emacs-29 6b2720778dc 1/2: Improve tree-sitter's prev-sibling indent anchor

[Yuan Fu]

branch: emacs-29
commit 6b2720778dc9531c0157bc7e773d2011bdf905e3
Author: Yuan Fu <casouri@gmail.com>
Commit: Yuan Fu <casouri@gmail.com>

    Improve tree-sitter's prev-sibling indent anchor
    
    Now it handles the case where NODE is nil when indenting an empty
    line: it tries to get the previous sibling nonetheless.
    
    * lisp/progmodes/c-ts-mode.el (c-ts-mode--anchor-prev-sibling):
    * lisp/treesit.el (treesit-simple-indent-presets): Add an or form to
    handle more cases.
---
 lisp/progmodes/c-ts-mode.el | 12 +++++++++---
 lisp/treesit.el             | 12 ++++++++++--
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index 53f7839e4af..cc99b8e213e 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -257,7 +257,7 @@ is actually the parent of point at the moment of 
indentation."
         0
       c-ts-mode-indent-offset)))
 
-(defun c-ts-mode--anchor-prev-sibling (node &rest _)
+(defun c-ts-mode--anchor-prev-sibling (node parent bol &rest _)
   "Return the start of the previous named sibling of NODE.
 
 This anchor handles the special case where the previous sibling
@@ -273,8 +273,14 @@ The anchor of \"int y = 2;\" should be \"int x = 1;\" 
rather than
 the labeled_statement.
 
 Return nil if a) there is no prev-sibling, or 2) prev-sibling
-doesn't have a child."
-  (when-let ((prev-sibling (treesit-node-prev-sibling node t)))
+doesn't have a child.
+
+PARENT and BOL are like other anchor functions."
+  (when-let ((prev-sibling
+              (or (treesit-node-prev-sibling node t)
+                  (treesit-node-prev-sibling
+                   (treesit-node-first-child-for-pos parent bol) t)
+                  (treesit-node-child parent -1 t))))
     (while (and prev-sibling
                 (equal "labeled_statement"
                        (treesit-node-type prev-sibling)))
diff --git a/lisp/treesit.el b/lisp/treesit.el
index fe9ed399773..dbd102d00b3 100644
--- a/lisp/treesit.el
+++ b/lisp/treesit.el
@@ -1237,9 +1237,17 @@ See `treesit-simple-indent-presets'.")
                                           (line-beginning-position))
                         (throw 'term (point)))
                       (setq parent (treesit-node-parent parent)))))))
-        (cons 'prev-sibling (lambda (node &rest _)
+        (cons 'prev-sibling (lambda (node parent bol &rest _)
                               (treesit-node-start
-                               (treesit-node-prev-sibling node))))
+                               (or (treesit-node-prev-sibling node t)
+                                   ;; If node is nil (indenting empty
+                                   ;; line), we still try to guess the
+                                   ;; previous sibling.
+                                   (treesit-node-prev-sibling
+                                    (treesit-node-first-child-for-pos
+                                     parent bol)
+                                    t)
+                                   (treesit-node-child parent -1 t)))))
         (cons 'no-indent (lambda (_n _p bol &rest _) bol))
         (cons 'prev-line (lambda (_n _p bol &rest _)
                            (save-excursion