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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="0_background_information.html" class="active"><strong aria-hidden="true">1.</strong> Background information</a></li><li><a href="1_futures_in_rust.html"><strong aria-hidden="true">2.</strong> Futures in Rust</a></li><li><a href="2_waker_context.html"><strong aria-hidden="true">3.</strong> Waker and Context</a></li><li><a href="3_generators_async_await.html"><strong aria-hidden="true">4.</strong> Generators and async/await</a></li><li><a href="4_pin.html"><strong aria-hidden="true">5.</strong> Pin</a></li><li><a href="6_future_example.html"><strong aria-hidden="true">6.</strong> Implementing Futures</a></li><li><a href="8_finished_example.html"><strong aria-hidden="true">7.</strong> Finished example (editable)</a></li><li class="affix"><a href="conclusion.html">Conclusion and exercises</a></li></ol>
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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<h1><a class="header" href="#some-background-information" id="some-background-information">Some Background Information</a></h1>
<p>Before we go into the details about Futures in Rust, let's take a quick look
at the alternatives for handling concurrent programming in general and some
pros and cons for each of them.</p>
<p>While we do that we'll also explain some aspects when it comes to concurrency which
will make it easier for us when we dive into Futures specifically.</p>
<blockquote>
<p>For fun, I've added a small snippet of runnable code with most of the examples.
If you're like me, things get way more interesting then and maybe you'll see some
things you haven't seen before along the way.</p>
</blockquote>
<h2><a class="header" href="#threads-provided-by-the-operating-system" id="threads-provided-by-the-operating-system">Threads provided by the operating system</a></h2>
<p>Now, one way of accomplishing concurrent programming is letting the OS take care
of everything for us. We do this by simply spawning a new OS thread for each
task we want to accomplish and write code like we normally would.</p>
<p>The runtime we use to handle concurrency for us is the operating system itself.</p>
<p><strong>Advantages:</strong></p>
<ul>
<li>Simple</li>
<li>Easy to use</li>
<li>Switching between tasks is reasonably fast</li>
<li>You get parallelism for free</li>
</ul>
<p><strong>Drawbacks:</strong></p>
<ul>
<li>OS level threads come with a rather large stack. If you have many tasks
waiting simultaneously (like you would in a web-server under heavy load) you'll
run out of memory pretty fast.</li>
<li>There are a lot of syscalls involved. This can be pretty costly when the number
of tasks is high.</li>
<li>The OS has many things it needs to handle. It might not switch back to your
thread as fast as you'd wish.</li>
<li>Might not be an option on some systems</li>
</ul>
<p><strong>Using OS threads in Rust looks like this:</strong></p>
<pre><pre class="playpen"><code class="language-rust">use std::thread;
fn main() {
println!(&quot;So we start the program here!&quot;);
let t1 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(200));
println!(&quot;We create tasks which gets run when they're finished!&quot;);
});
let t2 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(100));
println!(&quot;We can even chain callbacks...&quot;);
let t3 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(50));
println!(&quot;...like this!&quot;);
});
t3.join().unwrap();
});
println!(&quot;While our tasks are executing we can do other stuff here.&quot;);
t1.join().unwrap();
t2.join().unwrap();
}
</code></pre></pre>
<p>OS threads sure have some pretty big advantages. So why all this talk about
&quot;async&quot; and concurrency in the first place?</p>
<p>First, for computers to be <a href="https://en.wikipedia.org/wiki/Efficiency"><em>efficient</em></a> they need to multitask. Once you
start to look under the covers (like <a href="https://os.phil-opp.com/async-await/">how an operating system works</a>)
you'll see concurrency everywhere. It's very fundamental in everything we do.</p>
<p>Secondly, we have the web.</p>
<p>Web servers are all about I/O and handling small tasks
(requests). When the number of small tasks is large it's not a good fit for OS
threads as of today because of the memory they require and the overhead involved
when creating new threads.</p>
<p>This gets even more problematic when the load is variable which means the current number of tasks a
