Allow gap before first non-Lisp pseudovec member

Problem reported by Keith David Bershatsky in:
https://lists.gnu.org/r/emacs-devel/2019-04/msg00259.html
Solution suggested by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2019-04/msg00282.html
* src/buffer.h (BUFFER_LISP_SIZE): Simplify by using PSEUDOVECSIZE.
(BUFFER_REST_SIZE): Simplify by using VECSIZE and BUFFER_LISP_SIZE.
* src/lisp.h (PSEUDOVECSIZE): Base it on the last Lisp field,
not the first non-Lisp field.  All callers changed.  Callers
without Lisp fields changed to use ALLOCATE_PLAIN_PSEUDOVECTOR.
(ALLOCATE_PLAIN_PSEUDOVECTOR): New macro.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/window.h b/src/window.h
index 4235a6eade2..fdef407041b 100644
--- a/src/window.h
+++ b/src/window.h
@@ -212,9 +212,8 @@ struct window
     /* The help echo text for this window.  Qnil if there's none.  */
     Lisp_Object mode_line_help_echo;
 
-    /* No Lisp data may follow below this point without changing
-       mark_object in alloc.c.  The member current_matrix must be the
-       first non-Lisp member.  */
+    /* No Lisp data may follow this point; mode_line_help_echo must be
+       the last Lisp member.  */
 
     /* Glyph matrices.  */
     struct glyph_matrix *current_matrix;