Make sure NODE is not the root node in tree-sitter indent (bug#60602)

There are two possible ways to solve the problem raised in the bug
report: either make sure NODE is never the root (so that parent is
never nil), or allow parent to be nil.

If we go with the latter, a lot of matcher and anchor functions need
change (they need to guard against a null parent).  I tried it, and
needing to check for null parent is pretty annoying.  In comparison,
if NODE is never the root, it is very convenient for the user, and it
doesn't complicate the rule that much (and it's rather intuitive,
people usually don't think of the case where NODE is the root node).
So that's what I choose.

* doc/lispref/modes.texi (Parser-based Indentation): Update manual.
* lisp/treesit.el (treesit-indent-function): Update docstring.
(treesit--indent-1): Make sure NODE is not the root.

2 files changed, 12 insertions, 9 deletions

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index b2dd294ea28..ff372edb3ff 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -4936,10 +4936,10 @@ Each @var{matcher} or @var{anchor} is a function that takes three
 arguments: @var{node}, @var{parent}, and @var{bol}.  The argument
 @var{bol} is the buffer position whose indentation is required: the
 position of the first non-whitespace character after the beginning of
-the line.  The argument @var{node} is the largest (highest-in-tree)
-node that starts at that position; and @var{parent} is the parent of
-@var{node}.  However, when that position is in a whitespace or inside
-a multi-line string, no node can start at that position, so
+the line.  The argument @var{node} is the largest node that starts at
+that position (and is not a root node); and @var{parent} is the parent
+of @var{node}.  However, when that position is in a whitespace or
+inside a multi-line string, no node can start at that position, so
 @var{node} is @code{nil}.  In that case, @var{parent} would be the
 smallest node that spans that position.
 
diff --git a/lisp/treesit.el b/lisp/treesit.el
index 7205e43916d..e53d5d53bd0 100644
--- a/lisp/treesit.el
+++ b/lisp/treesit.el
@@ -1341,10 +1341,10 @@ and returns
     (ANCHOR . OFFSET).
 
 BOL is the position of the beginning of the line; NODE is the
-\"largest\" node that starts at BOL; PARENT is its parent; ANCHOR
-is a point (not a node), and OFFSET is a number.  Emacs finds the
-column of ANCHOR and adds OFFSET to it as the final indentation
-of the current line.")
+\"largest\" node that starts at BOL (and isn't a root node);
+PARENT is its parent; ANCHOR is a point (not a node), and OFFSET
+is a number.  Emacs finds the column of ANCHOR and adds OFFSET to
+it as the final indentation of the current line.")
 
 (defun treesit--indent-1 ()
   "Indent the current line.
@@ -1362,10 +1362,13 @@ Return (ANCHOR . OFFSET).  This function is used by
                 ((treesit-language-at (point))
                  (treesit-node-at bol (treesit-language-at (point))))
                 (t (treesit-node-at bol))))
+         (root (treesit-parser-root-node
+                (treesit-node-parser smallest-node)))
          (node (treesit-parent-while
                 smallest-node
                 (lambda (node)
-                  (eq bol (treesit-node-start node))))))
+                  (and (eq bol (treesit-node-start node))
+                       (not (treesit-node-eq node root)))))))
     (let*
         ((parser (if smallest-node
                      (treesit-node-parser smallest-node)