>> There is no code where to bind a dynamic variable, because its value
>> should be available for the next command in the command loop.
>> If you agree there is no other way to implement this than next-default-directory,
>> then I could bring up the discussion on emacs-devel.
>
> Before we dive into all that, why not try advice on 'command-execute'? For
> the PoC code at least. It's in Lisp since 2013.

Thanks for bringing up 'command-execute'.  I forgot it was moved from C to Lisp,
so the change is simpler and not needed to discuss on emacs-devel.  Then advice
on 'command-execute' will be required to support older Emacs versions in project.el.
But for Emacs 30 I modified my previous patch, and the next version is below:

> The comment in its body does say "Called directly from the C code", but I'm
> not sure if that has any direct implications for us.

Also interesting how 'command-execute' handles 'debug-on-next-call'
similar to 'next-default-directory'.