bug#68824: treesitter support for outline-minor-mode

[Juri Linkov <juri@linkov.net>]

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> Instead of using treesit-search-forward, can you use
> treesit-beginning-of-thing or treesit--navigate-thing to do what you want?
> They handle the “child before parent” problem for you, and handles some
> other edge cases.

Thanks, I tried and it works.

Probably treesit--navigate-thing is not internal anymore.
Also the patch below uses treesit--thing-at that doesn't look
internal either.  So maybe two dashes could be removed from names.

Also a remaining question: why treesit-parent-until simply calls
‘(funcall pred node)’ instead of supporting the standard format
that includes a regexp by using treesit-node-match-p?

This causes such an inconvenience that for treesit-outline-level
that uses treesit-parent-until there is a need to wrap a regexp
in a lambda such as for html-ts-mode below.


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diff --git a/lisp/treesit.el b/lisp/treesit.el
index fab2ddd88e6..921e61b2160 100644
--- a/lisp/treesit.el
+++ b/lisp/treesit.el
@@ -2854,6 +2854,53 @@ treesit-simple-imenu
                     index))))
             treesit-simple-imenu-settings)))
 
+;;; Outline minor mode
+
+(defvar-local treesit-outline-predicate nil
+  "Predicate used to find outline headings in the syntax tree.
+Intended to be set by a major mode.  When nil, the predicate
+is constructed from the value of `treesit-simple-imenu-settings'
+when a major mode sets it.")
+
+(defun treesit-outline-search (&optional bound move backward looking-at)
+  "Search for the next outline heading in the syntax tree.
+See the descriptions of arguments in `outline-search-function'."
+  (if looking-at
+      (when-let* ((node (or (treesit--thing-at (pos-eol) treesit-outline-predicate)
+                            (treesit--thing-at (pos-bol) treesit-outline-predicate)))
+                  (start (treesit-node-start node)))
+        (eq (pos-bol) (save-excursion (goto-char start) (pos-bol))))
+
+    (let* ((pos
+            ;; When function wants to find the current outline, point
+            ;; is at the beginning of the current line.  When it wants
+            ;; to find the next outline, point is at the second column.
+            (if (eq (point) (pos-bol))
+                (if (bobp) (point) (1- (point)))
+              (pos-eol)))
+           (found (treesit--navigate-thing pos (if backward -1 1) 'beg
+                                           treesit-outline-predicate)))
+      (if found
+          (if (or (not bound) (if backward (>= found bound) (<= found bound)))
+              (progn
+                (goto-char found)
+                (goto-char (pos-bol))
+                (set-match-data (list (point) (pos-eol)))
+                t)
+            (when move (goto-char bound))
+            nil)
+        (when move (goto-char (or bound (if backward (point-min) (point-max)))))
+        nil))))
+
+(defun treesit-outline-level ()
+  "Return the depth of the current outline heading."
+  (let* ((node (treesit-node-at (point)))
+         (level (if (treesit-node-match-p node treesit-outline-predicate t)
+                    1 0)))
+    (while (setq node (treesit-parent-until node treesit-outline-predicate))
+      (setq level (1+ level)))
+    (if (zerop level) 1 level)))
+
 ;;; Activating tree-sitter
 
 (defun treesit-ready-p (language &optional quiet)
@@ -2984,6 +3031,23 @@ treesit-major-mode-setup
     (setq-local imenu-create-index-function
                 #'treesit-simple-imenu))
 
+  ;; Outline minor mode.
+  (when (and (or treesit-outline-predicate treesit-simple-imenu-settings)
+             (not (seq-some #'local-variable-p
+                            '(outline-search-function
+                              outline-regexp outline-level))))
+    (unless treesit-outline-predicate
+      (setq treesit-outline-predicate
+            (lambda (node)
+              (seq-some
+               (lambda (setting)
+                 (and (string-match-p (nth 1 setting) (treesit-node-type node))
+                      (or (null (nth 2 setting))
+                          (funcall (nth 2 setting) node))))
+               treesit-simple-imenu-settings))))
+    (setq-local outline-search-function #'treesit-outline-search
+                outline-level #'treesit-outline-level))
+
   ;; Remove existing local parsers.
   (dolist (ov (overlays-in (point-min) (point-max)))
     (when-let ((parser (overlay-get ov 'treesit-parser)))
diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index e5835bdb62d..ac0682a2dc7 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -1259,6 +1259,15 @@ c-ts-base-mode
                                 eos)
                    c-ts-mode--defun-for-class-in-imenu-p nil))))
 
+  (setq-local treesit-outline-predicate
+              (lambda (node)
+                (and (treesit-node-match-p
+                      node "\\`function_declarator\\'" t)
+                     (when-let ((parent (treesit-node-parent node)))
+                       (treesit-node-match-p
+                        parent
+                        "\\`function_definition\\'" t)))))
+
   (setq-local treesit-font-lock-feature-list
               c-ts-mode--feature-list))
 
diff --git a/lisp/textmodes/html-ts-mode.el b/lisp/textmodes/html-ts-mode.el
index 301f3e8791c..71fed15f2fc 100644
--- a/lisp/textmodes/html-ts-mode.el
+++ b/lisp/textmodes/html-ts-mode.el
@@ -121,6 +121,16 @@ html-ts-mode
   ;; Imenu.
   (setq-local treesit-simple-imenu-settings
               '(("Element" "\\`tag_name\\'" nil nil)))
+
+  ;; Outline minor mode.
+  (setq-local treesit-outline-predicate
+              (lambda (node) (treesit-node-match-p
+                              node "\\`element\\'" t)))
+  ;; Restore default value for `treesit-outline-search'.
+  (kill-local-variable 'outline-regexp)
+  (kill-local-variable 'outline-heading-end-regexp)
+  (kill-local-variable 'outline-level)
+
   (treesit-major-mode-setup))
 
 (if (treesit-ready-p 'html)