Set alternate faces for some line numbers (bug#27734)

* lisp/faces.el (line-number-major-tick, line-number-minor-tick):
New faces.
* lisp/cus-start.el (display-line-numbers-major-tick)
(display-line-numbers-minor-tick): Add customization info.
* lisp/frame.el: Add `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick' to list of variables which
should trigger redisplay of the current buffer.

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-major-tick>
<display-line-numbers-major-tick>: Defvar new options.
(syms_of_xdisp) <line-number-major-tick, line-number-minor-tick>:
Defsym new faces.
(maybe_produce_line_number): Use new faces for line numbers
that are multiple of `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick'.

* etc/NEWS (value): Announce new feature.
* doc/emacs/display.texi (Display Custom): Describe it.  Wording by
Robert Pluim <rpluim@gmail.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index 84363d0f0d2..406feb8c127 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1871,7 +1871,12 @@ The line numbers are displayed in a special face @code{line-number}.
 The current line number is displayed in a different face,
 @code{line-number-current-line}, so you can make the current line's
 number have a distinct appearance, which will help locating the line
-showing point.
+showing point. Additional faces @code{line-number-major-tick} and
+@code{line-number-minor-tick} can be used to highlight the line numbers
+of lines which are a multiple of certain numbers.  Customize
+@code{display-line-numbers-major-tick} and
+@code{display-line-numbers-minor-tick} respectively to set those
+numbers.
 
 @vindex visible-bell
   If the variable @code{visible-bell} is non-@code{nil}, Emacs attempts