bug#66261: Disassembling a regexp's bytecode

[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?

Doesn't seem worth the trouble (and I must admit that the idea of using
a temp file hurts my sense of aesthetics, on top of it 😀.  Tho, it'd be
OK if it were done for me by gnulib).

>> - 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.)

OK, I'll make it conditional on ENABLE_CHECKING as well as on the
presence of `open_memstream`.


        Stefan