New tree-sitter indent anchor standalone-parent used by c-ts-mode

When writing c-ts-mode Theo used parent-bol which works well except
one case:

1 for (int i=0;
2      i < 5;
3      i++) {
4   func(i);
5 }

In this case, when indenting "func(i)", parent-bol returns the start
of "i++" on line 3, instead of the "correct" anchor, the start of
"for" on line 1.  parent-bol would have worked if the "for (...) {"
part is in one line.

To support this case I tried numerous things and added a bunch of
stuff, culminating in c-ts-common-statement-offset.  It's complicated,
requires extra setup, and slow.

Not anymore! I think the new anchor standalone-parent really captures
the logic behind how people expect indentation to work. It's simple
and fast, and requires no setup.

* doc/lispref/modes.texi (Parser-based Indentation): Update manual.

* lisp/progmodes/c-ts-mode.el:
(c-ts-mode--standalone-grandparent): New anchor.
(c-ts-mode--indent-styles): Replace c-ts-common-statement-offset with
standalone-parent.
(c-ts-base-mode): Add comment.

* lisp/treesit.el:
(treesit-simple-indent-presets): New anchor standalone-parent.

1 files changed, 9 insertions, 1 deletions

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 4c40f414ca0..11892aaa40e 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -5093,7 +5093,15 @@ This anchor is a function that is called with 3 arguments: @var{node},
 @item parent-bol
 This anchor is a function that is called with 3 arguments: @var{node},
 @var{parent}, and @var{bol}, and returns the first non-space character
-on the line of @var{parent}.
+on the line which @var{parent}'s start is on.
+
+@item parent-bol
+This anchor is a function that is called with 3 arguments: @var{node},
+@var{parent}, and @var{bol}.  It finds the first ancestor node
+(parent, grandparent, etc) of @var{node} that starts on its own line,
+and return the start of that node.  ``Starting on its own line'' means
+there is only whitespace character before the node on the line which
+the node's start is on.
 
 @item prev-sibling
 This anchor is a function that is called with 3 arguments: @var{node},