bug#66261: Disassembling a regexp's bytecode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 28 Sep 2023 22:28:16 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> - First, in order to easily use the same code between REGEX_EMACS_DEBUG
>   and my new `re--describe-compiled`, I need to print sometimes to
>   `stderr` and sometimes to a string, which I do using `open_memstream`.
>   AFAIK `open_memstream` is not directly available in Windows (and
>   maybe under some other Unixes either, tho it's in POSIX-2008, IIUC).
>   Could someone help me get an `opem_memstream` emulation working
>   (maybe via gnulib)?

Gnulib doesn't have such an emulation, AFAICT.

Why cannot you fall back to temporary files when open_memstream is not
available?

> - I'm thinking of always providing this function.  Another option would
>   be to do it under the control of a compilation flag, tho it doesn't
>   seem worth adding a new flag just for that.  I guess we could
>   reuse REGEX_EMACS_DEBUG (tho it's too invasive IMO), or
>   ENABLE_CHECKING, but I'd rather just always offer the function.
>   After all, it might encourage users to look more carefully at their
>   regexps and maybe even to help us improve our regexp engine, who knows.

I would suggest to have it under ENABLE_CHECKING first, and only
remove the condition if there's a demand.  (I assume that most people
who debug regexps build Emacs with --enable-checking.)