usage of Arc<T> where T is not Send or Sync

Currently cargo clippy produces this error:

$ cargo clippy 
    Checking rustls-ffi v0.10.0 (/home/jsha/rust/rustls-ffi)
error: usage of `Arc<T>` where `T` is not `Send` or `Sync`
   --> src/lib.rs:396:23
    |
396 |         Arc::into_raw(Arc::new(src)) as *const _
    |                       ^^^^^^^^^^^^^
    |
    = help: consider using `Rc<T>` instead or wrapping `T` in a std::sync type like `Mutex<T>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#arc_with_non_send_sync
    = note: `#[deny(clippy::arc_with_non_send_sync)]` on by default

The relevant code is:

pub(crate) trait CastConstPtr {
    type RustType;

    fn cast_const_ptr(ptr: *const Self) -> *const Self::RustType {
        ptr as *const _
    }
}

pub(crate) trait CastConstPtr {
    type RustType;

    fn cast_const_ptr(ptr: *const Self) -> *const Self::RustType {
        ptr as *const _
    }
}

/// Anything that qualifies for CastPtr also automatically qualifies for
/// CastConstPtr. Splitting out CastPtr vs CastConstPtr allows us to ensure
/// that Arcs are never cast to a mutable pointer.
impl<T, R> CastConstPtr for T
where
    T: CastPtr<RustType = R>,
    R: Send,
{
    type RustType = R;
}

pub(crate) trait ArcCastPtr: CastConstPtr + Sized + Send + Sync {
    ...
    fn to_const_ptr(src: Self::RustType) -> *const Self {
        Arc::into_raw(Arc::new(src)) as *const _
    }

I think the clippy finding is a good one. Somewhere along the line we should probably be guaranteeing that the RustType for an ArcCastPtr is at least Send. I tried a couple of quick stabs at expressing this in the type system but haven't yet succeeded, so I'm filing this issue to track it.

Start with the CastConstPtr and ArcCastPtr definitions in src/lib.rs, especially the to_const_ptr code around line 396, and run cargo clippy to reproduce the warning. Determine how the relevant RustType bound should be expressed; done means the intended ArcCastPtr types satisfy the required Send or Sync guarantees and cargo clippy passes.