Description
Some consumers of streams expect streams to implement Sync. This is true, for example, of tonic GRPC servers when the result of a method is a stream.
Currently, an AsyncStream type automatically gets Sync if the generator also implicitly implements Sync. But it's easy to write a generator that doesn't implement Sync. Just call any async function on a trait that uses async_trait. The return type of such a function is Pin<Box<dyn Future + Send + 'async>>. Notice it doesn't include + Sync.
My guess is that async_trait is doing the right thing here, not adding unnecessary trait bounds. And I don't know why one would want to access a future simultaneously from multiple threads anyway.
For that matter, I don't know why one would want to access a stream simultaneously from multiple threads. But if I understand Sync correctly, it would be safe to explicitly implement it on AsyncStream (though it would require an unsafe block), because the generator is only ever accessed inside a method that takes a mutable reference to the AsyncStream, so IIUC only one thread at a time should be able to call that method.
Note: I'm still pretty new to Rust, so my understanding may be way off. Currently I'm stuck trying to call a Rusoto function inside a try_stream! block, and I'm getting a compiler error because the resulting generator doesn't implement Sync and tonic needs Sync on the stream.