How to print / output from within async tokio test

Question

I am writing a new test and when I run this code, the println's do not output in the terminal.

#[tokio::test]
async fn can_fetch_child_keys() {
    if std::option_env!("TEST_WS").is_none() {
        return
    }
    init_logger();
    let child_keys = Builder::<Block>::new()
        .mode(Mode::Online(OnlineConfig {
            transport: std::option_env!("TEST_WS").unwrap().to_owned().into(),
            pallets: vec!["Contracts".to_owned()],
            child_trie: true,
            ..Default::default()
        }))
        .build()
        .await
        .unwrap()
        .execute_with(|| {
            let child_keys = sp_io::storage::next_key(&[]).expect("some key must exist in the snapshot");
            assert!(sp_io::storage::get(&child_keys).is_some());
            child_keys
        });
    assert!(child_keys.len() > 0);
    
    println!("child_keys: {:?}", child_keys);
    println!("👩‍👦 child_keys.len(): {:?}", child_keys.len());
}

I have tried using:

RUST_LOG=debug cargo test --features=remote-test -p frame-remote-externalities can_fetch_child_keys -- --nocapture

Any suggestions on how to output in the terminal for debugging purposes?

This is in specific to the tests found here:

• https://github.com/paritytech/polkadot-sdk/blob/73ff1c3647024d3a5e94727e7fb74714c3a98b47/substrate/utils/frame/remote-externalities/src/lib.rs#L1165

Answer 1

That test requires #[cfg(all(test, feature = "remote-test"))].

Try: cargo test --features remote-test -- --nocapture

Answer 2

Make sure you have TEST_WS environment variable defined:

RUST_LOG=debug TEST_WS=ws://127.0.0.1:9944 cargo test --features=remote-test -p frame-remote-externalities -- --nocapture