Re: Native line numbers landed on master

[Juanma Barranquero <lekktu@gmail.com>]

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This implements display-line-numbers-offset, and adds info, NEWS, etc.
Better wording welcome, as always. The variable is automatically
buffer-local because I don't imagine that there's a good default value
(other than zero) and each buffer that uses it will want its own.

I've done some testing, but more eyes would be great.

As an aside, this allows some dirty tricks, like numbering from 0 (by
setting it to -1), or this funny monstrosity, which allows to reverse-count
the buffer:

  (let ((old nil)
        (buf (current-buffer)))
    (add-hook 'before-change-functions
              (lambda (beg end)
                (when (eq buf (current-buffer))
                  (setq old (count-lines beg end)))))
    (add-hook 'after-change-functions
              (lambda (beg end _len)
                (when (and (eq buf (current-buffer))
                           old
                           (/= old (count-lines beg end)))
                  (setq display-line-numbers-offset (- -1
                                                     (count-lines
(point-min)

(point-max)))
                        old nil)))))

Of course, if the buffer is immutable, this is just

  (setq display-line-numbers-offset (- -1 (count-lines (point-min)
(point-max))))

in some mode hook.

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From 223ebc2a9a2a7cecbfae5e884e0e3bdc8995e000 Mon Sep 17 00:00:00 2001
From: Juanma Barranquero <lekktu@gmail.com>
Date: Mon, 7 Oct 2019 01:22:14 +0200
Subject: [PATCH] Implement offsets for absolute line numbers

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-offset>:
New variable to add an offset to absolute line numbers.
(syms_of_xdisp) <display-line-numbers>: Mention it in docstring.
(maybe_produce_line_number): Use it.

* doc/emacs/display.texi (Display Custom): Document it.

* etc/NEWS (value): Announce it.

* lisp/frame.el: Add `display-line-numbers-offset' to list of
variables to which should trigger redisplay of the current buffer.
---
 doc/emacs/display.texi |  7 +++++++
 etc/NEWS               |  4 ++++
 lisp/frame.el          |  1 +
 src/xdisp.c            | 43 +++++++++++++++++++++++++++++++++++-------
 4 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index 406feb8c12..cb37ef448e 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1855,6 +1855,13 @@ Display Custom
 value, line numbers will disregard any narrowing and will start at the
 first character of the buffer.
 
+@vindex display-line-numbers-offset
+If the value of @code{display-line-numbers-offset} is non-zero, it is
+added to each absolute line number, and lines are counted from the
+beginning of the buffer, as if @code{display-line-numbers-widen} were
+non-@code{nil}.  It has no effect when set to zero, or when line
+numbers are not absolute.
+
 @vindex display-line-numbers-width-start
 @vindex display-line-numbers-grow-only
 @vindex display-line-numbers-width
diff --git a/etc/NEWS b/etc/NEWS
index 4a32300268..a40eb06b85 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -560,11 +560,15 @@ now prompts the user for the directory containing the desktop file.
 
 +++
 ** display-line-numbers-mode
+
 *** New faces 'line-number-major-tick' and 'line-number-minor-tick',
 and customizable variables 'display-line-numbers-major-tick' and
 'display-line-numbers-minor-tick' can be used to highlight the line
 numbers of lines multiple of certain numbers.
 
+*** New variable `display-line-numbers-offset', when non-zero, adds
+an offset to absolute line numbers.
+
 +++
 ** winner
 *** A new variable, 'winner-boring-buffers-regexp', has been added.
diff --git a/lisp/frame.el b/lisp/frame.el
index 51b3b621ff..018c2f578e 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2726,6 +2726,7 @@ 'automatic-hscrolling
         display-line-numbers-widen
         display-line-numbers-major-tick
         display-line-numbers-minor-tick
+        display-line-numbers-offset
         display-fill-column-indicator
         display-fill-column-indicator-column
         display-fill-column-indicator-character
diff --git a/src/xdisp.c b/src/xdisp.c
index 1f3a8136f8..563cf473cf 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22506,10 +22506,22 @@ maybe_produce_line_number (struct it *it)
   ptrdiff_t start_from, bytepos;
   ptrdiff_t this_line;
   bool first_time = false;
-  ptrdiff_t beg_byte = display_line_numbers_widen ? BEG_BYTE : BEGV_BYTE;
-  ptrdiff_t z_byte = display_line_numbers_widen ? Z_BYTE : ZV_BYTE;
+  ptrdiff_t beg_byte;
+  ptrdiff_t z_byte;
+  bool line_numbers_wide;
   void *itdata = bidi_shelve_cache ();
 
