bug#47711: bug#48841: bug#47711: [PATCH VERSION 2] Add new `completion-filter-completions` API and deferred highlighting

[Eli Zaretskii <eliz@gnu.org>]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Sat, 14 Aug 2021 05:47:43 +0300
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 48841@debbugs.gnu.org,
>  47711@debbugs.gnu.org
> 
> I thought I explained the problem with this previously.
> 
> It's basically this: we cannot mutate what we don't own. Across all of 
> completion functions out there, there will be such that return "shared" 
> strings (meaning, not copied or newly allocated) from their completion 
> tables. And modifying them is bad, with consequences which can present 
> themselves in unexpected, often subtle ways.
> 
> Since up until now completion-pcm--hilit-commonality copied all strings 
> before modifying, completion tables such as described (with "shared" 
> strings) have all been "legal". Suddenly deciding to stop supporting 
> them would be a major API breakage with consequences that are hard to 
> predict. And while I perhaps agree that it's an inconvenience, I don't 
> think it's a choice we can simply make as this stage in c-a-p-f's 
> development.

This sounds like an argument against Daniel's approach as well, no?
Because if a completion API returns strings it "doesn't own", there
will be restrictions on Lisp programs that use those strings, because
those Lisp programs previously could do anything they liked with those
strings, and now they cannot.  Or am I missing something?

>    1. (setq s (symbol-name 'car))
> 
>    2. (put-text-property 1 3 'face 'error s)
> 
>    3. Switch to a buffer in fundamental mode
> 
>    4. (insert (symbol-name 'car)) --> see the error face in the buffer
> 
> Now imagine that some completion table collects symbol names by passing 
> obarray through #'symbol-name rather than #'all-completions, and voila, 
> if the completion machinery adds properties (any properties, not just 
> face) to those strings, you have just modified a bunch of global values. 
> That's not good.

How is this different from Daniel's proposal of returning the original
strings?  AFAIU, he just shifts the responsibility from the completion
code to the caller of the completion code, but basically leaves the
problem still very much real and pretty much into our face.