macros: select! not evaluate async expression if precondition fails

[kezhuw]

Motivation

This permits following code without a panic.

let mut value = Some(5);
loop {
  select! {
      _ = ready(value.take().unwrap()), if value.is_some() => {},
      else => break,
  };
}

Currently, <async expression> in select! is evaluated regardless of its precondition. I, personally, think usually async expression is guarded for a reason that it is probably not in a valid state. There are might be cases the evaluation of async expression is pure with side effect, but it is not always. So, I think it might be better to guard evaluation by its precondition.

Solution

Fill futures tuple with None initially and fill them up only if precondition succeed.

Question

I have not polished the code. Before that, I want to make up an agreement.

Does this sound and do we want this ? This is most likely a breaking change. No sure whether we are depending on this subtlety.

Origins

This comes from my experiment in async_select::select.

[kezhuw]

Another behavior changed is that it drop future eagerly after Poll::Ready. But I think this is optional and pure implementation detail.

[Darksonn]

This is probably what we would want, but I'm concerned about breaking changes.