Allow offset in tree-sitter indent rules to be functions

This is needed for fixing C indentation.  See next comment.

* doc/lispref/modes.texi (Parser-based Indentation): Update manual.
* lisp/treesit.el (treesit-simple-indent): Try evaluating OFFSET as a
function if it's not integer nor variable.

2 files changed, 17 insertions, 10 deletions

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index ff372edb3ff..fe5eb8a1b8d 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -4926,8 +4926,7 @@ the current line to @var{matcher}; if it returns non-@code{nil}, this
 rule is applicable.  Then Emacs passes the node to @var{anchor}, which
 returns a buffer position.  Emacs takes the column number of that
 position, adds @var{offset} to it, and the result is the indentation
-column for the current line.  @var{offset} can be an integer or a
-variable whose value is an integer.
+column for the current line.
 
 The @var{matcher} and @var{anchor} are functions, and Emacs provides
 convenient defaults for them.
@@ -4943,10 +4942,13 @@ 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.
 
-Emacs finds @var{bol}, @var{node} and @var{parent} and
-passes them to each @var{matcher} and @var{anchor}.  @var{matcher}
-should return non-@code{nil} if the rule is applicable, and
-@var{anchor} should return a buffer position.
+@var{matcher} should return non-@code{nil} if the rule is applicable,
+and @var{anchor} should return a buffer position.
+
+@var{offset} can be an integer, a variable whose value is an integer,
+or a function that returns an integer.  If it is a function, it is
+passed @var{node}, @var{parent}, and @var{bol}, like matchers and
+anchors.
 @end defvar
 
 @defvar treesit-simple-indent-presets
diff --git a/lisp/treesit.el b/lisp/treesit.el
index 5b306354465..ffc78b93032 100644
--- a/lisp/treesit.el
+++ b/lisp/treesit.el
@@ -1511,10 +1511,15 @@ OFFSET."
                return
                (let ((anchor-pos
                       (treesit--simple-indent-eval
-                       (list anchor node parent bol))))
-                 (cons anchor-pos (if (symbolp offset)
-                                      (symbol-value offset)
-                                    offset)))
+                       (list anchor node parent bol)))
+                     (offset-val
+                      (cond ((numberp offset) offset)
+                            ((and (symbolp offset)
+                                  (boundp offset))
+                             (symbol-value offset))
+                            (t (treesit--simple-indent-eval
+                                (list offset node parent bol))))))
+                 (cons anchor-pos offset-val))
                finally return
                (progn (when treesit--indent-verbose
                         (message "No matched rule"))