Implement 1-based column numbering in mode line

* src/xdisp.c (decode_mode_spec): Implement the %C construct.

* lisp/bindings.el (column-number-indicator-zero-based): New
defcustom.
(mode-line-position): Use %C when
column-number-indicator-zero-based is nil.

* src/xdisp.c (syms_of_xdisp) <frame-title-format>:
* src/buffer.c (syms_of_buffer) <mode-line-format>:
* doc/lispref/modes.texi (%-Constructs):
* doc/lispref/frames.texi (Frame Titles): Document the %C
construct.

* doc/emacs/display.texi (Optional Mode Line): Document
'column-number-indicator-zero-based'.

* etc/NEWS: Mention 'column-number-indicator-zero-based' and the
%C construct.

2 files changed, 11 insertions, 6 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 33ffdf8eb8e..80dbd3318dc 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5486,8 +5486,11 @@ A string is printed verbatim in the mode line except for %-constructs:
         For a modified read-only buffer, %* gives % and %+ gives *.
   %s -- print process status.   %l -- print the current line number.
   %c -- print the current column number (this makes editing slower).
+        Columns are numbered starting from the left margin, and the
+        leftmost column is displayed as zero.
         To make the column number update correctly in all cases,
         `column-number-mode' must be non-nil.
+  %C -- Like %c, but the leftmost column is displayed as one.
   %i -- print the size of the buffer.
   %I -- like %i, but use k, M, G, etc., to abbreviate.
   %p -- print percent of buffer above top of window, or Top, Bot or All.
diff --git a/src/xdisp.c b/src/xdisp.c
index c730cdae054..cdea20993c7 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -476,7 +476,7 @@ int windows_or_buffers_changed;
    used to track down the cause for this full-redisplay).
 
    Since the frame title uses the same %-constructs as the mode line
-   (except %c and %l), if this variable is non-zero, we also consider
+   (except %c, %C, and %l), if this variable is non-zero, we also consider
    redisplaying the title of each frame, see x_consider_frame_title.
 
    The `redisplay' bits are the same as those used for
@@ -11530,7 +11530,7 @@ window_buffer_changed (struct window *w)
   return (BUF_SAVE_MODIFF (b) < BUF_MODIFF (b)) != w->last_had_star;
 }
 
-/* True if W has %c in its mode line and mode line should be updated.  */
+/* True if W has %c or %C in its mode line and mode line should be updated.  */
 
 static bool
 mode_line_update_needed (struct window *w)
@@ -23690,7 +23690,8 @@ decode_mode_spec (struct window *w, register int c, int field_width,
       break;
 
     case 'c':
-      /* %c and %l are ignored in `frame-title-format'.
+    case 'C':
+      /* %c, %C, and %l are ignored in `frame-title-format'.
          (In redisplay_internal, the frame title is drawn _before_ the
          windows are updated, so the stuff which depends on actual
          window contents (such as %l) may fail to render properly, or
@@ -23700,8 +23701,9 @@ decode_mode_spec (struct window *w, register int c, int field_width,
       else
         {
           ptrdiff_t col = current_column ();
+          int disp_col = (c == 'C') ? col + 1 : col;
           w->column_number_displayed = col;
-          pint2str (decode_mode_spec_buf, width, col);
+          pint2str (decode_mode_spec_buf, width, disp_col);
           return decode_mode_spec_buf;
         }
 
@@ -23749,7 +23751,7 @@ decode_mode_spec (struct window *w, register int c, int field_width,
         ptrdiff_t topline, nlines, height;
         ptrdiff_t junk;
 
-        /* %c and %l are ignored in `frame-title-format'.  */
+        /* %c, %C, and %l are ignored in `frame-title-format'.  */
         if (mode_line_target == MODE_LINE_TITLE)
           return "";
 
@@ -31725,7 +31727,7 @@ This variable is not guaranteed to be accurate except while processing
 \(Assuming the window manager supports this feature.)
 
 This variable has the same structure as `mode-line-format', except that
-the %c and %l constructs are ignored.  It is used only on frames for
+the %c, %C, and %l constructs are ignored.  It is used only on frames for
 which no explicit name has been set (see `modify-frame-parameters').  */);
 
   DEFVAR_LISP ("icon-title-format", Vicon_title_format,