bug#63564: 29.0.91; (setcdr) behaves differently between natively and byte compiled code

[Mattias Engdegård]

18 maj 2023 kl. 19.45 skrev Jimmy Wong <wyuenho@gmail.com>:

> As Mattias has noted, this commit on markdown-mode did fix the issues, it 
> would still be nice to know why the natively compiled version behaves 
> differently from the byte compiled version tho. This could be good learning 
> for occasional elisp devs such as myself to know what to watch out for.

The code was mutating a program constant (quoted list) which is a no-no in 
Elisp. As luck has it, a new warning in Emacs 30 discovered it and a 
markdown-mode maintainer changed the code accordingly (the commit message says 
that he 'fixed a warning' but he really fixed broken code; the warning is fine).

Technically, the error likely occurred because the native compiler propagated 
that constant to its points of use where it underwent some compile-time 
evaluation. Essentially:

(let ((root '(nil)))
  ...
   (let ((sibling-alist (last (cdr root))))  ; sibling-alist = nil
     (dotimes (_ (1- level))
       (setq sibling-alist (last (cdar sibling-alist)))) ; still nil
       (setcdr sibling-alist alist)      ; boom

In Emacs 30 the byte-compiler is able to do some of that propagation as well, 
and rightly so.
Don't mutate program constants. It's not safe, and it has never been safe.