bug#36472: 27.0.50; Convey information by showing line numbers using different colors?

[Eli Zaretskii]

> Cc: 36472@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 2 Jul 2019 18:09:01 +0300
> 
> On 02.07.2019 17:30, Eli Zaretskii wrote:
> 
>  > I wouldn't hold my breath, because this will mean any display routine
> that calculates layout will have to call some non-trivial Lisp,
> possibly even running a subprocess.
> 
> Not really. In my use case it would only look up overlays on the lines 
> corresponding to the numbers.

I don't understand: who would need to look up overlays?  And how will
"it" know that it needs to look up overlays?

IOW, I don't think I understand the API you had in mind.  You
originally said "a hook", which implies the display engine would call
a hook variable, but now it sounds like you had something else in
mind.