program has at any point in time is unpredictable. That's why you'll see so many async web
frameworks and database drivers today.</p>
<p>However, for a huge number of problems, the standard OS threads will often be the
right solution. So, just think twice about your problem before you reach for an
async library.</p>
<p>Now, let's look at some other options for multitasking. They all have in common
that they implement a way to do multitasking by having a &quot;userland&quot;
runtime.</p>
<h2><a class="header" href="#green-threads" id="green-threads">Green threads</a></h2>
<p>Green threads use the same mechanism as an OS does by creating a thread for
each task, setting up a stack, saving the CPU's state, and jumping from one
task(thread) to another by doing a &quot;context switch&quot;.</p>
<p>We yield control to the scheduler (which is a central part of the runtime in
such a system) which then continues running a different task.</p>
<p>Rust had green threads once, but they were removed before it hit 1.0. The state
of execution is stored in each stack so in such a solution there would be no
need for <code>async</code>, <code>await</code>, <code>Future</code> or <code>Pin</code>.</p>
<p><strong>The typical flow looks like this:</strong></p>
<ol>
<li>Run some non-blocking code.</li>
<li>Make a blocking call to some external resource.</li>
<li>CPU &quot;jumps&quot; to the &quot;main&quot; thread which schedules a different thread to run and
&quot;jumps&quot; to that stack.</li>
<li>Run some non-blocking code on the new thread until a new blocking call or the
task is finished.</li>
<li>CPU &quot;jumps&quot; back to the &quot;main&quot; thread, schedules a new thread which is ready
to make progress, and &quot;jumps&quot; to that thread.</li>
</ol>
<p>These &quot;jumps&quot; are known as <strong>context switches</strong>. Your OS is doing it many times each
second as you read this.</p>
<p><strong>Advantages:</strong></p>
<ol>
<li>Simple to use. The code will look like it does when using OS threads.</li>
<li>A &quot;context switch&quot; is reasonably fast.</li>
<li>Each stack only gets a little memory to start with so you can have hundreds of
thousands of green threads running.</li>
<li>It's easy to incorporate <a href="https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/green-threads#preemptive-multitasking"><em>preemption</em></a>
which puts a lot of control in the hands of the runtime implementors.</li>
</ol>
<p><strong>Drawbacks:</strong></p>
<ol>
<li>The stacks might need to grow. Solving this is not easy and will have a cost.</li>
<li>You need to save all the CPU state on every switch.</li>
<li>It's not a <em>zero cost abstraction</em> (Rust had green threads early on and this
was one of the reasons they were removed).</li>
<li>Complicated to implement correctly if you want to support many different
platforms.</li>
</ol>
<p>A green threads example could look something like this:</p>
<blockquote>
<p>The example presented below is an adapted example from an earlier gitbook I
wrote about green threads called <a href="https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/">Green Threads Explained in 200 lines of Rust.</a>
If you want to know what's going on you'll find everything explained in detail
in that book. The code below is wildly unsafe and it's just to show a real example.
It's not in any way meant to showcase &quot;best practice&quot;. Just so we're on
the same page.</p>
</blockquote>
<p><em><strong>Press the expand icon in the top right corner to show the example code.</strong></em></p>
<pre><pre class="playpen"><code class="language-rust edition2018"># #![feature(asm, naked_functions)]
# use std::ptr;
#
# const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2;
# const MAX_THREADS: usize = 4;
# static mut RUNTIME: usize = 0;
#
# pub struct Runtime {
# threads: Vec&lt;Thread&gt;,
# current: usize,
# }
#
# #[derive(PartialEq, Eq, Debug)]
# enum State {
# Available,
# Running,
# Ready,
# }
#
# struct Thread {
# id: usize,
# stack: Vec&lt;u8&gt;,
# ctx: ThreadContext,
# state: State,
# task: Option&lt;Box&lt;dyn Fn()&gt;&gt;,
# }
#
# #[derive(Debug, Default)]
# #[repr(C)]
# struct ThreadContext {
# rsp: u64,
# r15: u64,
# r14: u64,
# r13: u64,
# r12: u64,
# rbx: u64,
# rbp: u64,
# thread_ptr: u64,
# }
#
# impl Thread {
# fn new(id: usize) -&gt; Self {
# Thread {
# id,
# stack: vec![0_u8; DEFAULT_STACK_SIZE],
# ctx: ThreadContext::default(),
# state: State::Available,
# task: None,
# }
# }
# }
#
# impl Runtime {
# pub fn new() -&gt; Self {
