ffi/unsafe: add void/reference-sink

pkgs/racket-doc/scribblings/foreign/pointers.scrbl

@@ -409,6 +409,17 @@ be used with @racket[ptr-add] to create a substring of a Racket byte
 string, because the offset pointer would be to the middle of a
 collectable object (which is not allowed).}
 
+
+@defproc[(void/reference-sink [v any/c] ...) void?]{
+
+Returns @|void-const|, but unlike calling the @racket[void] function
+where the compiler may optimize away the call and replace it with a
+@|void-const| result, calling @racket[void/reference-sink] ensures
+that the arguments are considered reachable by the garbage collector
+until the call returns.
+
+@history[#:added "6.10.1.2"]}
+
 @; ----------------------------------------------------------------------
 
 @section{Pointer Structure Property}

pkgs/racket-test-core/tests/racket/foreign-test.rktl

@@ -1225,6 +1225,15 @@
 
 ;; ----------------------------------------
 
+;; Check `void/reference-sink`
+(let* ([sym (gensym)]
+       [wb (make-weak-box sym)])
+  (collect-garbage)
+  (void/reference-sink sym)
+  (test #f not (weak-box-value wb)))
+
+;; ----------------------------------------
+
 (let ()
   (unless (eq? (system-type) 'windows)
     (define-ffi-definer define-test-lib test-lib

racket/collects/ffi/unsafe.rkt

@@ -22,7 +22,8 @@
          _bool _stdbool _pointer _gcpointer _scheme (rename-out [_scheme _racket]) _fpointer function-ptr
          memcpy memmove memset
          malloc-immobile-cell free-immobile-cell
-         make-late-weak-box make-late-weak-hasheq)
+         make-late-weak-box make-late-weak-hasheq
+         void/reference-sink)
 
 (define-syntax define*
   (syntax-rules ()
@@ -1988,3 +1989,13 @@
                            (let loop () (will-execute killer-executor) (loop))))))
                      (retry-loop)))))))
       (will-register killer-executor obj finalizer))))
+
+;; The same as `void`, but written so that the compiler cannot
+;; optimize away the call or arguments, so that calling
+;; `void/reference-sink` ensures that arguments are retained.
+(define* void/reference-sink
+  (let ([proc void])
+    (case-lambda
+      [(v) (proc v)]
+      [(v1 v2) (proc v1 v2)]
+      [args (set! proc (lambda args (void))) (proc args)])))