Re: minor mode for highlighting character classes such as non-ascii (bug 47455)

[Eli Zaretskii <eliz@gnu.org>]

> From: Óscar Fuentes <ofv@wanadoo.es>
> Date: Tue, 01 Jun 2021 18:32:59 +0200
> 
> "Roland Winkler" <winkler@gnu.org> writes:
> 
> > In the context of bug 47455 the idea emerged that it can be useful
> > to have a minor mode that highlights non-ascii characters.  I want
> > to ask hear for feedback: does such a mode already exist?  Can it be
> > useful to have this minor mode more general such that the highlighted
> > character classes become configurable?  Are there possible use cases
> > for this beyond non-ascii characters?
> 
> Highlighting characters that does not belong to certain coding system
> would be useful to me.

Character cannot belong to a coding system.  I think you mean
"characters that cannot be safely encoded by a coding system".

> Sometimes I work on source code encoded as UTF-8 but they must not
> contain characters not encodable by windows-1252. More precisely, string
> literals must be composed of characters encodable by windows-1252.

We already make such tests when you save the buffer, so the code to do
that exists, and can be reused.  But I'm not sure this highlighting
couldn't turn out to be somewhat expensive.