# let base_thread = Thread {
# id: 0,
# stack: vec![0_u8; DEFAULT_STACK_SIZE],
# ctx: ThreadContext::default(),
# state: State::Running,
# task: None,
# };
#
# let mut threads = vec![base_thread];
# threads[0].ctx.thread_ptr = &amp;threads[0] as *const Thread as u64;
# let mut available_threads: Vec&lt;Thread&gt; = (1..MAX_THREADS).map(|i| Thread::new(i)).collect();
# threads.append(&amp;mut available_threads);
#
# Runtime {
# threads,
# current: 0,
# }
# }
#
# pub fn init(&amp;self) {
# unsafe {
# let r_ptr: *const Runtime = self;
# RUNTIME = r_ptr as usize;
# }
# }
#
# pub fn run(&amp;mut self) -&gt; ! {
# while self.t_yield() {}
# std::process::exit(0);
# }
#
# fn t_return(&amp;mut self) {
# if self.current != 0 {
# self.threads[self.current].state = State::Available;
# self.t_yield();
# }
# }
#
# fn t_yield(&amp;mut self) -&gt; bool {
# let mut pos = self.current;
# while self.threads[pos].state != State::Ready {
# pos += 1;
# if pos == self.threads.len() {
# pos = 0;
# }
# if pos == self.current {
# return false;
# }
# }
#
# if self.threads[self.current].state != State::Available {
# self.threads[self.current].state = State::Ready;
# }
#
# self.threads[pos].state = State::Running;
# let old_pos = self.current;
# self.current = pos;
#
# unsafe {
# switch(&amp;mut self.threads[old_pos].ctx, &amp;self.threads[pos].ctx);
# }
# true
# }
#
# pub fn spawn&lt;F: Fn() + 'static&gt;(f: F){
# unsafe {
# let rt_ptr = RUNTIME as *mut Runtime;
# let available = (*rt_ptr)
# .threads
# .iter_mut()
# .find(|t| t.state == State::Available)
# .expect(&quot;no available thread.&quot;);
#
# let size = available.stack.len();
# let s_ptr = available.stack.as_mut_ptr();
# available.task = Some(Box::new(f));
# available.ctx.thread_ptr = available as *const Thread as u64;
# ptr::write(s_ptr.offset((size - 8) as isize) as *mut u64, guard as u64);
# ptr::write(s_ptr.offset((size - 16) as isize) as *mut u64, call as u64);
# available.ctx.rsp = s_ptr.offset((size - 16) as isize) as u64;
# available.state = State::Ready;
# }
# }
# }
#
# fn call(thread: u64) {
# let thread = unsafe { &amp;*(thread as *const Thread) };
# if let Some(f) = &amp;thread.task {
# f();
# }
# }
#
# #[naked]
# fn guard() {
# unsafe {
# let rt_ptr = RUNTIME as *mut Runtime;
# let rt = &amp;mut *rt_ptr;
# println!(&quot;THREAD {} FINISHED.&quot;, rt.threads[rt.current].id);
# rt.t_return();
# };
# }
#
# pub fn yield_thread() {
# unsafe {
# let rt_ptr = RUNTIME as *mut Runtime;
# (*rt_ptr).t_yield();
# };
# }
#
# #[naked]
# #[inline(never)]
# unsafe fn switch(old: *mut ThreadContext, new: *const ThreadContext) {
# asm!(&quot;
# mov %rsp, 0x00($0)
# mov %r15, 0x08($0)
# mov %r14, 0x10($0)
# mov %r13, 0x18($0)
# mov %r12, 0x20($0)
# mov %rbx, 0x28($0)
# mov %rbp, 0x30($0)
#
# mov 0x00($1), %rsp
# mov 0x08($1), %r15
# mov 0x10($1), %r14
# mov 0x18($1), %r13
# mov 0x20($1), %r12
# mov 0x28($1), %rbx
# mov 0x30($1), %rbp
# mov 0x38($1), %rdi
# ret
# &quot;
# :
# : &quot;r&quot;(old), &quot;r&quot;(new)
# :
# : &quot;alignstack&quot;
# );
# }
# #[cfg(not(windows))]
fn main() {
let mut runtime = Runtime::new();
runtime.init();
Runtime::spawn(|| {
println!(&quot;I haven't implemented a timer in this example.&quot;);
yield_thread();
println!(&quot;Finally, notice how the tasks are executed concurrently.&quot;);
});
Runtime::spawn(|| {
println!(&quot;But we can still nest tasks...&quot;);
Runtime::spawn(|| {
println!(&quot;...like this!&quot;);
})
});
runtime.run();
}
# #[cfg(windows)]
# fn main() { }
</code></pre></pre>
<p>Still hanging in there? Good. Don't get frustrated if the code above is
difficult to understand. If I hadn't written it myself I would probably feel
the same. You can always go back and read the book which explains it later.</p>
<h2><a class="header" href="#callback-based-approaches" id="callback-based-approaches">Callback based approaches</a></h2>
<p>You probably already know what we're going to talk about in the next paragraphs
from JavaScript which I assume most know.</p>
<blockquote>
<p>If your exposure to JavaScript callbacks has given you any sorts of PTSD earlier
in life, close your eyes now and scroll down for 2-3 seconds. You'll find a link
there that takes you to safety.</p>
</blockquote>
<p>The whole idea behind a callback based approach is to save a pointer to a set of
instructions we want to run later together with whatever state is needed. In Rust this
would be a <code>closure</code>. In the example below, we save this information in a <code>HashMap</code>
but it's not the only option.</p>
