Make out param stack local, fix for zero sized types

Co-authored-by: Daniel Henry-Mantilla <daniel.henry.mantilla@gmail.com>

src/send-and-sync.md

@@ -93,10 +93,11 @@ struct Carton<T>(NonNull<T>);
 impl<T> Carton<T> {
     pub fn new(value: T) -> Self {
         // Allocate enough memory on the heap to store one T.
-        let memptr = &mut null_mut() as *mut *mut T;
+        assert_ne!(size_of::<T>(), 0, "Zero-sized types are out of the scope of this example);
+        let memptr: *mut c_void = ptr::null_mut();
         unsafe {
             let ret = libc::posix_memalign(
-                memptr as *mut *mut c_void,
+                &mut memptr,
                 align_of::<T>(),
                 size_of::<T>()
             );
@@ -107,7 +108,7 @@ impl<T> Carton<T> {
         let mut ptr = unsafe {
             // Safety: memptr is dereferenceable because we created it from a
             // reference and have exclusive access.
-            NonNull::new(*memptr)
+            NonNull::new(memptr.cast::<T>())
                 .expect("Guaranteed non-null if posix_memalign returns 0")
         };