Optional display of overlay-arrow in margin (bug#81109)

* lisp/emacs-lisp/edebug.el (edebug-prepare-margin): New
function to set up left margin for Edebug.
(edebug--display-1): Use it.
* src/xdisp.c (display_line): Try to use left margin to display
overlay arrow and fallback to the text area otherwise.
* doc/lispref/display.texi (Overlay Arrow):
* doc/lispref/edebug.texi (Using Edebug):
* doc/emacs/building.texi (Debugger Operation): Document the
change.
* etc/NEWS: Announce the change.

doc/emacs/building.texi

@@ -746,13 +746,13 @@ for special commands that can be used in the GUD interaction buffer.
   As you debug a program, Emacs displays the relevant source files by
 visiting them in Emacs buffers, with an arrow in the left fringe
 indicating the current execution line.  (On a text terminal, the arrow
-appears as @samp{=>}, overlaid on the first two text columns.)  Moving
+appears as @samp{=>}, in the left margin.)  Moving point in such a
-point in such a buffer does not move the arrow.  You are free to edit
+buffer does not move the arrow.  You are free to edit these source
-these source files, but note that inserting or deleting lines will
+files, but note that inserting or deleting lines will throw off the
-throw off the arrow's positioning, as Emacs has no way to figure out
+arrow's positioning, as Emacs has no way to figure out which edited
-which edited source line corresponds to the line reported by the
+source line corresponds to the line reported by the debugger subprocess.
-debugger subprocess.  To update this information, you typically have
+To update this information, you typically have to recompile and restart
-to recompile and restart the program.
+the program.
 
 @cindex GUD and hl-line-mode
 @cindex highlighting execution lines in GUD

doc/lispref/display.texi

@@ -5065,11 +5065,12 @@ to the left of the display area.
 @defvar overlay-arrow-position
 This variable holds a marker that indicates where to display the overlay
 arrow.  It should point at the beginning of a line.  On a non-graphical
-display, or when the left fringe is not shown, the arrow text
+display, or when the left fringe is not shown, the arrow text appears at
-appears at the beginning of that line, overlaying any text that would
+the beginning of that line, overlaying any text that would otherwise
-otherwise appear.  Since the arrow is usually short, and the line
+appear.  Since the arrow is usually short, and the line usually begins
-usually begins with indentation, normally nothing significant is
+with indentation, normally nothing significant is overwritten.  If the
-overwritten.
+window has a left margin with enough space, Emacs displays the arrow
+there instead and the text area is not overwritten.
 
 The overlay-arrow string is displayed in any given buffer if the value
 of @code{overlay-arrow-position} in that buffer points into that

doc/lispref/edebug.texi

@@ -105,8 +105,10 @@ the Lisp code you are debugging.  This is referred to as the @dfn{source
 code buffer}, and it is temporarily read-only.
 
   An arrow in the left fringe indicates the line where the function is
-executing.  Point initially shows where within the line the function is
+executing.  On a non-graphical display (or when the left fringe is not
-executing, but this ceases to be true if you move point yourself.
+shown), this arrow is displayed in the left margin instead.  Point
+initially shows where within the line the function is executing, but
+this ceases to be true if you move point yourself.
 
   If you instrument the definition of @code{fac} (shown below) and then
 execute @code{(fac 3)}, here is what you would normally see.  Point is

etc/NEWS

@@ -72,6 +72,13 @@ accept or ignore the value.
 unconditionally.  The previous behavior, toggling the mode, was
 neither reliable nor generally desirable.
 
++++
+** Emacs tries to display overlay arrow in the left margin.
+On a non-graphical display (or when the left fringe is not shown), if a
+left margin is present, Emacs will now display the overlay arrow into
+this margin.  Edebug is now using this feature by explicitly setting up
+a left margin for it.
+
 
 * Editing Changes in Emacs 32.1
 

lisp/emacs-lisp/edebug.el

@@ -2721,6 +2721,9 @@ when edebug becomes active."
                               edebug-function)
                 ))
 
+          ;; Margin setup for overlay arrow.
+          (edebug-prepare-margin)
+
           ;; Make sure we bind those in the right buffer (bug#16410).
           (let ((overlay-arrow-position overlay-arrow-position)
                 (overlay-arrow-string overlay-arrow-string))
@@ -3001,6 +3004,17 @@ when edebug becomes active."
     )
   "Association list of arrows for each edebug mode.")
 
+(defun edebug-prepare-margin ()
+  "Increase (or set) left margin with the size of the longest arrow string."
+  (let ((arrow-len (apply #'max (mapcar (lambda (x)
+                                          (string-width (cdr x)))
+                                        edebug-arrow-alist)))
+        (margins (window-margins)))
+    ;; Set or increase left margin.
+    (if (numberp (car margins))
+        (set-window-margins nil (+ (car margins) arrow-len))
+      (set-window-margins nil arrow-len))))
+
 (defun edebug-overlay-arrow ()
   ;; Set up the overlay arrow at beginning-of-line in current buffer.
   ;; The arrow string is derived from edebug-arrow-alist and
@@ -3011,7 +3025,6 @@ when edebug becomes active."
     (setq overlay-arrow-position (make-marker))
     (set-marker overlay-arrow-position pos (current-buffer))))
 
-
 (defun edebug-toggle-save-all-windows ()
   "Toggle the saving and restoring of all windows.
 Also, each time you toggle it on, the inside and outside window

src/xdisp.c

@@ -26753,8 +26753,23 @@ display_line (struct it *it, int cursor_vpos)
 	    = get_overlay_arrow_glyph_row (it->w, overlay_arrow_string);
 	  struct glyph *glyph = arrow_row->glyphs[TEXT_AREA];
 	  struct glyph *arrow_end = glyph + arrow_row->used[TEXT_AREA];
-	  struct glyph *p = row->glyphs[TEXT_AREA];
+	  struct glyph *p, *p2, *end, *where;
-	  struct glyph *p2, *end;
+	  short *p_used;
+
+	  /* When possible, put the arrow glyphs at the start of the
+	     left margin.  Otherwise put them at the start of the text
+	     area.  */
+	  if (WINDOW_LEFT_MARGIN_WIDTH (it->w) >= arrow_row->used[TEXT_AREA])
+	    {
+	      p = where = row->glyphs[LEFT_MARGIN_AREA];
+	      p_used = &(row->used[LEFT_MARGIN_AREA]);
+	      row->used[LEFT_MARGIN_AREA] += arrow_row->used[TEXT_AREA];
+	    }
+	  else
+	    {
+	      p = where = row->glyphs[TEXT_AREA];
+	      p_used = &(row->used[TEXT_AREA]);
+	    }
 
 	  /* Copy the arrow glyphs.  */
 	  while (glyph < arrow_end)
@@ -26762,14 +26777,14 @@ display_line (struct it *it, int cursor_vpos)
 
 	  /* Throw away padding glyphs.  */
 	  p2 = p;
-	  end = row->glyphs[TEXT_AREA] + row->used[TEXT_AREA];
+	  end = where + *p_used;
 	  while (p2 < end && CHAR_GLYPH_PADDING_P (*p2))
 	    ++p2;
 	  if (p2 > p)
 	    {
 	      while (p2 < end)
 		*p++ = *p2++;
-	      row->used[TEXT_AREA] = p2 - row->glyphs[TEXT_AREA];
+	      *p_used = p2 - where;
 	    }
 	}
       else