bug#60805: [PATCH] Extend go-ts-mode with command to add docstring to function

[Evgeni Kolev]

I'm sending an updated patch. The improvement is that empty lines are
not considered comments. Example:
```
// unrelated comment

// docstring
func sum(a, b int) int {
    return a + b
}
```
With point in the function body, C-c C-d jumps to "// docstring"
instead of "// unrelated comment".

Patch is below.

>From 0c19a235c5775857cbf15676e3ee7196b158f0d1 Mon Sep 17 00:00:00 2001
From: Evgeni Kolev <evgenysw@gmail.com>
Date: Sat, 14 Jan 2023 08:28:06 +0200
Subject: [PATCH] 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.
---
 etc/NEWS                     |  8 ++++++++
 lisp/progmodes/go-ts-mode.el | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/etc/NEWS b/etc/NEWS
index f111d401df8..1715cbb83b1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -178,6 +178,14 @@ 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

diff --git a/lisp/progmodes/go-ts-mode.el b/lisp/progmodes/go-ts-mode.el
index 64e761d2f72..be5a69c2ec4 100644
--- a/lisp/progmodes/go-ts-mode.el
+++ b/lisp/progmodes/go-ts-mode.el
@@ -177,9 +177,16 @@ go-ts-mode--font-lock-settings
 ;;;###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 @@ go-ts-mode--other-type-node-p
    (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
-- 
2.30.2