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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Mon, Nov 05, 2018 at 17:57:35 +0200, Eli Zaretskii wrote:
> > Date: Mon, 5 Nov 2018 10:53:02 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: emacs-devel@gnu.org

> > 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.

> Sounds gross to me.

What is done at the moment is no less gross.  Just to clarify, the above
acton of read would only be done when in byte compilation, a bit like
how the current list of source symbols is also only for when in
compilation.

I've spend many hours at my PC, trying to figure out a neat way of
solving this problem.  The above is the best I've been able to come up
with, so far.

Why do you think the idea is gross, given the difficulty of the
underlying problem?  The idea should work with only moderate amendment
of the byte-compiler/macro routines, and virtually no change outside of
that, bar amending the reader and the lowest level functions like `cons'
and `car'.

> Did you consider using mint_ptr objects instead?  That'd be still be
> gross, but at least we won't introduce another type of Lisp_Object.

The using up of the last available object type is a severe disadvantage,
yes.  I wasn't aware of mint_ptrs until you just mentioned them.  I'll
need to read up on them to get the hang of what they're about.

> Also, what about keeping the source position in some other way, like a
> property of some symbol?

Difficult.  Essentially, these source positions are properties of
Lisp_Objects, such as conses, not of symbols.  A typical symbol is used
several or many times in a compilation unit.  Some means has to be found
of attaching properties (in this case, source positions), to arbitrary
Lisp_Objects.

It's gradually become clear to me that what I proposed this morning is a
special case of attaching a property list to an arbitrary object.  Maybe
an actual property list, being more general, would be a better idea.

Alternatively, it may be possible to use a vector or pseudovector type
rather than using Lisp_Type 1 to implement basically the same idea.
This would be slower at run time, however, possibly not significantly.

-- 
Alan Mackenzie (Nuremberg, Germany).