I’m trying to use the select! macro’s pre-condition to only include a branch if an Option is the Some variant.

I observe behavour which surprises me: the unwrap is eagerly evaluated causing panic! unless contained within async { }.

That is, this works as expected:

async fn foo(x: u32) {}
let maybe: Option<u32> = None;
select! {
    _ = async { foo(maybe.unwrap()).await }, if maybe.is_some() => (),
    _ = async { some_other_branch().await } => (),
}

…whereas this causes the thread to panic!:

async fn foo(x: u32) {}
let maybe: Option<u32> = None;
select! {
    _ = foo(maybe.unwrap()), if maybe.is_some() => (),
    _ = async { some_other_branch().await } => (),
}

What’s the reason for the difference in behaviour here?