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

[Alan Mackenzie]

Hello, Stefan.

On Thu, Nov 01, 2018 at 18:45:00 -0400, Stefan Monnier wrote:
> > The third idea is to amend the reader so that whereas it now produces a
> > form, in a byte compiler special mode, it would produce the cons (form .
> > offset).  So, for example, the text "(not a)" currently gets read into

> Sounds good.  I have the vague feeling that I mentioned it already, but
> in case I haven't: please make sure the positions are character-precise
> rather than line-precise, so that we can (eventually) ditch Edebug's
> Elisp-reimplementation-of-the-reader which returns the same kind of info
> (and needs character-precise location info).

Actually this idea was not good; macros could not handle such a form
without severe changes in the way macros work.  (A research project,
perhaps).

I have come up with an improved scheme, which may well work.

The reader would produce, in place of the Lisp_Objects it currently
does, an object with Lisp_Type 1 (which is currently unused).  The rest
of the object would be an address pointing at two Lisp_Objects, one
being the "real" read object, the other being a source position.

The low level routines, like CONSP, and a million others in lisp.h would
need amendment.  But the Lisp system would continue with 8-byte objects,
and the higher level bits (nearly all of it) would not need changes.
The beauty of this scheme is that, outside of byte compilation, nothing
else would change.

One or two extra functions would be needed, such as `big-object' which
would create a new-type object out of a source offset and "ordinary"
object, `big-object-p', `big-offset' to get the source offset from a big
object, and possibly one or two others.

These would naturally be available to byte-compile-warn and friends,
supplying the source position.  To cope with the times when no source
position would be available (e.g. in forms expanded from macros), the
new variable `byte-compile-containing-form' would be bound at strategic
places in the byte compiler.  This would provide a fallback source
position.

The extra indirection involved in these "big objects" would naturally
slow down byte compilation somewhat.  I've no idea how much, but it
might not be much at all.

And yes, the source positions used would be character-precise.

What do you think?

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).