Fix assertion failure when displaying stretch glyphs under linum-mode.

 src/xdisp.c (pop_it): Allow it->object that is a cons cell to pass
 the xassert test.
 src/dispextern.h (struct it): Update the comment documenting what
 can it->OBJECT be.

1 files changed, 13 insertions, 3 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index 6528412158..3c157371ef 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2398,9 +2398,19 @@ struct it
   Lisp_Object font_height;
 
   /* Object and position where the current display element came from.
-     Object can be a Lisp string in case the current display element
-     comes from an overlay string, or it is buffer.  It may also be nil
-     during mode-line update.  Position is a position in object.  */
+     Object is normally the buffer which is being rendered, but it can
+     also be a Lisp string in case the current display element comes
+     from an overlay string or from a display string (before- or
+     after-string).  It may also be nil when a C string is being
+     rendered, e.g., during mode-line or header-line update.  It can
+     also be a cons cell of the form `(space ...)', when we produce a
+     stretch glyph from a `display' specification.  Finally, it can be
+     a zero-valued Lisp integer, but only temporarily, when we are
+     producing special glyphs for display purposes, like truncation
+     and continuation glyphs, or blanks that extend each line to the
+     edge of the window on a TTY.
+
+     Position is the current iterator position in object.  */
   Lisp_Object object;
   struct text_pos position;