The Rust Programming Language

Working With Any Number of Futures

When we switched from using two futures to three in the previous section, we also had to switch from using join to using join3. It would be annoying to have to call a different function every time we changed the number of futures we wanted to join. Happily, we have a macro form of join to which we can pass an arbitrary number of arguments. It also handles awaiting the futures itself. Thus, we could rewrite the code from Listing 17-13 to use join! instead of join3, as in Listing 17-14:

extern crate trpl; // required for mdbook test

use std::time::Duration;

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = async move {
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let rx_fut = async {
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        };

        let tx_fut = async move {
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        trpl::join!(tx1_fut, tx_fut, rx_fut);
    });
}

This is definitely a nice improvement over needing to swap between join and join3 and join4 and so on! However, even this macro form only works when we know the number of futures ahead of time. In real-world Rust, though, pushing futures into a collection and then waiting on some or all the futures in that collection to complete is a common pattern.

To check all the futures in some collection, we will need to iterate over and join on all of them. The trpl::join_all function accepts any type which implements the Iterator trait, which we learned about back in Chapter 13, so it seems like just the ticket. Let’s try putting our futures in a vector, and replace join! with join_all.

extern crate trpl; // required for mdbook test

use std::time::Duration;

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = async move {
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let rx_fut = async {
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        };

        let tx_fut = async move {
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let futures = vec![tx1_fut, rx_fut, tx_fut];

        trpl::join_all(futures).await;
    });
}

Unfortunately, this does not compile. Instead, we get this error:

error[E0308]: mismatched types
  --> src/main.rs:43:37
   |
8  |           let tx1_fut = async move {
   |  _______________________-
9  | |             let vals = vec![
10 | |                 String::from("hi"),
11 | |                 String::from("from"),
...  |
19 | |             }
20 | |         };
   | |_________- the expected `async` block
21 |
22 |           let rx_fut = async {
   |  ______________________-
23 | |             while let Some(value) = rx.recv().await {
24 | |                 println!("received '{value}'");
25 | |             }
26 | |         };
   | |_________- the found `async` block
...
43 |           let futures = vec![tx1_fut, rx_fut, tx_fut];
   |                                       ^^^^^^ expected `async` block, found a different `async` block
   |
   = note: expected `async` block `{async block@src/main.rs:8:23: 20:10}`
              found `async` block `{async block@src/main.rs:22:22: 26:10}`
   = note: no two async blocks, even if identical, have the same type
   = help: consider pinning your async block and and casting it to a trait object

This might be surprising. After all, none of them returns anything, so each block produces a Future<Output = ()>. However, Future is a trait, not a concrete type. The concrete types are the individual data structures generated by the compiler for async blocks. You cannot put two different hand-written structs in a Vec, and the same thing applies to the different structs generated by the compiler.

To make this work, we need to use trait objects, just as we did in “Returning Errors from the run function” in Chapter 12. (We will cover trait objects in detail in Chapter 18.) Using trait objects lets us treat each of the anonymous futures produced by these types as the same type, since all of them implement the Future trait.

We start by wrapping each of the futures in the vec! in a Box::new, as shown in Listing 17-16.

extern crate trpl; // required for mdbook test

use std::time::Duration;

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = async move {
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let rx_fut = async {
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        };

        let tx_fut = async move {
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let futures =
            vec![Box::new(tx1_fut), Box::new(rx_fut), Box::new(tx_fut)];

        trpl::join_all(futures).await;
    });
}

Unfortunately, this still does not compile. In fact, we have the same basic error we did before, but we get one for both the second and third Box::new calls, and we also get new errors referring to the Unpin trait. We will come back to the Unpin errors in a moment. First, let’s fix the type errors on the Box::new calls, by explicitly providing the type of futures as a trait object (Listing 17-17).

extern crate trpl; // required for mdbook test

use std::{future::Future, time::Duration};

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = async move {
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let rx_fut = async {
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        };

        let tx_fut = async move {
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        };

        let futures: Vec<Box<dyn Future<Output = ()>>> =
            vec![Box::new(tx1_fut), Box::new(rx_fut), Box::new(tx_fut)];

        trpl::join_all(futures).await;
    });
}

The type we had to write here is a little involved, so let’s walk through it:

• The innermost type is the future itself. We note explicitly that the output of the future is the unit type () by writing Future<Output = ()>.
• Then we annotate the trait with dyn to mark it as dynamic.
• The entire trait reference is wrapped in a Box.
• Finally, we state explicitly that futures is a Vec containing these items.

