bug#25987: 25.2; support gcc fixit notes

[Tom Tromey]

With -fdiagnostics-parseable-fixits, GCC can emit "fixit" notes that
tell how to apply a patch to fix a problem.

For example for this:

struct x {
  int field;
};

struct x y;

int main() {
  return y.feld;
}

I get:

q.c: In function ‘main’:
q.c:8:12: error: ‘struct x’ has no member named ‘feld’; did you mean ‘field’?
   return y.feld;
            ^~~~
            field
fix-it:"q.c":{8:12-8:16}:"field"


The documentation, from the gcc manual:

================================================================
@item -fdiagnostics-parseable-fixits
@opindex fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption
by IDEs.  For each fix-it, a line will be printed after the relevant
diagnostic, starting with the string ``fix-it:''.  For example:

@smallexample
fix-it:"test.c":@{45:3-45:21@}:"gtk_widget_show_all"
@end smallexample

The location is expressed as a half-open range, expressed as a count of
bytes, starting at byte 1 for the initial column.  In the above example,
bytes 3 through 20 of line 45 of ``test.c'' are to be replaced with the
given string:

@smallexample
00000000011111111112222222222
12345678901234567890123456789
  gtk_widget_showall (dlg);
  ^^^^^^^^^^^^^^^^^^
  gtk_widget_show_all
@end smallexample

The filename and replacement string escape backslash as ``\\", tab as ``\t'',
newline as ``\n'', double quotes as ``\"'', non-printable characters as octal
(e.g. vertical tab as ``\013'').

An empty replacement string indicates that the given range is to be removed.
An empty range (e.g. ``45:3-45:3'') indicates that the string is to
be inserted at the given position.
================================================================


I think Emacs should support this in compilation mode buffers.
A few thoughts on the implementation:

* The filename quoting stuff will require a bit of unquoting
* The offsets are byte offsets, not columns, which is a bit of a pain
  (or at least, it is for me since I don't know how to handle that in
  elisp)
* One way for this to work would be to display the buffer and 
  show the proposed change as an overlay; and then use y-or-n-p
  to ask whether it should be applied.  I was thinking something like:

(defun compilation--fixit-make-overlay (start end text)
  (let ((overlay (make-overlay start end)))
    (overlay-put overlay 'face 'diff-removed)
    (overlay-put overlay 'after-string
                 (propertize text 'face 'diff-added))
    overlay))

  With this approach perhaps nothing could be done if the fixit was
  already visited, or if the buffer text already matches the
  replacement.

* However other approaches are possible, like letting the user request
  no confirmation, or request "auto-apply all changes" or something.
  Maybe a fixit should be associated with the previous error somehow.

* Allowing "fixit" to be handled specially in compile mode might mean
  adding a new "type" value, not sure.

Tom



In GNU Emacs 25.2.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.22.8)
 of 2017-03-02 built on bapiya
Repository revision: 6e788ef0e262fafc014c21f4ad52cc5dc9f1715b
Windowing system distributor 'Fedora Project', version 11.0.11901000
System Description:     Fedora release 25 (Twenty Five)

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