Re: Thoughts on getting correct line numbers in the byte compiler's warning messages

[Alan Mackenzie]

Hello, Stefan.

On Thu, Nov 08, 2018 at 22:13:11 +0000, Alan Mackenzie wrote:
> On Thu, Nov 08, 2018 at 12:02:01 -0500, Stefan Monnier wrote:

[ .... ]

> OK.  I've hacked together some basic infrastructure in alloc.c, lread.c,
> print.c, and lisp.h.  I can now read a small test file and get back the
> form with "located symbols".  I've called the new function which does
> this read-locating-symbols, but that might want to change.

> As soon as I've sorted out SYMBOLP and XSYMBOL, I'll create a new branch
> under /scratch, commit what I've got, and then we can play with it.

I've now got this working, and created the new, optimistically named,
branch /scratch/accurate-warning-pos.

To use this, do something like:

    M-: (setq bar (read-locating-symbols (current-buffer)))

with point at the beginning of a (smallish) buffer.  The following form,
from Roland Winkler's bug #9109, works well:

    (unwind-protect
        (let ((foo "foo"))
          (insert foo))
      (setq foo "bar"))

.  (car bar), for example, is now a "located symbol".

Direct symbol functions are "protected" by an enabling flag
located-symbols-enabled.  This is needed, partly to minimise the run time
taken when the facility is not being used, but more pertinently to enable
Emacs to build without a segfault.  Currently this flag guards only
SYMBOLP and XSYMBOL.

So, try M-: (symbolp (car bar)).  This is nil.  But

    M-: (let ((located-symbols-enabled t)) (symbolp (car bar)))

is t.  Similarly, set, symbol-value, symbol-function, symbol-plist need
that flag to be non-nil.

> > > One thing we'd need to watch out for is using equal, not eq, when we
> > > compare symbols.  (eq 'foo #<symbol foo with position 73>) will surely
> > > be nil, but (equal ....) would be t.  Same with member and memq.

> > Indeed.

`equal' has been enhanced so that M-: (equal (car bar) 'unwind-protect)
is t.

Additionally, there are defuns only-symbol-p, located-symbol-p,
located-symbol-sym, located-symbol-loc, which do the obvious.

> > > We'd also need to make sure that the reader's enabling flag for creating
> > > these extended symbols is bound to nil whenever we suspend the byte
> > > compiler to do something else (edebug, for example).

> > Rather than a dynamically-scoped var, it might be a better option to
> > either use a new function `read-with-positions`, or else use an
> > additional argument to `read`.

As noted above I've currently got a rather untidy mixture of these two
approaches.

There's a lot left to do, but this is a start.

Incidentally, I timed a make bootstrap in this branch, comparing it with
master.  The branch was ~0.5% slower.  This might be real, it might just
be random noise.

Comments and criticism welcome!

> >         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).