That already made a big difference. Now when we run the compiler, we only have the errors mentioning Unpin. Although there are three of them, notice that each is very similar in its contents.

error[E0277]: `{async block@src/main.rs:8:23: 20:10}` cannot be unpinned
   --> src/main.rs:46:24
    |
46  |         trpl::join_all(futures).await;
    |         -------------- ^^^^^^^ the trait `Unpin` is not implemented for `{async block@src/main.rs:8:23: 20:10}`, which is required by `Box<{async block@src/main.rs:8:23: 20:10}>: std::future::Future`
    |         |
    |         required by a bound introduced by this call
    |
    = note: consider using the `pin!` macro
            consider using `Box::pin` if you need to access the pinned value outside of the current scope
    = note: required for `Box<{async block@src/main.rs:8:23: 20:10}>` to implement `std::future::Future`
note: required by a bound in `join_all`
   --> /Users/chris/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/future/join_all.rs:105:14
    |
102 | pub fn join_all<I>(iter: I) -> JoinAll<I::Item>
    |        -------- required by a bound in this function
...
105 |     I::Item: Future,
    |              ^^^^^^ required by this bound in `join_all`

error[E0277]: `{async block@src/main.rs:8:23: 20:10}` cannot be unpinned
  --> src/main.rs:46:9
   |
46 |         trpl::join_all(futures).await;
   |         ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Unpin` is not implemented for `{async block@src/main.rs:8:23: 20:10}`, which is required by `Box<{async block@src/main.rs:8:23: 20:10}>: std::future::Future`
   |
   = note: consider using the `pin!` macro
           consider using `Box::pin` if you need to access the pinned value outside of the current scope
   = note: required for `Box<{async block@src/main.rs:8:23: 20:10}>` to implement `std::future::Future`
note: required by a bound in `JoinAll`
  --> /Users/chris/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/future/join_all.rs:29:8
   |
27 | pub struct JoinAll<F>
   |            ------- required by a bound in this struct
28 | where
29 |     F: Future,
   |        ^^^^^^ required by this bound in `JoinAll`

error[E0277]: `{async block@src/main.rs:8:23: 20:10}` cannot be unpinned
  --> src/main.rs:46:33
   |
46 |         trpl::join_all(futures).await;
   |                                 ^^^^^ the trait `Unpin` is not implemented for `{async block@src/main.rs:8:23: 20:10}`, which is required by `Box<{async block@src/main.rs:8:23: 20:10}>: std::future::Future`
   |
   = note: consider using the `pin!` macro
           consider using `Box::pin` if you need to access the pinned value outside of the current scope
   = note: required for `Box<{async block@src/main.rs:8:23: 20:10}>` to implement `std::future::Future`
note: required by a bound in `JoinAll`
  --> /Users/chris/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/future/join_all.rs:29:8
   |
27 | pub struct JoinAll<F>
   |            ------- required by a bound in this struct
28 | where
29 |     F: Future,
   |        ^^^^^^ required by this bound in `JoinAll`

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.

That is a lot to digest, so let’s pull it apart. The first part of the message tell us that the first async block (src/main.rs:8:23: 20:10) does not implement the Unpin trait, and suggests using pin! or Box::pin to resolve it. Later in the chapter, we will dig into a few more details about Pin and Unpin. For the moment, though, we can just follow the compiler’s advice to get unstuck! In Listing 17-18, we start by updating the type annotation for futures, with a Pin wrapping each Box. Second, we use Box::pin to pin the futures themselves.

extern crate trpl; // required for mdbook test

use std::{
    future::Future,
    pin::{pin, Pin},
    time::Duration,
};

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = pin!(async move {
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        });

        let rx_fut = pin!(async {
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        });

        let tx_fut = pin!(async move {
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        });

        let futures: Vec<Pin<Box<dyn Future<Output = ()>>>> =
            vec![Box::pin(tx1_fut), Box::pin(rx_fut), Box::pin(tx_fut)];

        trpl::join_all(futures).await;
    });
}

If we compile and run this, we finally get the output we hoped for:

received 'hi'
received 'more'
received 'from'
received 'messages'
received 'the'
received 'for'
received 'future'
received 'you'

Phew!

