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>

3 files changed, 50 insertions, 0 deletions

diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index e61c1954a1f..89a96a9f51c 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -655,6 +655,20 @@ since it could result in memory overflow and make Emacs crash."
                (const :tag "Count lines from beginning of narrowed region"
                       :value nil))
               "26.1")
+             (display-line-numbers-major-tick
+              display-line-numbers
+              (choice
+               (const :tag "No line" 0)
+               (integer :tag "Multiples of"
+                        :value 10))
+              "27.1")
+             (display-line-numbers-minor-tick
+              display-line-numbers
+              (choice
+               (const :tag "No line" 0)
+               (integer :tag "Multiples of"
+                        :value 5))
+              "27.1")
 
              (display-fill-column-indicator
               display-fill-column-indicator
diff --git a/lisp/faces.el b/lisp/faces.el
index 9c5ffe1e590..58b6349ccfd 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -2390,6 +2390,40 @@ unwanted effects."
   :group 'basic-faces
   :group 'display-line-numbers)
 
+(defface line-number-major-tick
+  '((t :inherit line-number))
+  "Face for highlighting \"major ticks\" (as in a ruler).
+When `display-line-numbers-major-tick' is positive, highlight
+the line numbers of lines which are a multiple of its value.
+This face is used when `display-line-numbers' is non-nil.
+
+If you customize the font of this face, make sure it is a
+monospaced font, otherwise line numbers will not line up,
+and text lines might move horizontally as you move through
+the buffer.  Similarly, making this face's font different
+from that of the `line-number' face could produce such
+unwanted effects."
+  :version "27.1"
+  :group 'basic-faces
+  :group 'display-line-numbers)
+
+(defface line-number-minor-tick
+  '((t :inherit line-number))
+  "Face for highlighting \"minor ticks\" (as in a ruler).
+When `display-line-numbers-minor-tick' is positive, highlight
+the line numbers of lines which are a multiple of its value.
+This face is used when `display-line-numbers' is non-nil.
+
+If you customize the font of this face, make sure it is a
+monospaced font, otherwise line numbers will not line up,
+and text lines might move horizontally as you move through
+the buffer.  Similarly, making this face's font different
+from that of the `line-number' face could produce such
+unwanted effects."
+  :version "27.1"
+  :group 'basic-faces
+  :group 'display-line-numbers)
+
 ;; Definition stolen from display-line-numbers.
 (defface fill-column-indicator
   '((t :inherit shadow :weight normal :slant normal
diff --git a/lisp/frame.el b/lisp/frame.el
index 0c68fc378b9..51b3b621ff4 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2724,6 +2724,8 @@ See also `toggle-frame-maximized'."
         display-line-numbers-width
         display-line-numbers-current-absolute
         display-line-numbers-widen
+        display-line-numbers-major-tick
+        display-line-numbers-minor-tick
         display-fill-column-indicator
         display-fill-column-indicator-column
         display-fill-column-indicator-character