object_drop_front: object_drop_front::<E>,

        if let Some(error) = self.downcast_ref::<E>() {

        if self.is::<E>() {
            let outer = ManuallyDrop::new(self);

                let error = ptr::read(error);
                drop(ptr::read(&self.inner));
                mem::forget(self);

                let error = ptr::read(&outer.inner.error as *const () as *const E);
                let inner = ptr::read(&outer.inner);
                let erased = ManuallyDrop::into_inner(inner);
                (erased.vtable.object_drop_front)(erased);

    object_drop_front: unsafe fn(Box<ErrorImpl<()>>),

    drop(unerased);
}
unsafe fn object_drop_front<E>(e: Box<ErrorImpl<()>>) {
    // Drop the fields of ErrorImpl other than E as well as the Box allocation,
    // without dropping E itself. This is used by downcast after doing a
    // ptr::read to take ownership of the E.
    let unerased = mem::transmute::<Box<ErrorImpl<()>>, Box<ErrorImpl<ManuallyDrop<E>>>>(e);