Rust futures executes in multi-threaded runtime.  Because of this mongodb driver must support multi-threaded runtime. It's means that almost every structure in mongodb driver must implement Sync and Send traits. Also it's means that every future that produces this driver must implement Sync and Send traits. But currently mongodb driver does't do this.

Steps to reproduce:

1) Write function test_send_sync

fn test_send_sync<T>(_: T) where T: Send + Sync {}

2) Create connection and open database. Create future and pass it into test_send_sync.

fn test_send_sync<T>(_: T) where T: Send + Sync {}

async fn test_run_command() -> Result<()> {    
let client = Client::with_uri_str("mongodb://localhost:27017").await?;    
let database = client.database("_");    
let future = database.run_command(bson::doc!{}, None);    
test_send_sync(future);    
return Ok(());
}

3) Code will not compiled because future is not implemented Send and Sync trait

Expectations:

This code will be compiled correctly. All futures must implement Send + Sync!