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

[Ruijie Yu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hello,

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.

Currently, the code that handles interactive codes is written as part of
`call-interactively'.  This is in src/callint.c,
DEFUN("call-interactively").

In fact, there is already _a way_ to do it (I consider it more like a
workaround).  However, I think my proposal might be more concise than
the workaround for readers.  And also more performant, since the
workaround unnecessarily creates a lambda and then extracts its
interactive form, only to make use of the result of its interactive
code.

--8<---------------cut here---------------start------------->8---
(call-interactively (lambda (f) (interactive "f") f))
--8<---------------cut here---------------end--------------->8---

If people are in favor of exposing the interactive codes, I imagine it
can be defined as `interactive-handle-code (code &optional prompt)', and
I would be able to do this:

--8<---------------cut here---------------start------------->8---
(interactive-handle-code ?f)
(interactive-handle-code ?M "Insert some random text: ")
;; etc
--8<---------------cut here---------------end--------------->8---

Alternatively, if we believe that my c-i + λ workaround is sufficiently
small, we could advise people in the same boat to follow suit?  That
implies modifying the `interactive' docstring and (info "(elisp)
Interactive Codes").

-- 
Best,


RY