bug#66912: With `require', the byte compiler reports the wrong file for errors.

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> I was thinking we wouldn't bother, just like the we don't bother
>> emptying the match-data after we used it.
> I'm getting problems when debug-on-error is non-nil, and a *Backtrace*
> is on the screen.  At this stage, Vloads_in_progress is still non-nil.

Right, because we're still in the process of loading those files.

> So when another error occurs, even a trivial one like "next-line: End of
> buffer", its error message gets prefixed by the "While loading ..."
> strings.

Of course.

This is an instance of the problem I described as follows:

    But I don't think it would be correct in all cases: if file A loads
    file B which compiles file C which loads file D which signals an
    error we want the compiler error message to say "error in D loaded
    from C" and not "error in D loaded from C loaded from B loaded from A".

> This is a Bad Thing.  Do you have any suggestions for a fix?

Other than changing to another approach (e.g. one that incrementally
adds the context info via `handler-bind`s so you only get the context
that found "between" the error and the place where we actually handle
the error), I think the easy solution is to consider not the whole
`Vloads_in_progress_of_last_error` but only the part which is not still
present in `Vloads_in_progress`.

Something like

    (while (not (equal Vloads_in_progress_of_last_error Vloads_in_progress))
      (setq msg (concat msg " while loading "
                        (pop Vloads_in_progress_of_last_error))))

> I've tried it, and the above problem seems definitely to make it less
> simple than the approach I originally took (which currently works).

AFAIK, your previous approach suffered from the exact same problem, tho
maybe in fewer cases.


        Stefan