bug#25987: 25.2; support gcc fixit notes

[Eli Zaretskii]

> From: David Malcolm <dmalcolm@redhat.com>
> Cc: 25987@debbugs.gnu.org
> Date: Tue, 06 Oct 2020 14:17:55 -0400
> 
> In GCC 11 we've revamped the column number handling in how we emit
> diagnostics; see:
>   https://gcc.gnu.org/pipermail/gcc-patches/2020-October/555632.html
> 
> GCC 11 diagnostics now (by default) should use actual column numbers,
> rather than byte counts.

That's good news, thanks.

> We haven't changed -fdiagnostics-parseable-fixits; it still emits its
> range information in terms of byte offsets (and e.g. Eclipse already
> consumes that option); this is bug-for-bug compatible with clang, I
> believe (which had that option first).

So fixit hints will still count bytes?  Or will GCC 11 emit such
hints even without -fdiagnostics-parseable-fixits on the command line?

> Note that characters != columns, or, at least, we have to be careful
> about what we mean.  Consider the case of 🙂 aka SLIGHTLY SMILING FACE
> (U+1F642) which is a single unicode code point, but occupies two
> columns, with its UTF-8 encoding requiring four bytes.
> 
> When I type it into an Emacs buffer, and look at the column number I
> see that Emacs appears to treat the character as occupying two columns.
>  Is that the kind of column numbering you would want for machine-
> readable fix-it hints?

Yes.  Emacs computes the width of each character by using UCD, the
Unicode Character Database (specifically, the EastAsianWidth.txt file
that is part of the UCD).  If GCC gets its column counts from a
similar DB, then it will match what Emacs does.

> Similarly, the column numbering emitted by GCC 11 diagnostics is
> affected by tab characters as tab stops, which seems to reflect Emacs
> behavior as well.

Yes.

> I can add an additional option for fix-it hints that's similar to
> -fdiagnostics-parseable-fixits, but with a different output format (or
> to add an argument to that option, perhaps).

If an option is needed for getting the hints, then a special option
which reports columns in hints will be appreciated, as it will make
the Emacs support for processing those hints 100% accurate and devoid
of encoding guesswork.

> Before I do that, I wanted to check that it would be consumable by
> Emacs.  What works for you?  Would it help to send the proposed GCC
> patch to this bug address (or to the emacs-devel list?).

I don't know how many people here build their own GCC, and thus could
try the patch, but if sending the patch is not too much trouble,
perhaps posting it on emacs-devel would be a good idea.  If you do
that, please cite this bug report, so that people who try that could
respond here with their experience.

> Alternatively, we already have a JSON output option (-fdiagnostics-
> format=json); perhaps something like that could be used?

Emacs can parse JSON.  What are the pros and cons of the JSON
alternative wrt to the text alternative?

> Feature freeze for GCC 11 is about a month away; I'd love for Emacs to
> be able to consume GCC fix-it hints (and have GCC and Emacs fix my
> typos for me)

Agreed; let's try to make that happen.

Thanks.