Macro unsafe_pinned_drop_ffi 

macro_rules! unsafe_pinned_drop_ffi {
    ($type:ty, $ffi_fn:expr) => { ... };
}

A helper macro to implement PinnedDrop using a raw FFI destructor.

This generates a PinnedDrop implementation that calls the FFI function with a type-cast pointer of the object.

Safety

The FFI function must take a pointer to the object and correctly clean it up.