<p>The basic idea of <em>not</em> involving threads as a primary way to achieve concurrency
is the common denominator for the rest of the approaches. Including the one
Rust uses today which we'll soon get to.</p>
<p><strong>Advantages:</strong></p>
<ul>
<li>Easy to implement in most languages</li>
<li>No context switching</li>
<li>Relatively low memory overhead (in most cases)</li>
</ul>
<p><strong>Drawbacks:</strong></p>
<ul>
<li>Since each task must save the state it needs for later, the memory usage will grow
linearly with the number of callbacks in a chain of computations.</li>
<li>Can be hard to reason about. Many people already know this as &quot;callback hell&quot;.</li>
<li>It's a very different way of writing a program, and will require a substantial
rewrite to go from a &quot;normal&quot; program flow to one that uses a &quot;callback based&quot; flow.</li>
<li>Sharing state between tasks is a hard problem in Rust using this approach due
to its ownership model.</li>
</ul>
<p>An extremely simplified example of a how a callback based approach could look
like is:</p>
<pre><pre class="playpen"><code class="language-rust">fn program_main() {
println!(&quot;So we start the program here!&quot;);
set_timeout(200, || {
println!(&quot;We create tasks with a callback that runs once the task finished!&quot;);
});
set_timeout(100, || {
println!(&quot;We can even chain sub-tasks...&quot;);
set_timeout(50, || {
println!(&quot;...like this!&quot;);
})
});
println!(&quot;While our tasks are executing we can do other stuff instead of waiting.&quot;);
}
fn main() {
RT.with(|rt| rt.run(program_main));
}
use std::sync::mpsc::{channel, Receiver, Sender};
use std::{cell::RefCell, collections::HashMap, thread};
thread_local! {
static RT: Runtime = Runtime::new();
}
struct Runtime {
callbacks: RefCell&lt;HashMap&lt;usize, Box&lt;dyn FnOnce() -&gt; ()&gt;&gt;&gt;,
next_id: RefCell&lt;usize&gt;,
evt_sender: Sender&lt;usize&gt;,
evt_reciever: Receiver&lt;usize&gt;,
}
fn set_timeout(ms: u64, cb: impl FnOnce() + 'static) {
RT.with(|rt| {
let id = *rt.next_id.borrow();
*rt.next_id.borrow_mut() += 1;
rt.callbacks.borrow_mut().insert(id, Box::new(cb));
let evt_sender = rt.evt_sender.clone();
thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(ms));
evt_sender.send(id).unwrap();
});
});
}
impl Runtime {
fn new() -&gt; Self {
let (evt_sender, evt_reciever) = channel();
Runtime {
callbacks: RefCell::new(HashMap::new()),
next_id: RefCell::new(1),
evt_sender,
evt_reciever,
}
}
fn run(&amp;self, program: fn()) {
program();
for evt_id in &amp;self.evt_reciever {
let cb = self.callbacks.borrow_mut().remove(&amp;evt_id).unwrap();
cb();
if self.callbacks.borrow().is_empty() {
break;
}
}
}
}
</code></pre></pre>
<p>We're keeping this super simple, and you might wonder what's the difference
between this approach and the one using OS threads and passing in the callbacks
to the OS threads directly.</p>
<p>The difference is that the callbacks are run on the
same thread using this example. The OS threads we create are basically just used
as timers but could represent any kind of resource that we'll have to wait for.</p>
<h2><a class="header" href="#from-callbacks-to-promises" id="from-callbacks-to-promises">From callbacks to promises</a></h2>
<p>You might start to wonder by now, when are we going to talk about Futures?</p>
<p>Well, we're getting there. You see Promises, Futures and other names for
deferred computations are often used interchangeably.</p>
<p>There are formal differences between them, but we won't cover those
here. It's worth explaining <code>promises</code> a bit since they're widely known due to
their use in JavaScript. Promises also have a lot in common with Rust's Futures.</p>
<p>First of all, many languages have a concept of promises, but I'll use the one
from JavaScript in the examples below.</p>
<p>Promises are one way to deal with the complexity which comes with a callback
based approach.</p>
<p>Instead of:</p>
<pre><code class="language-js ignore">setTimer(200, () =&gt; {
setTimer(100, () =&gt; {
setTimer(50, () =&gt; {
console.log(&quot;I'm the last one&quot;);
});
});
});
</code></pre>
<p>We can do this:</p>
<pre><code class="language-js ignore">function timer(ms) {
return new Promise((resolve) =&gt; setTimeout(resolve, ms));
}
timer(200)
.then(() =&gt; return timer(100))
.then(() =&gt; return timer(50))
.then(() =&gt; console.log(&quot;I'm the last one&quot;));
</code></pre>
<p>The change is even more substantial under the hood. You see, promises return
