bug#62563: [FR] Expose `interactive' arg handling as an Elisp function

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 31 Mar 2023 15:27:47 +0800
> From:  Ruijie Yu via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I find myself sometimes needing to manually write code that do the same
> job as the string-form `interactive' would do, like "read for an
> existing file name" (the "f" form), etc.  This happens because I want to
> do the following conversion.  
> 
> --8<---------------cut here---------------start------------->8---
> (defun foo (fname)
>   (interactive "f")
>   (ignore fname))
> 
> (defun foo (fname bar)
>   (interactive
>    (list (simulate-interactive-f)
>          (get-bar)))
>   (ignore fname bar))
> --8<---------------cut here---------------end--------------->8---
> 
> In short, this is useful when I need to add an interactive argument that
> is not already covered by interactive codes, so I have to use the more
> verbose interactive list form.

I don't think I understand the rationale, so please tell more.  The
interactive spec can be used like this:

  (interactive "bBuffer to rename: \nsRename buffer %s to: ")

So basically, you can prompt for anything using the 's' descriptor and
the following prompt string with %-constructs.  In what use cases is
this not enough, so much so that it would require exposing the guts of
this to Lisp?