bug#36967: 27.0.50; Duplicate lines in xref output

[Dmitry Gutov]

On 03.12.2020 23:30, Juri Linkov wrote:
> > > Here is the patch that makes the broken
> >
> > Pretty harsh there.
>
> But true: I can't use it in the current form, and was waiting
> when someone will fix it.

It's great to see you get this ball rolling.

> > Here's an alternative proposal:
> >
> > Combine the lines inside the rendering code instead.
> >
> > So each xref will have a separate location, but then xref--insert-xrefs
> > will see that xref-location-line value repeats across some consecutive
> > locations, and will combine them into single line with some text property
> > magic (basically, copying the summary from one of them, and then applying
> > 'xref-item and 'face properties appropriately). This retains the xref item
> > semantics (as opposed to, say, associating an xref item with multiple
> > locations). And _hopefully_ the replace-related code won't need
> > any changes.
>
> I tried to improve xref--insert-xrefs to group matches by lines
> by using the most convenient function seq-group-by.  But then
> noticed that xref.el doesn't rely on seq.el.  Even xref--alistify
> that groups matches by files could be replaced by seq-group-by.
> Is it a requirement to avoid using seq functions in xref.el?

Not really, no. project.el pulls in seq already, why not have xref do it 
too.

I'm _slightly_ worried about extra garbage if we do seq-group-by twice 
(with an extra list for every line, even those that don't need it), but 
that's what benchmarking is for (can do that later).