Fix some crashes on self-modifying Elisp code
Prompted by a problem report by Alex in: http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00143.html * src/eval.c (For, Fprogn, Fsetq, FletX, eval_sub): Compute XCDR (x) near XCAR (x); although this doesn't fix any bugs, it is likely to run a bit faster with typical hardware caches. (Fif): Use Fcdr instead of XCDR, to avoid crashing on self-modifying S-expressions. (Fsetq, Flet, eval_sub): Count the number of arguments as we go instead of trusting an Flength prepass, to avoid problems when the code is self-modifying. (Fquote, Ffunction, Fdefvar, Fdefconst): Prefer !NILP to CONSP where either will do. This is mostly to document the fact that the value must be a proper list. It's also a tiny bit faster on typical machines nowadays. (Fdefconst, FletX): Prefer XCAR+XCDR to Fcar+Fcdr when either will do. (eval_sub): Check that the args are a list as opposed to some other object that has a length. This prevents e.g. (if . "string") from making Emacs dump core in some cases. * test/src/eval-tests.el (eval-tests--if-dot-string) (eval-tests--let-with-circular-defs, eval-tests--mutating-cond): New tests.
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