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, 24 insertions, 0 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 8aa38c8034c..1f3a8136f8b 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22664,6 +22664,14 @@ maybe_produce_line_number (struct it *it)
              empty lines beyond EOB.  */
           && it->what != IT_EOB)
         tem_it.face_id = current_lnum_face_id;
+      else if (display_line_numbers_major_tick > 0
+               && (lnum_to_display % display_line_numbers_major_tick == 0))
+        tem_it.face_id = merge_faces (it->w, Qline_number_major_tick,
+                                      0, DEFAULT_FACE_ID);
+      else if (display_line_numbers_minor_tick > 0
+               && (lnum_to_display % display_line_numbers_minor_tick == 0))
+        tem_it.face_id = merge_faces (it->w, Qline_number_minor_tick,
+                                      0, DEFAULT_FACE_ID);
       else
         tem_it.face_id = lnum_face_id;
       if (beyond_zv
@@ -34125,6 +34133,8 @@ be let-bound around code that needs to disable messages temporarily. */);
   /* Names of the faces used to display line numbers.  */
   DEFSYM (Qline_number, "line-number");
   DEFSYM (Qline_number_current_line, "line-number-current-line");
+  DEFSYM (Qline_number_major_tick, "line-number-major-tick");
+  DEFSYM (Qline_number_minor_tick, "line-number-minor-tick");
   /* Name of a text property which disables line-number display.  */
   DEFSYM (Qdisplay_line_numbers_disable, "display-line-numbers-disable");
 
@@ -34760,6 +34770,20 @@ if the font in fill-column-indicator face does not support Unicode characters.
   DEFSYM (Qdisplay_fill_column_indicator_character, "display-fill-column-indicator-character");
   Fmake_variable_buffer_local (Qdisplay_fill_column_indicator_character);
 
+  DEFVAR_INT ("display-line-numbers-major-tick", display_line_numbers_major_tick,
+    doc: /* If an integer N > 0, highlight line number of every Nth line.
+The line number is shown with the `line-number-major-tick' face.
+Otherwise, no special highlighting is done every Nth line.
+Note that major ticks take precedence over minor ticks.  */);
+  display_line_numbers_major_tick = 0;
+
+  DEFVAR_INT ("display-line-numbers-minor-tick", display_line_numbers_minor_tick,
+    doc: /* If an integer N > 0, highlight line number of every Nth line.
+The line number is shown with the `line-number-minor-tick' face.
+Otherwise, no special highlighting is done every Nth line.
+Note that major ticks take precedence over minor ticks.  */);
+  display_line_numbers_minor_tick = 0;
+
   DEFVAR_BOOL ("inhibit-eval-during-redisplay", inhibit_eval_during_redisplay,
     doc: /* Non-nil means don't eval Lisp during redisplay.  */);
   inhibit_eval_during_redisplay = false;