There is a bit more we can explore here. For one thing, using Pin<Box<T>> comes with a small amount of extra overhead from putting these futures on the heap with Box—and we are only doing that to get the types to line up. We don’t actually need the heap allocation, after all: these futures are local to this particular function. As noted above, Pin is itself a wrapper type, so we can get the benefit of having a single type in the Vec—the original reason we reached for Box—without doing a heap allocation. We can use Pin directly with each future, using the std::pin::pin macro.

However, we must still be explicit about the type of the pinned reference; otherwise Rust will still not know to interpret these as dynamic trait objects, which is what we need them to be in the Vec. We therefore pin! each future when we define it, and define futures as a Vec containing pinned mutable references to the dynamic Future type, as in Listing 17-19.

extern crate trpl; // required for mdbook test

use std::{
    future::Future,
    pin::{pin, Pin},
    time::Duration,
};

fn main() {
    trpl::run(async {
        let (tx, mut rx) = trpl::channel();

        let tx1 = tx.clone();
        let tx1_fut = pin!(async move {
            // --snip--
            let vals = vec![
                String::from("hi"),
                String::from("from"),
                String::from("the"),
                String::from("future"),
            ];

            for val in vals {
                tx1.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        });

        let rx_fut = pin!(async {
            // --snip--
            while let Some(value) = rx.recv().await {
                println!("received '{value}'");
            }
        });

        let tx_fut = pin!(async move {
            // --snip--
            let vals = vec![
                String::from("more"),
                String::from("messages"),
                String::from("for"),
                String::from("you"),
            ];

            for val in vals {
                tx.send(val).unwrap();
                trpl::sleep(Duration::from_secs(1)).await;
            }
        });

        let futures: Vec<Pin<&mut dyn Future<Output = ()>>> =
            vec![tx1_fut, rx_fut, tx_fut];

        trpl::join_all(futures).await;
    });
}

We got this far by ignoring the fact that we might have different Output types. For example, in Listing 17-20, the anonymous future for a implements Future<Output = u32>, the anonymous future for b implements Future<Output = &str>, and the anonymous future for c implements Future<Output = bool>.

extern crate trpl; // required for mdbook test

fn main() {
    trpl::run(async {
        let a = async { 1u32 };
        let b = async { "Hello!" };
        let c = async { true };

        let (a_result, b_result, c_result) = trpl::join!(a, b, c);
        println!("{a_result}, {b_result}, {c_result}");
    });
}

We can use trpl::join! to await them, because it allows you to pass in multiple future types and produces a tuple of those types. We cannot use trpl::join_all, because it requires the futures passed in all to have the same type. Remember, that error is what got us started on this adventure with Pin!

This is a fundamental tradeoff: we can either deal with a dynamic number of futures with join_all, as long as they all have the same type, or we can deal with a set number of futures with the join functions or the join! macro, even if they have different types. This is the same as working with any other types in Rust, though. Futures are not special, even though we have some nice syntax for working with them, and that is a good thing.

Racing futures

When we “join” futures with the join family of functions and macros, we require all of them to finish before we move on. Sometimes, though, we only need some future from a set to finish before we move on—kind of like racing one future against another.

In Listing 17-21, we once again use trpl::race to run two futures, slow and fast, against each other. Each one prints a message when it starts running, pauses for some amount of time by calling and awaiting sleep, and then prints another message when it finishes. Then we pass both to trpl::race and wait for one of them to finish. (The outcome here won’t be too surprising: fast wins!) Unlike when we used race back in Our First Async Program, we just ignore the Either instance it returns here, because all of the interesting behavior happens in the body of the async blocks.

extern crate trpl; // required for mdbook test

use std::time::Duration;

fn main() {
    trpl::run(async {
        let slow = async {
            println!("'slow' started.");
            trpl::sleep(Duration::from_millis(100)).await;
            println!("'slow' finished.");
        };

        let fast = async {
            println!("'fast' started.");
            trpl::sleep(Duration::from_millis(50)).await;
            println!("'fast' finished.");
        };

        trpl::race(slow, fast).await;
    });
}

Notice that if you flip the order of the arguments to race, the order of the “started” messages changes, even though the fast future always completes first. That is because the implementation of this particular race function is not fair. It always runs the futures passed as arguments in the order they are passed. Other implementations are fair, and will randomly choose which future to poll first. Regardless of whether the implementation of race we are using is fair, though, one of the futures will run up to the first await in its body before another task can start.

