tracing can't track the execution duration of futures passed to futures-concurrency

[yoshuawuyts]

An example provided by @jstarks:

let fut1 = async { ... }.instrument(info_span!("span1"));
let fut2 = async { ... }.instrument(info_span!("span2"));
(fut1, fut2).join();

The problem is that both spans are always measured to have the same duration, because the wrapping future returned by .instrument keeps the span alive until the future is dropped, not until the future's poll() returns ready.

On our end the change we can make is to more eagerly drop futures on completion, rather than only dropping them when the final future resolves. Conservatively tagging this as a bug, since the resulting behavior is not right - even if we aren't failing to uphold any of Rust's async invariants.

tasks

• impl Join for Array Drop futures as soon as they're done for array::join #138
• impl Join for Vec Drop futures as soon as they're done for vec::join #141
• impl Join for Tuple tuple::join - drop futures in-place #143
• impl TryJoin for Array Drop futures as soon as they're done for {array, vec}::try_join #142
• impl TryJoin for Vec Drop futures as soon as they're done for {array, vec}::try_join #142
• impl TryJoin for Tuple tuple::try_join - drop futures in-place #145
• impl Race for Array (N/A)
• impl Race for Vec (N/A)
• impl Race for Tuple (N/A)
• impl RaceOk for Array
• impl RaceOk for Vec
• impl RaceOk for Tuple