wanted: support for multiple-values in (setf mem-ref)

[3b]

It would be nice to be able to define accessors for simple structures that didn't require allocating temporary containers.

For example with

(defcstruct vec3
  (v :float :count 3))

it would be nice to be able to do something like

(with-foreign-object (vecs '(:struct vec3) 1000)
  (loop for i below 1000
        do (multiple-value-bind (x y z) (mem-aref vecs '(:struct vec3) i)
             (setf (mem-aref vecs '(:struct vec3) i) (values z y x)))))

without any allocation in the loop.

Not sure if a type defined to be used like that could be completely general purpose, but it might still be worth having even if it only worked with for example mem-ref/mem-aref. In that case, the user could define a primary type that translates to/from a sequence, and have a parallel type for the specific optimization use case.

[3b (00003b)]

Probably would require implementation of EXPAND-INTO-FOREIGN-MEMORY as mentioned in the MEM-SET compiler macro, since it can't currently do compile time translation of that even without multiple-values.