When running an application which is trying to be completely single-threaded (by using tokio's current_thread runtime, and then running everything within a LocalSet to allow for !Send types) there is some unexpected behaviour when the futures are dropped:

When tokio::task::spawn is used in a struct's Drop implementation - it panics with there is no reactor running, must be called from the context of a Tokio 1.x runtime if the parent task was spawned with tokio::task::spawn_local:

task started with	spawn used in Drop	panic	expected
tokio::task::spawn	tokio::task::spawn	No	Yes ✅
tokio::task::spawn	tokio::task::spawn_local	Yes: if not running within LocalSet	Yes ✅
tokio::task::spawn_local	tokio::task::spawn	Yes: no reactor running	No ❌
tokio::task::spawn_local	tokio::task::spawn_local	No	Yes ✅

This repository runs each of the combinations in the above table:

$ cargo run --quiet
SpawnMethod::Select + DropMethod::Spawn

SpawnMethod::Select + DropMethod::SpawnLocal

SpawnMethod::Spawn + DropMethod::Spawn

SpawnMethod::Spawn + DropMethod::SpawnLocal
thread 'main' panicked at '`spawn_local` called from outside of a `task::LocalSet`', src/main.rs:88:39
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

SpawnMethod::SpawnLocal + DropMethod::Spawn
thread 'main' panicked at 'there is no reactor running, must be called from the context of a Tokio 1.x runtime', src/main.rs:87:34

SpawnMethod::SpawnLocal + DropMethod::SpawnLocal