Add assertion to STRING_SET_CHARS

* src/lisp.h (STRING_SET_CHARS): Add an assertion and commentary
to prevent incorrect usage.  For details, see this discussion:
http://lists.gnu.org/archive/html/emacs-devel/2017-04/msg00412.html.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 678e261c1da..e0bad58f758 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1369,6 +1369,10 @@ SBYTES (Lisp_Object string)
 INLINE void
 STRING_SET_CHARS (Lisp_Object string, ptrdiff_t newsize)
 {
+  /* This function cannot change the size of data allocated for the
+     string when it was created.  */
+  eassert ((STRING_MULTIBYTE (string) && newsize <= SBYTES (string))
+           || (!STRING_MULTIBYTE (string) && newsize == SCHARS (string)));
   XSTRING (string)->size = newsize;
 }