Module async::combinator

async: combinators (join, select, timeout, map, and_then)

Futures built from other futures, as in Rust's futures::future and tokio's join!/select!/timeout. Each takes its children by value (move them in before the first poll: futures are pinned once polled) and owns them: it polls them, and drops the ones it abandons — select drops the loser, timeout drops the inner future when time runs out, and drop() of a combinator drops every child still running.

Rust infers a combinator's output from its children; SafeC generics can't name "the T of F's Poll<T>", so the output types are type parameters too, written in the declaration and inferred from it:

struct Join2<struct Sleep, struct Fetch, Unit, int> j =
    join2(Sleep::new(50LL), Fetch::new());
AWAIT_INTO(1, self.both, self.j);        // self.both: struct Both<Unit, int>

struct Timeout<struct Fetch, int> t = timeout(Fetch::new(), 100LL);
AWAIT_INTO(2, self.res, self.t);          // self.res: ?int (null = timed out)

Nothing here allocates. JoinAll and SelectAny take caller-provided slices of futures (and of output slots), up to 64 futures.