Recall from Our First Async Program that at each await point, Rust gives a runtime a chance to pause the task and switch to another one if the future being awaited is not ready. The inverse is also true: Rust only pauses async blocks and hands control back to a runtime at an await point. Everything between await points is synchronous.

That means if you do a bunch of work in an async block without an await point, that future will block any other futures from making progress. You may sometimes hear this referred to as one future starving other futures. In some cases, that may not be a big deal. However, if you are doing some kind of expensive setup or long-running work, or if you have a future which will keep doing some particular task indefinitely, you will need to think about when and where to hand control back to the runtime.

By the same token, if you have long-running blocking operations, async can be a useful tool for providing ways for different parts of the program to relate to each other.

But how would you hand control back to the runtime in those cases?

Yielding

Let’s simulate a long-running operation. Listing 17-22 introduces a slow function. It uses std::thread::sleep instead of trpl::sleep so that calling slow will block the current thread for some number of milliseconds. We can use slow to stand in for real-world operations which are both long-running and blocking.

extern crate trpl; // required for mdbook test

use std::{thread, time::Duration};

fn main() {
    trpl::run(async {
        // We will call `slow` here later
    });
}

fn slow(name: &str, ms: u64) {
    thread::sleep(Duration::from_millis(ms));
    println!("'{name}' ran for {ms}ms");
}

In Listing 17-23, we use slow to emulate doing this kind of CPU-bound work in a pair of futures. To begin, each future only hands control back to the runtime after carrying out a bunch of slow operations.

extern crate trpl; // required for mdbook test

use std::{thread, time::Duration};

fn main() {
    trpl::run(async {
        let a = async {
            println!("'a' started.");
            slow("a", 30);
            slow("a", 10);
            slow("a", 20);
            trpl::sleep(Duration::from_millis(50)).await;
            println!("'a' finished.");
        };

        let b = async {
            println!("'b' started.");
            slow("b", 75);
            slow("b", 10);
            slow("b", 15);
            slow("b", 350);
            trpl::sleep(Duration::from_millis(50)).await;
            println!("'b' finished.");
        };

        trpl::race(a, b).await;
    });
}

fn slow(name: &str, ms: u64) {
    thread::sleep(Duration::from_millis(ms));
    println!("'{name}' ran for {ms}ms");
}

If you run this, you will see this output:

'a' started.
'a' ran for 30ms
'a' ran for 10ms
'a' ran for 20ms
'b' started.
'b' ran for 75ms
'b' ran for 10ms
'b' ran for 15ms
'b' ran for 350ms
'a' finished.

As with our earlier example, race still finishes as soon as a is done. There is no interleaving between the two futures, though. The a future does all of its work until the trpl::sleep call is awaited, then the b future does all of its work until its own trpl::sleep call is awaited, and then the a future completes. To allow both futures to make progress between their slow tasks, we need await points so we can hand control back to the runtime. That means we need something we can await!

We can already see this kind of handoff happening in Listing 17-23: if we removed the trpl::sleep at the end of the a future, it would complete without the b future running at all. Maybe we could use the sleep function as a starting point?

extern crate trpl; // required for mdbook test

use std::{thread, time::Duration};

fn main() {
    trpl::run(async {
        let one_ms = Duration::from_millis(1);

        let a = async {
            println!("'a' started.");
            slow("a", 30);
            trpl::sleep(one_ms).await;
            slow("a", 10);
            trpl::sleep(one_ms).await;
            slow("a", 20);
            trpl::sleep(one_ms).await;
            println!("'a' finished.");
        };

        let b = async {
            println!("'b' started.");
            slow("b", 75);
            trpl::sleep(one_ms).await;
            slow("b", 10);
            trpl::sleep(one_ms).await;
            slow("b", 15);
            trpl::sleep(one_ms).await;
            slow("b", 35);
            trpl::sleep(one_ms).await;
            println!("'b' finished.");
        };

        trpl::race(a, b).await;
    });
}

fn slow(name: &str, ms: u64) {
    thread::sleep(Duration::from_millis(ms));
    println!("'{name}' ran for {ms}ms");
}

In Listing 17-24, we add trpl::sleep calls with await points between each call to slow. Now the two futures’ work is interleaved:

'a' started.
'a' ran for 30ms
'b' started.
'b' ran for 75ms
'a' ran for 10ms
'b' ran for 10ms
'a' ran for 20ms
'b' ran for 15ms
'a' finished.

