bug#58342: 29.0.50; noverlay branch is O(N) for important calls

[Drew Adams]

> > Does anybody know of an Emacs package that uses a large number of
> > overlays that span large amounts of the buffer in complex ways?
> 
> Sure.  zones.el does (especially the version
> I use, which I haven't yet exposed externally).
> 
> The point of zones.el is to manipulate arbitrary
> sets of buffer zones, which can be implemented
> as overlays (as one possibility).
> 
> They can overlap in any way, as the library is
> a utility that you can use in any way.  And you
> can sort zones, unite/coalesce them, etc.  It's
> not at all unusual to deal with many overlapping
> zones, e.g., overlays.
> 
> Dunno what the "noverlay" branch is.  I haven't
> seen any description of it or its purpose,
> despite the many, many emails here and in
> emacs-devel with "noverlay" in the Subject line.
> As a result, those many messages get only an
> uninformed glance from me.
> 
> But if the "noverlay" branch is supposed to deal
> with overlays _in general_ in some way, then I'd
> think that the case of many overlapping overlays
> wouldn't necessarily be rare.  Why would it be?
> 
> You can use an overlay for anything: store any
> information on for buffer zone.  An overlay is
> just two buffer positions plus a set of
> properties - any properties.  _Super_ general.

As an example just one use of arbitrarily
positioned overlays, you can save narrowings
to a set of zones (which can be overlays), and
later reactivate any of them on demand.

Narrowings can have any limits, of course.  The
overlays in a zones set can overlap in arbitrary
ways, like Venn diagrams.