RE: Emacs pretest: highlight line number in display-line-numbers-mode?

[Drew Adams <drew.adams@oracle.com>]

> This face is by default the same as line-number because of 2 reasons:
> 
>   . linum-mode did the same

...chasing that rabbit: Why did `linum-mode' do that?

Two wrongs don't necessarily make a right.

>   . using a different face makes redisplay slightly slower
>     (because it disallows some redisplay optimizations)

Would those optimizations still be available if a
user chose the same face for both (but the faces
were different by default)?

If so, I'd suggest making the faces different by
default.  And if the performance difference is
important then perhaps mention it in the doc,
so users who want that optimization at the cost
of not highlighting the current line number can
get it.  IOW, opt-in for the loss of a difference.

> > To find the face used, I use `list-faces-display' and look
> > for either a face name that is a likely candidate
> > (`line-number' is pretty obvious, in this case) or a face
> > appearance that matches what I see.
> 
> There are other places where this doesn't work, e.g. on the
> mode line.

What do you mean by "this"?  Using `list-faces-display'
or the fact that you can't use `C-u C-x =' to learn
the face?

You gave that reply after my mention of the former. 
Aren't all of the faces used in the mode line shown in
`list-faces-display'?  At least all faces with "mode-line"
in their name are listed there.

There are certainly several places where you cannot use
`C-u C-x =' to see what the face is.  (In particular,
places where there is no cursor/buffer position.)