The a future still runs for a bit before handing off control to b, because it calls slow before ever calling trpl::sleep, but after that the futures swap back and forth each time one of them hits an await point. In this case, we have done that after every call to slow, but we could break up the work however makes the most sense to us.

We do not really want to sleep here, though: we want to make progress as fast as we can. We just need to hand back control to the runtime. We can do that directly, using the yield_now function. In Listing 17-25, we replace all those sleep calls with yield_now.

extern crate trpl; // required for mdbook test

use std::{thread, time::Duration};

fn main() {
    trpl::run(async {
        let a = async {
            println!("'a' started.");
            slow("a", 30);
            trpl::yield_now().await;
            slow("a", 10);
            trpl::yield_now().await;
            slow("a", 20);
            trpl::yield_now().await;
            println!("'a' finished.");
        };

        let b = async {
            println!("'b' started.");
            slow("b", 75);
            trpl::yield_now().await;
            slow("b", 10);
            trpl::yield_now().await;
            slow("b", 15);
            trpl::yield_now().await;
            slow("b", 35);
            trpl::yield_now().await;
            println!("'b' finished.");
        };

        trpl::race(a, b).await;
    });
}

fn slow(name: &str, ms: u64) {
    thread::sleep(Duration::from_millis(ms));
    println!("'{name}' ran for {ms}ms");
}

This is both clearer about the actual intent and can be significantly faster than using sleep, because timers like the one used by sleep often have limits to how granular they can be. The version of sleep we are using, for example, will always sleep for at least a millisecond, even if we pass it a Duration of one nanosecond. Again, modern computers are fast: they can do a lot in one millisecond!

You can see this for yourself by setting up a little benchmark, like the one in Listing 17-26. (This is not an especially rigorous way to do performance testing, but it suffices to show the difference here.) Here, we skip all the status printing, pass a one-nanosecond Duration to trpl::sleep, and let each future run by itself, with no switching between the futures. Then we run for 1,000 iterations and see how long the future using trpl::sleep takes compared to the future using trpl::yield_now.

extern crate trpl; // required for mdbook test

use std::time::{Duration, Instant};

fn main() {
    trpl::run(async {
        let one_ns = Duration::from_nanos(1);
        let start = Instant::now();
        async {
            for _ in 1..1000 {
                trpl::sleep(one_ns).await;
            }
        }
        .await;
        let time = Instant::now() - start;
        println!(
            "'sleep' version finished after {} seconds.",
            time.as_secs_f32()
        );

        let start = Instant::now();
        async {
            for _ in 1..1000 {
                trpl::yield_now().await;
            }
        }
        .await;
        let time = Instant::now() - start;
        println!(
            "'yield' version finished after {} seconds.",
            time.as_secs_f32()
        );
    });
}

The version with yield_now is way faster!

This means that async can be useful even for compute-bound tasks, depending on what else your program is doing, because it provides a useful tool for structuring the relationships between different parts of the program. This is a form of cooperative multitasking, where each future has both the power to determine when it hands over control via await points. Each future therefore also has the responsibility to avoid blocking for too long. In some Rust-based embedded operating systems, this is the only kind of multitasking!

In real-world code, you will not usually be alternating function calls with await points on every single line, of course. While yielding control like this is relatively inexpensive, it is not free! In many cases, trying to break up a compute-bound task might make it significantly slower, so sometimes it is better for overall performance to let an operation block briefly. You should always measure to see what your code’s actual performance bottlenecks are. The underlying dynamic is an important one to keep in mind if you are seeing a lot of work happening in serial that you expected to happen concurrently, though!

Building Our Own Async Abstractions

We can also compose futures together to create new patterns. For example, we can build a timeout function with async building blocks we already have. When we are done, the result will be another building block we could use to build up yet further async abstractions.

