Time to merge scratch/correct-warning-pos into master, perhaps?

[Alan Mackenzie]

Hello, Emacs.

The branch scratch/correct-warning-pos is working and is almost
finished.  Its intention is to get the byte compiler to report correct
source positions in warnings and errors it emits.  It remains just to
write some documentation about symbols with position and to fix any last
minute bugs which turn up.

There is a minor bug in test/lisp/emacs-lisp/comp-tests.el, which should
be easier to resolve when Andrea is back again.

To use it, just merge it and use it.  You shouldn't notice much
difference.  If there are any byte compiler warning positions which
remain uncorrect, please report them.

Thie branch's predecessor was rejected in 2018 since it slowed down
Emacs too much.  My benchmarks this afternoon show a much smaller
slowdown, in the region of 1%.

In these benchmarks, I used the following function:

    (defun time-scroll-b (&optional arg)
      (condition-case nil
          (while t
            (if arg (scroll-down) (scroll-up))
            (sit-for 0))
        (error nil)))

in conjunction with benchmark-run in native-compiled Emacsen started
with emacs -Q on buffer src/xdisp.c.  Both repositories were brought up
to date yesterday evening, and master merged into the branch.

Before each timing run I typed and deleted the character at the start of
the buffer (to wipe out the fontification) and executed M-x
garbage-collect.

Here are the four timings I did on master:
      o - (20.798601883 460 7.728701306)
      o - (20.947118356 295 7.1172684969999995)
      o - (20.941589929 293 7.144901186)
      o - (20.917180235 293 7.136285445000002)

..  Here are the same four timings on scratch/correct-warning-pos):
      o - (20.854543266 480 7.691123986)
      o - (21.064465459 320 7.189660959000001)
      o - (21.143813105 318 7.287708998000001)
      o - (21.115932422 318 7.266432223999999)

..  It can be seen that the branch is around 1% slower than master.  This
minor slowdown should be worth it to get correct positions in compile
warning messages.

So, I would propose to merge the branch into master now.  What do people
(in particular Eli) say about this?

-- 
Alan Mackenzie (Nuremberg, Germany).