Eli,

Thanks for taking the time to respond. The separator character I use is simply the 'pipe' character, "|". The purpose for me is to clearly delineate the text/code from the line numbers in a way that makes it easier to grok the text being displayed. My brain sees the numbers as part of the code with the current formatting of display-line-numbers, but maybe I'm uniquely incapable.

I completely understand feature creep, and the reason I've switched to display-line-numbers *because* of performance. Using a combination of outline-minor-mode (with folding) and linum-mode made emacs run at 100% cpu while moving the cursor in the terminal. Not to mention other linum bugs.

Best.

On Fri, Sep 10, 2021 at 6:31 AM Eli Zaretskii <eliz@gnu.org> wrote:

> Date: Fri, 10 Sep 2021 01:40:45 -0600
> From: "Michael Gallagher (CIRES/NOAA)" via "Bug reports for GNU Emacs,
> the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>
> linum mode has several performance issues but has the convenient ability
> to format the line numbers in a way chosen by the user. It would be
> wonderful to be able to add a "separator" character of choosing to
> display-line-numbers-mode.

Please tell more about this "separator character": what should it be
and how will it be displayed? And what is its purpose in the first
place?

I'd like to avoid adding a general format-style extension to the
native line numbers, that's a kind of creeping featurism that will
eventually slow down redisplay too much, something that the
display-line-numbers implementation explicitly attempts to avoid.
People who must have the full-fledged format capability could simply
use linum-mode instead.

> I would be happy to try to contribute code but I'm unsure of where
> to start with such a task.

The code is in xdisp.c, if that's what you meant to ask.

Thanks.