Rust: What is the difference between select! and join!?

select! — races futures. join! — runs all futures.

select!:

• Returns first completed.
• Cancels others.
• For racing.

join!:

• Runs all concurrently.
• Returns when all complete.
• For parallelism.

1use tokio::{select, join};
2
3select! {
4    val = fetch_a() => println!("A: {}", val),
5    val = fetch_b() => println!("B: {}", val),
6}
7
8let (a, b) = join!(fetch_a(), fetch_b());

Key: select for racing, join for parallel execution.