bug#64442: 29.0.92; treesit-beginning-of-defun fails in DEFUN functions in C

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Tue, 4 Jul 2023 01:41:22 -0700
> Cc: 64442@debbugs.gnu.org
> 
> The problem with DEFUN’s is that a DEFUN is really made of two nodes in the parse tree. One for the DEFUN part, one for the body, and there isn’t a parent node that encloses the two. 
> 
> The defun movement functions are not designed to handle a construct made of two adjacent nodes. They can find a node, go to the beginning/end of it; they can’t find a node, and go to the end of the next node.
> 
> It sounds easy to add some hack to handle it, but really isn’t. Defun movement need to support forward/backward to beg/end, that’s four movement types;

Why cannot we look for a top-level expression_statement node which is
a call_expression whose function identifier is "DEFUN" and whose
position is between the place where C-M-a was invoked and the place
where it does find a defun?

> on top of that you have nested defun’s.

DEFUN's cannot be nested, so we don't need to consider that.

Thanks.