error[E0277]: `*const cxx::void` cannot be sent between threads safely
  --> tests/ui/vector_autotraits.rs:20:19
   |
20 |     assert_send::<CxxVector<ffi::NotThreadSafe>>();
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*const cxx::void` cannot be sent between threads safely
   |
   = help: within `CxxVector<NotThreadSafe>`, the trait `Send` is not implemented for `*const cxx::void`, which is required by `CxxVector<NotThreadSafe>: Send`
   = note: required because it appears within the type `[*const cxx::void; 0]`
note: required because it appears within the type `cxx::private::Opaque`
  --> src/opaque.rs
   |
   | pub struct Opaque {
   |            ^^^^^^
note: required because it appears within the type `NotThreadSafe`
  --> tests/ui/vector_autotraits.rs:7:14
   |
7  |         type NotThreadSafe;
   |              ^^^^^^^^^^^^^
   = note: required because it appears within the type `[NotThreadSafe]`
note: required because it appears within the type `PhantomData<[NotThreadSafe]>`
  --> $RUST/core/src/marker.rs
   |
   | pub struct PhantomData<T: ?Sized>;
   |            ^^^^^^^^^^^
note: required because it appears within the type `CxxVector<NotThreadSafe>`
  --> src/cxx_vector.rs
   |
   | pub struct CxxVector<T> {
   |            ^^^^^^^^^
note: required by a bound in `assert_send`
  --> tests/ui/vector_autotraits.rs:16:19
   |
16 | fn assert_send<T: Send>() {}
   |                   ^^^^ required by this bound in `assert_send`