Extend go-ts-mode with command to add docstring to function

go-ts-mode is extended with command go-ts-mode-docstring which adds
docstring comment to the defun at point. If a comment already exists,
the point is instead moved to the top-most comment line. The command
is bound to "C-c C-d".

* lisp/progmodes/go-ts-mode.el (go-ts-mode): Extend docstring.
(go-ts-mode-docstring): New function.
(go-ts-mode--comment-on-previous-line-p): New function.
(go-ts-mode-map): New map variable.
* etc/NEWS: Mention the change.

(bug#60805)

etc/NEWS

@@ -181,6 +181,14 @@ the new argument NEW-BUFFER non-nil, it will use a new buffer instead.
 Interactively, invoke 'eww-open-file' with a prefix argument to
 activate this behavior.
 
+** go-ts-mode
+
++++
+*** New command 'go-ts-mode-docstring'.
+This command adds a docstring comment to the current defun.  If a
+comment already exists, point is only moved to the comment.  It is
+bound to 'C-c C-d' in 'go-ts-mode'.
+
 
 * New Modes and Packages in Emacs 30.1
 

lisp/progmodes/go-ts-mode.el

@@ -177,9 +177,16 @@
 ;;;###autoload
 (add-to-list 'auto-mode-alist '("\\.go\\'" . go-ts-mode))
 
+(defvar-keymap go-ts-mode-map
+  :doc "Keymap used in Go mode, powered by tree-sitter"
+  :parent prog-mode-map
+  "C-c C-d" #'go-ts-mode-docstring)
+
 ;;;###autoload
 (define-derived-mode go-ts-mode prog-mode "Go"
-  "Major mode for editing Go, powered by tree-sitter."
+  "Major mode for editing Go, powered by tree-sitter.
+
+\\{go-ts-mode-map}"
   :group 'go
   :syntax-table go-ts-mode--syntax-table
 
@@ -274,6 +281,32 @@ Return nil if there is no name or if NODE is not a defun node."
    (not (go-ts-mode--struct-node-p node))
    (not (go-ts-mode--alias-node-p node))))
 
+(defun go-ts-mode-docstring ()
+  "Add a docstring comment for the current defun.
+The added docstring is prefilled with the defun's name.  If the
+comment already exists, jump to it."
+  (interactive)
+  (when-let ((defun-node (treesit-defun-at-point)))
+    (goto-char (treesit-node-start defun-node))
+    (if (go-ts-mode--comment-on-previous-line-p)
+        ;; go to top comment line
+        (while (go-ts-mode--comment-on-previous-line-p)
+          (forward-line -1))
+      (insert "// " (treesit-defun-name defun-node))
+      (newline)
+      (backward-char))))
+
+(defun go-ts-mode--comment-on-previous-line-p ()
+  "Return t if the previous line is a comment."
+  (when-let ((point (- (pos-bol) 1))
+             ((> point 0))
+             (node (treesit-node-at point)))
+    (and
+     ;; check point is actually inside the found node
+     ;; treesit-node-at can return nodes after point
+     (<= (treesit-node-start node) point (treesit-node-end node))
+     (string-equal "comment" (treesit-node-type node)))))
+
 ;; go.mod support.
 
 (defvar go-mod-ts-mode--syntax-table