bug#591: 23.0.60; lisp-complete-symbol erases extra text

[Chong Yidong <cyd@stupidchicken.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> This sets completion-base-size to 0, which causes the completions buffer
>> to delete everything in the Lisp buffer when you make a selection.
>
> The problem is that leaving it nil will revert to the use of
> a heuristic.  Fixing it right is a bit more difficult.

I don't understand what you mean :-P

>> I don't understand why we need the (completely undocumented) hack
>> introduced here, where the cdr of the last item on the completions list
>> gives completion-base-size.  Does anything else in Emacs depend on this?
>> Why not simply add a new optional argument to display-completion-list?
>
> Yes the base-size in the cdr is a hack, and we be able to get rid of it
> now that I've added the new `boundaries' action.  But passing it as an
> additional argument won't make any difference for the bug at hand.

Why not?  That would allow lisp-complete-symbol to pass the correct
value of completion-base-size, which it has already computed (i.e. the
variable `beg' in lisp-complete-symbol) to display-completion-list.  No?