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

[Alan Mackenzie]

Hello, Stefan.

On Sun, Nov 12, 2023 at 12:28:13 -0500, Stefan Monnier wrote:
> > Why not?  We're not in the compilation any more, we're loading a file.
> > Some error has prevented that file loading, so we want a backtrace just
> > as we would get with M-x load-file foo.elc RET.

> Hmm... that's a good point.

> >> I think for this specific problem being discussed (which is indeed
> >> a fairly common occurrence in my experience), the better solution is to
> >> change `load` so it adds the "context" (i.e. filename and ideally also
> >> the approximate file position info) to errors.
> >> [ This may require something like `handler-bind`.  ]
> > Another solution would be to dispense with
> > display-byte-compile-warnings, just letting compiler errors generate
> > backtraces.

> > The problem here is that there is no distinction in bytecomp.el between
> > "external" errors (such as from require) and errors detected by the
> > compiler in the source file being compiled.

> These are two fairly "clear" cases, admittedly.

> But there are also cases in between where it's less clear, mostly with
> errors during macro-expansion where the internal/external distinction is
> not always that clear since some macros come from outside but others
> come from the very file we're compiling, and where we can't easily tell
> if an error is due to a bug in the macro definition or a bug in the use
> of the macro.

Question: will the user be able to identify the macro and its source
file if we just print the bare error message as enforced by
displaying-byte-compile-warnings?  It the answer is no or not really, we
should give her the backtrace to get started on.

> > The first decidedly want a backtrace, the second probably not.
> > All these errors are handled as though they were "internal" errors
> > detected by the compiler.  This is suboptimal.

> Also there are 2 questions:

> - whether to give a backtrace (and/or enter the debugger).
> - when we don't show a backtrace, what info do we put in the error message.

> For the first, the current "solution" is to set `byte-compile-debug`.
> It's not ideal, and we should improve it, but at least we do have
> a solution for it.

I suspect byte-compile-debug isn't widely known.  Its name is also a bit
discordant - it's not necessarily about debugging byte-compile, it's
just to get sensible error messages when something goes wrong,
especially when that something is not part of the byte compiler.

> For the second we currently don't show a good enough info and in my
> previous response I focused on that part.

Indeed, for the error message which provoked this bug report, the
current information is poor indeed.  Considering that require's can be
nested, we only tell the user the identity of the outermost one.

> > Perhaps we should report the second type of error (detected by the
> > compiler) by calling a warning function, as we do for warnings, and
> > removing the damaging condition-case's as suggested two paragraphs back.

> If the user is not asking to see backtraces, the current treatment seems
> cleaner than without any `condition-case`.

It's "neat and tidy", but at the cost of discarding all useful
information.  There are other common situations in Emacs where
the debugger is entered, or a backtrace output without debug-on-error
having to be set.  Perhaps this one should join them.

> So maybe those `condition-case` should be turned into
> `condition-case-unless-debug`?

I think this would be a very useful first step.  I think it likely a
user will set debug-on-error on encountering any unhelpful error
message.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).