Listing 17-27 shows how we would expect this timeout to work with a slow future.

extern crate trpl; // required for mdbook test

use std::time::Duration;

fn main() {
    trpl::run(async {
        let slow = async {
            trpl::sleep(Duration::from_millis(100)).await;
            "I finished!"
        };

        match timeout(slow, Duration::from_millis(10)).await {
            Ok(message) => println!("Succeeded with '{message}'"),
            Err(duration) => {
                println!("Failed after {} seconds", duration.as_secs())
            }
        }
    });
}

Let’s implement this! To begin, let’s think about the API for timeout:

• It needs to be an async function itself so we can await it.
• Its first parameter should be a future to run. We can make it generic to allow it to work with any future.
• Its second parameter will be the maximum time to wait. If we use a Duration, that will make it easy to pass along to trpl::sleep.
• It should return a Result. If the future completes successfully, the Result will be Ok with the value produced by the future. If the timeout elapses first, the Result will be Err with the duration that the timeout waited for.

Listing 17-28 shows this declaration.

extern crate trpl; // required for mdbook test

use std::{future::Future, time::Duration};

fn main() {
    trpl::run(async {
        let slow = async {
            trpl::sleep(Duration::from_secs(5)).await;
            "Finally finished"
        };

        match timeout(slow, Duration::from_millis(10)).await {
            Ok(message) => println!("Succeeded with '{message}'"),
            Err(duration) => {
                println!("Failed after {} seconds", duration.as_secs())
            }
        }
    });
}

async fn timeout<F: Future>(
    future_to_try: F,
    max_time: Duration,
) -> Result<F::Output, Duration> {
    // Here is where our implementation will go!
}

That satisfies our goals for the types. Now let’s think about the behavior we need: we want to race the future passed in against the duration. We can use trpl::sleep to make a timer future from the duration, and use trpl::race to run that timer with the future the caller passes in.

We also know that race is not fair, and polls arguments in the order they are passed. Thus, we pass future_to_try to race first so it gets a chance to complete even if max_time is a very short duration. If future_to_try finishes first, race will return Left with the output from future. If timer finishes first, race will return Right with the timer’s output of ().

In Listing 17-29, we match on the result of awaiting trpl::race. If the future_to_try succeeded and we get a Left(output), we return Ok(output). If the sleep timer elapsed instead and we get a Right(()), we ignore the () with _ and return Err(max_time) instead.

extern crate trpl; // required for mdbook test

use std::{future::Future, time::Duration};

use trpl::Either;

// --snip--

fn main() {
    trpl::run(async {
        let slow = async {
            trpl::sleep(Duration::from_secs(5)).await;
            "Finally finished"
        };

        match timeout(slow, Duration::from_secs(2)).await {
            Ok(message) => println!("Succeeded with '{message}'"),
            Err(duration) => {
                println!("Failed after {} seconds", duration.as_secs())
            }
        }
    });
}

async fn timeout<F: Future>(
    future_to_try: F,
    max_time: Duration,
) -> Result<F::Output, Duration> {
    match trpl::race(future_to_try, trpl::sleep(max_time)).await {
        Either::Left(output) => Ok(output),
        Either::Right(_) => Err(max_time),
    }
}

With that, we have a working timeout, built out of two other async helpers. If we run our code, it will print the failure mode after the timeout:

Failed after 2 seconds

Because futures compose with other futures, you can build really powerful tools using smaller async building blocks. For example, you can use this same approach to combine timeouts with retries, and in turn use those with things like network calls—one of the examples from the beginning of the chapter!

In practice, you will usually work directly with async and await, and secondarily with functions and macros like join, join_all, race, and so on. You will only need to reach for pin now and again to use them with those APIs.

We have now seen a number of ways to work with multiple futures at the same time. Up next, we will look at how we can work with multiple futures in a sequence over time, with streams. Here are a couple more things you might want to consider first, though:

• We used a Vec with join_all to wait for all of the futures in some group to finish. How could you use a Vec to process a group of futures in sequence, instead? What are the tradeoffs of doing that?

• Take a look at the futures::stream::FuturesUnordered type from the futures crate. How would using it be different from using a Vec? (Don’t worry about the fact that it is from the stream part of the crate; it works just fine with any collection of futures.)