Cargo.toml

[package]
name = "foo"
version = "0.1.0"

[dependencies]
futures = "0.1"

ice.rs

extern crate futures;

use futures::{future, Future};

fn foo() -> impl Future<Item = (), Error = ()> + Send {
    future::join_all(vec![0].into_iter().map(|_| bar())).and_then(|_: Vec<_>| Ok(()))
}

fn bar() -> Box<Future<Item = (), Error = ()> + Send> {
    unimplemented!()
}

fn main() {}