bug#81533: c-ts-mode: Improved block comment indentation

From 95c85e3deb1a63eacc0ba47b7e92498823b5ad2a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Lindqvist?= <[email protected]>
Date: Sun, 2 Aug 2026 02:18:22 +0200
Subject: [PATCH] c-ts-mode: Improved block comment indentation

1) lines beginning with "*" are aligned to the first "*"
2) lines following the first or blank lines are indented three spaces
3) other lines are aligned to the previous line

* lisp/progmodes/c-ts-mode.el (c-ts-mode--simple-indent-rules): Improved
  block comment indentation.
---
 lisp/progmodes/c-ts-mode.el                   | 28 +++++++++++------
 .../progmodes/c-ts-mode-resources/indent.erts | 30 +++++++++++++------
 2 files changed, 40 insertions(+), 18 deletions(-)

diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index 5f064716a89..7c281c1e508 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -473,6 +473,24 @@ c-ts-mode--emacs-macro-rules
     (cons (treesit-node-start parent)
           c-ts-indent-offset))))
 
+(defun c-ts-mode--block-comment-offset (_n parent bol &rest _)
+  "Indentation offset for lines in block comments.
+
+One space if line starts with \"*\", three if the previous line is blank
+or the first line of the comment, and otherwise same as previous line."
+  (save-excursion
+    (beginning-of-line)
+    (let* ((c-point (treesit-node-start parent))
+	   (starred? (looking-at-p (rx (* blank) "*")))
+	   (first-or-second? (<= (line-beginning-position 0) c-point)))
+      (forward-line -1)
+      (let ((prev-indent (current-indentation)))
+        (cond (starred? 1)
+              ((or first-or-second? (= prev-indent 0)) 3)
+              (t (- prev-indent (progn (goto-char c-point)
+                                       (current-column)))))))))
+
+
 (defun c-ts-mode--simple-indent-rules (mode style)
   "Return the indent rules for MODE and STYLE.
 
@@ -513,15 +531,7 @@ c-ts-mode--simple-indent-rules
            ;; ((match nil "function_declarator" "parameters") parent 0)
            ;; ((parent-is "template_declaration") parent 0)
 
-           ;; `c-ts-common-looking-at-star' has to come before
-           ;; `c-ts-common-comment-2nd-line-matcher'.
-           ;; FIXME: consolidate into a single rule.
-           ((and (parent-is "comment") c-ts-common-looking-at-star)
-            c-ts-common-comment-start-after-first-star -1)
-           (c-ts-common-comment-2nd-line-matcher
-            c-ts-common-comment-2nd-line-anchor
-            1)
-           ((parent-is "comment") prev-adaptive-prefix 0)
+           ((parent-is "comment") parent c-ts-mode--block-comment-offset)
 
            ;; Preproc directives
            ((node-is "preproc_arg") no-indent)
diff --git a/test/lisp/progmodes/c-ts-mode-resources/indent.erts b/test/lisp/progmodes/c-ts-mode-resources/indent.erts
index 3d875e3113e..fc956b770e8 100644
--- a/test/lisp/progmodes/c-ts-mode-resources/indent.erts
+++ b/test/lisp/progmodes/c-ts-mode-resources/indent.erts
@@ -342,11 +342,11 @@ Name: Multiline Block Comments 1 (bug#60270)
  */
 =-=-=
 
-Name: Multiline Block Comments 2 (bug#60270)
+Name: Multiline Block Comments 2 (bug#60270, bug#81533)
 
 =-=
 /*
-  some comment
+   some comment
  */
 =-=-=
 
@@ -365,7 +365,7 @@ Name: Multiline Block Comments 4 (bug#60270)
  */
 =-=-=
 
-Name: Multiline Block Comments 5 (bug#60270)
+Name: Multiline Block Comments 5 (bug#60270, bug#81533)
 
 =-=
 /*
@@ -374,12 +374,24 @@ line 2
  */
 =-=
 /*
-  line one
-  line 2
+   line one
+   line 2
  */
 =-=-=
 
-Name: Block Comment prefixes (Bug#61314)
+Name: Dangler (bug#81533)
+
+=-=
+int put; /* line one
+
+line 2 */
+=-=
+int put; /* line one
+
+            line 2 */
+=-=-=
+
+Name: Block Comment prefixes (Bug#61314, bug#81533)
 
 =-=-=
 /*
@@ -389,9 +401,9 @@ Name: Block Comment prefixes (Bug#61314)
 */
 =-=-=
 /*
-  - item1
-  - item2
-  - item3
+   - item1
+   - item2
+   - item3
  */
 =-=-=
 
-- 
2.55.0