Merge from origin/emacs-25

b73f466 * lisp/cus-start.el (exec-path): Handle nil elements.  (Bug#2...
55ebb70 Catch the imenu-unavailable error in sh-mode completion table
993acb5 ; Minor fix for last change in characters.el
30c4bb5 More char-width fixes
4eb4463 Fix char-width-table values for some Emoji
528997d Keep point when switching from and to *terminal* buffer
2130005 * INSTALL: Use correct Emacs release number 25.
10835b1 Avoid crashes due to objects read with the #n=object form
4de671d Improve doc string of 'completion-at-point-functions'
ceb46f0 Fix crash in evaluating functions
d8374c4 * src/filelock.c (current_lock_owner): Update comment.

1 files changed, 12 insertions, 1 deletions

diff --git a/src/lread.c b/src/lread.c
index 10eec3b1be0..58d518ce40b 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2917,7 +2917,18 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
                   if (c == '=')
                     {
                       /* Make a placeholder for #n# to use temporarily.  */
-                      AUTO_CONS (placeholder, Qnil, Qnil);
+                      /* Note: We used to use AUTO_CONS to allocate
+                         placeholder, but that is a bad idea, since it
+                         will place a stack-allocated cons cell into
+                         the list in read_objects, which is a
+                         staticpro'd global variable, and thus each of
+                         its elements is marked during each GC.  A
+                         stack-allocated object will become garbled
+                         when its stack slot goes out of scope, and
+                         some other function reuses it for entirely
+                         different purposes, which will cause crashes
+                         in GC.  */
+                      Lisp_Object placeholder = Fcons (Qnil, Qnil);
                       Lisp_Object cell = Fcons (make_number (n), placeholder);
                       read_objects = Fcons (cell, read_objects);