nats.rs: Can't spawn asynk::Subscription

I’m using smol and nats, and just migrated from rants to this official client since it now has direct async support.

With rants, I was creating a subscription stream and mapping messages to their deserialized data, then spawning a task to run through that stream in the background. After migrating, I’m getting this:

error[E0277]: `std::sync::MutexGuard<'_, std::collections::HashSet<u64>>` cannot be sent between threads safely
   --> detect/src/main.rs:72:5
    |
72  |     Task::spawn(async move {
    |     ^^^^^^^^^^^ `std::sync::MutexGuard<'_, std::collections::HashSet<u64>>` cannot be sent between threads safely
    |
   ::: /Users/andrewbanchich/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/future/mod.rs:55:43
    |
55  | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
    |                                           ------------------------------- within this `impl core::future::future::Future`
    |
   ::: /Users/andrewbanchich/.cargo/registry/src/github.com-1ecc6299db9ec823/async-executor-0.1.2/src/lib.rs:421:31
    |
421 |     pub fn spawn(future: impl Future<Output = T> + Send + 'static) -> Task<T>
    |                               ------------------ required by this bound in `async_executor::Task::<T>::spawn`