bug#28280: Maximally unhelpful diagnostic message.

[Lars Ingebrigtsen <larsi@gnus.org>]

npostavs@users.sourceforge.net writes:

>> Yes, but that's a separate problem from the essence of the bug report.
>> Regardless of the lack of progress on bug #18913, the information
>> provided by an assert being triggered is unhelpful and inadequate.
>
> Here is a possible M-x cl-show-assert command:

[...]

> * lisp/emacs-lisp/cl-lib.el (cl-show-assert): New command.
> (cl-assert-last-backtrace): New variable.
> * lisp/emacs-lisp/cl-preloaded.el (cl--assertion-failed): Set it.

[...]

> +    (debugger-insert-backtrace cl-assert-last-backtrace t)

This patch still applies, but this function was removed in 2018, so if
we want something like this, we have to refactor out that function
again.

But...  I'm not sure a command like this makes sense.  As others noted
in this thread, if you want a backtrace (from an error or assertion),
then you set debug-on-error.

So if we want this, I think it should be even more general -- a command
to give you a backtrace of the previous error signalled, no matter
whether it was an error or an assertion.

Which I think would be a really nice thing to have -- I run Emacs with
debug-on-error on by default because of this, but if it were possible to
say `M-x debug-last-error', then I wouldn't have to.

Would this be possible to implement in an easy way, I wonder?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no