i'm confused as to why these fields exist, since the typical
implementation of a "temporary alloc" would be something like alloca(),
and alloca() is usually just inlined code that adjusts the stack
pointer, which is not a function whose address could be taken.

this breaks the API/ABI and will require a note in the ChangeLog.

possibly we could revisit the idea of "temporary allocations" by using
C99 variable length arrays. this would have the advantage of being
standardized, unlike alloca().