bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org,  jidanni@jidanni.org
> Date: Mon, 17 Jan 2022 18:38:48 +0100
> 
> I'm looking at the Confusable section now. 
> 
> https://www.unicode.org/reports/tr39/#Confusable_Detection
> 
> Looks easy enough to implement (and the ELPA package already does the
> parsing, so I'll be reusing bits from that).
> 
> But...  I'm wondering what the higher level interface would be?  I mean,
> quite a lot of strings are confusable with something else, but which
> ones are interesting?  The only thing that seems immediately interesting
> to check for is whether a string is confusable with ASCII?
> 
> That is,
> 
> (textsec-confusable-with-ascii-p "C𝗂𝗋𝖼𝗅𝖾")
> => t
> 
> Because the ASCII characters are the ones that people rely on when doing
> ... things, like email and browsing the web.
> 
> But I mean, "C𝗂𝗋𝖼𝗅𝖾" is confusable with "СігсӀе" (the latter is
> Cyrillic), and if you're writing Russian, that might also be
> interesting.  So perhaps a 
> 
> (textsec-confusable-with-script-p "C𝗂𝗋𝖼𝗅𝖾" 'cyrillic)
> => t
> 
> ?  But...  I'm not sure in which contexts that would actually be vital
> to know.  Hm.

I think we should first determine what kinds of applications may need
this, and take it from there.  The initial number of "confusability
with" classes can be very small, and we can add more as we discover
interesting use cases.  The full number is pretty much infinite, I
think, but I'm not sure Emacs needs  to support all of them OOTB.  We
could support some of the popular ones, and provide infrastructure for
developing more.