bug#25987: 25.2; support gcc fixit notes

[Andrea Corallo via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

David Malcolm <dmalcolm@redhat.com> writes:

> On Tue, 2020-10-06 at 21:37 +0300, Eli Zaretskii wrote:
>> > 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?
>
> The existing "-fdiagnostics-format=json" GCC option replaces the
> existing diagnostic output with a big blob of JSON to stderr, all on
> one line.  Although I implemented it, I now feel it to be rather half-
> baked.
>
> I like how -fdiagnostics-parseable-fixits adds extra lines of output,
> with a prefix that's ought to be easy to detect.
>
> BTW, does Emacs set anything in the environment that GCC could detect?

Hi Dave,

If we are searching for a way to ask GCC to dump a file instead of
logging to sterr I think Emacs could easily set the env variable before
invoking the compilation.  Which one I think is to be agreed as would be
probably used by a flymake back-end or same other special case as the
one discussed.

>> > 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.
>
> I put together a test file showing various features to try to ensure
> that GCC and Emacs interoperate on this.  I'm attaching it.
>
> This can also be seen on Compiler Explorer at:
>   https://godbolt.org/z/zazejq
> which adds the existing
>   -fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
> options.
>
> Ideas for other test cases are most welcome.
>
> Does Emacs have an automated test suite that could test this feature?
> A long-postponed goal for me for GCC's testsuite is to ensure that fix-
> it hints apply and actually fix the problem (clang's testsuite has
> tests that verify this for clang's fix-it hints).

Yes we have a testsuite, we could have a test that runs only with like
GCC versions >= 11 or keep in the testsuite some pre cooked GCC output.
Some tests should be easy to write.

  Andrea