 functions work correctly.  Lisp callers of any other `comment-*'
 function should first call this function explicitly."
+  (when (functionp comment-setup-function)
+    (funcall comment-setup-function))
   (unless (and (not comment-start) noerror)
     (unless comment-start
       (let ((cs (read-string "No comment syntax is defined.  Use: ")))
diff --git a/lisp/textmodes/mhtml-ts-mode.el b/lisp/textmodes/mhtml-ts-mode.el
index d41f93e2682..6b261205409 100644
--- a/lisp/textmodes/mhtml-ts-mode.el
+++ b/lisp/textmodes/mhtml-ts-mode.el
@@ -356,6 +356,26 @@ Return nil if there is no name or if NODE is not a defun node."
      (js-name js-name)
      (css-name css-name))))
 
+(defvar-local mhtml-ts-mode--comment-current-lang nil)
+
+(defun mhtml-ts-mode--comment-setup ()
+  (let ((lang (treesit-language-at (point))))
+    (unless (eq mhtml-ts-mode--comment-current-lang lang)
+      (setq mhtml-ts-mode--comment-current-lang lang)
+      (pcase lang
+        ('html
+         (setq-local comment-start "<!-- ")
+         (setq-local comment-start-skip nil)
+         (setq-local comment-end " -->")
+         (setq-local comment-end-skip nil))
+        ('css
+         (setq-local comment-start "/*")
+         (setq-local comment-start-skip "/\\*+[ \t]*")
+         (setq-local comment-end "*/")
+         (setq-local comment-end-skip "[ \t]*\\*+/"))
+        ('javascript
+         (c-ts-common-comment-setup))))))
+
 ;;; Flymake integration
 
 (defvar-local mhtml-ts-mode--flymake-process nil
@@ -431,9 +451,8 @@ Powered by tree-sitter."
     ;; just like it's done in the original mode.
 
     ;; Comment.
-    ;; indenting settings for js-ts-mode.
-    (c-ts-common-comment-setup)
     (setq-local comment-multi-line t)
+    (setq-local comment-setup-function #'mhtml-ts-mode--comment-setup)
 
     ;; Font-lock.