Ensure opaque types are !Unpin
diff --git a/src/cxx_vector.rs b/src/cxx_vector.rs
index 5fb0807..d25ea91 100644
--- a/src/cxx_vector.rs
+++ b/src/cxx_vector.rs
@@ -1,7 +1,7 @@
 use crate::cxx_string::CxxString;
 use core::ffi::c_void;
 use core::fmt::{self, Display};
-use core::marker::PhantomData;
+use core::marker::{PhantomData, PhantomPinned};
 use core::mem;
 use core::ptr;
 use core::slice;
@@ -17,6 +17,7 @@
 #[repr(C, packed)]
 pub struct CxxVector<T> {
     _private: [T; 0],
+    _pinned: PhantomData<PhantomPinned>,
 }
 
 impl<T> CxxVector<T>