Re: [PATCH] (Updated) Run hook when variable is set

[Kelly Dean]

I wrote:
> it now spends half the time doing garbage collection.

I figured out how to solve the voidness problem without consing or specbinding 
every time a hooked variable is set, and without usurping a keyword to 
represent void. Simply use an uninterned symbol to represent void. Specifically:
  DEFSYM (Qvoid_sentinel, "void-sentinel");
  DEFVAR_LISP ("void-sentinel", Vvoid_sentinel,
               doc: /* Representation of voidness for hooked variables.
The value of this constant is an uninterned Lisp symbol that represents void
when passed to or returned from `symbol-setter-function'.
When a variable is hooked, Emacs can't distinguish between setting it to this
value and making it unbound. Therefore, to prevent a difference of behavior
for hooked and unhooked variables, don't set any variable to this value.  */);
  Vvoid_sentinel = Fmake_symbol (build_string ("::void::"));
  XSYMBOL (Qvoid_sentinel)->declared_special = true;
  XSYMBOL (Qvoid_sentinel)->vetted = SYM_CONST;

Then at the end of run_varhook:
  oldval = EQ (oldval, Qunbound) ? Vvoid_sentinel : oldval;
  newval = EQ (newval, Qunbound) ? Vvoid_sentinel : newval;
  newval = call4 (Vsymbol_setter_function, symbol, env, oldval, newval);
  return EQ (newval, Vvoid_sentinel) ? Qunbound : newval;

Semantically, that's worse than consing or specbinding, but not as bad as 
usurping a keyword. Is it ok to do it that way?