+  if (display_line_numbers_offset
+      && !display_line_numbers_widen
+      && !EQ (Vdisplay_line_numbers, Qvisual)
+      && !EQ (Vdisplay_line_numbers, Qrelative))
+    line_numbers_wide = true;
+  else
+    line_numbers_wide = display_line_numbers_widen;
+
+  beg_byte = line_numbers_wide ? BEG_BYTE : BEGV_BYTE;
+  z_byte = line_numbers_wide ? Z_BYTE : ZV_BYTE;
+
   if (EQ (Vdisplay_line_numbers, Qvisual))
     this_line = display_count_lines_visually (it);
   else
@@ -22524,7 +22536,7 @@ maybe_produce_line_number (struct it *it)
 		 numbers, so we cannot use its data if the user wants
 		 line numbers that disregard narrowing, or if the
 		 buffer's narrowing has just changed.  */
-	      && !(display_line_numbers_widen
+	      && !(line_numbers_wide
 		   && (BEG_BYTE != BEGV_BYTE || Z_BYTE != ZV_BYTE))
 	      && !current_buffer->clip_changed)
 	    {
@@ -22614,6 +22626,8 @@ maybe_produce_line_number (struct it *it)
     lnum_offset = it->pt_lnum;
   else if (EQ (Vdisplay_line_numbers, Qvisual))
     lnum_offset = 0;
+  else if (display_line_numbers_offset)
+    lnum_offset -= display_line_numbers_offset;
 
   /* Under 'relative', display the absolute line number for the
      current line, unless the user requests otherwise.  */
@@ -34701,12 +34715,18 @@ syms_of_xdisp (void)
 
   DEFVAR_LISP ("display-line-numbers", Vdisplay_line_numbers,
     doc: /* Non-nil means display line numbers.
+
 If the value is t, display the absolute number of each line of a buffer
 shown in a window.  Absolute line numbers count from the beginning of
-the current narrowing, or from buffer beginning.  If the value is
-`relative', display for each line not containing the window's point its
-relative number instead, i.e. the number of the line relative to the
-line showing the window's point.
+the current narrowing, or from buffer beginning.  The variable
+`display-line-numbers-offset', if non-zero, is a signed offset added
+to each absolute line number; it also forces line numbers to be counted
+from the beginning of the buffer, as if `display-line-numbers-wide'
+were non-nil.  It has no effect when line numbers are not absolute.
+
+If the value is `relative', display for each line not containing the
+window's point its relative number instead, i.e. the number of the line
+relative to the line showing the window's point.
 
 In either case, line numbers are displayed at the beginning of each
 non-continuation line that displays buffer text, i.e. after each newline
@@ -34747,6 +34767,15 @@ syms_of_xdisp (void)
   DEFSYM (Qdisplay_line_numbers_widen, "display-line-numbers-widen");
   Fmake_variable_buffer_local (Qdisplay_line_numbers_widen);
 
+  DEFVAR_INT ("display-line-numbers-offset", display_line_numbers_offset,
+    doc: /* A signed integer added to each absolute line number.
+When this variable is non-zero, line numbers are always counted from
+the beginning of the buffer even if `display-line-numbers-widen' is nil.
+It has no effect when set to 0, or when line numbers are not absolute.  */);
+  display_line_numbers_offset = 0;
+  DEFSYM (Qdisplay_line_numbers_offset, "display-line-numbers-offset");
+  Fmake_variable_buffer_local (Qdisplay_line_numbers_offset);
+
   DEFVAR_BOOL ("display-fill-column-indicator", Vdisplay_fill_column_indicator,
     doc: /* Non-nil means display the fill column indicator.  */);
   Vdisplay_fill_column_indicator = false;
-- 
2.23.0.windows.1