From https://github.com/mongodb/mongo-rust-driver/issues/1287, with 3.2.0 users have started seeing an error message pointing to Client::drop's use of spawn. Notably, this only seems to happen in test scenarios where Tokio runtimes and clients are being spun up and torn down repeatedly.

My current theory is that this is a recurrence of the issue we've seen repeatedly where a drop impl can race the destruction of the Tokio runtime and sometimes lose.