a state machine which can be in one of three states: <code>pending</code>, <code>fulfilled</code> or
<code>rejected</code>.</p>
<p>When we call <code>timer(200)</code> in the sample above, we get back a promise in the state <code>pending</code>.</p>
<p>Since promises are re-written as state machines, they also enable an even better
syntax which allows us to write our last example like this:</p>
<pre><code class="language-js ignore">async function run() {
await timer(200);
await timer(100);
await timer(50);
console.log(&quot;I'm the last one&quot;);
}
</code></pre>
<p>You can consider the <code>run</code> function as a <em>pausable</em> task consisting of several
sub-tasks. On each &quot;await&quot; point it yields control to the scheduler (in this
case it's the well-known JavaScript event loop). </p>
<p>Once one of the sub-tasks changes state to either <code>fulfilled</code> or <code>rejected</code>, the
task is scheduled to continue to the next step.</p>
<p>Syntactically, Rust's Futures 0.1 was a lot like the promises example above, and
Rust's Futures 0.3 is a lot like async/await in our last example.</p>
<p>Now this is also where the similarities between JavaScript promises and Rust's
Futures stop. The reason we go through all this is to get an introduction and
get into the right mindset for exploring Rust's Futures.</p>
<blockquote>
<p>To avoid confusion later on: There's one difference you should know. JavaScript
promises are <em>eagerly</em> evaluated. That means that once it's created, it starts
running a task. Rust's Futures on the other hand are <em>lazily</em> evaluated. They
need to be polled once before they do any work.</p>
</blockquote>
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<h1><a class="header" href="#futures-in-rust" id="futures-in-rust">Futures in Rust</a></h1>
<blockquote>
<p><strong>Overview:</strong></p>
<ul>
<li>Get a high level introduction to concurrency in Rust</li>
<li>Know what Rust provides and not when working with async code</li>
<li>Get to know why we need a runtime-library in Rust</li>
<li>Understand the difference between &quot;leaf-future&quot; and a &quot;non-leaf-future&quot;</li>
<li>Get insight on how to handle CPU intensive tasks</li>
</ul>
</blockquote>
<h2><a class="header" href="#futures" id="futures">Futures</a></h2>
<p>So what is a future?</p>
<p>A future is a representation of some operation which will complete in the
future.</p>
<p>Async in Rust uses a <code>Poll</code> based approach, in which an asynchronous task will
have three phases.</p>
<ol>
<li><strong>The Poll phase.</strong> A Future is polled which result in the task progressing until
a point where it can no longer make progress. We often refer to the part of the
runtime which polls a Future as an executor.</li>
<li><strong>The Wait phase.</strong> An event source, most often referred to as a reactor,
registers that a Future is waiting for an event to happen and makes sure that it
will wake the Future when that event is ready.</li>
<li><strong>The Wake phase.</strong> The event happens and the Future is woken up. It's now up
to the executor which polled the Future in step 1 to schedule the future to be
polled again and make further progress until it completes or reaches a new point
where it can't make further progress and the cycle repeats.</li>
</ol>
<p>Now, when we talk about futures I find it useful to make a distinction between
<strong>non-leaf</strong> futures and <strong>leaf</strong> futures early on because in practice they're
pretty different from one another.</p>
<h3><a class="header" href="#leaf-futures" id="leaf-futures">Leaf futures</a></h3>
<p>Runtimes create <em>leaf futures</em> which represents a resource like a socket.</p>
<pre><code class="language-rust ignore noplaypen">// stream is a **leaf-future**
let mut stream = tokio::net::TcpStream::connect(&quot;127.0.0.1:3000&quot;);
</code></pre>
<p>Operations on these resources, like a <code>Read</code> on a socket, will be non-blocking
and return a future which we call a leaf future since it's the future which
we're actually waiting on.</p>
<p>It's unlikely that you'll implement a leaf future yourself unless you're writing
a runtime, but we'll go through how they're constructed in this book as well.</p>
<p>It's also unlikely that you'll pass a leaf-future to a runtime and run it to
completion alone as you'll understand by reading the next paragraph.</p>
<h3><a class="header" href="#non-leaf-futures" id="non-leaf-futures">Non-leaf-futures</a></h3>
<p>Non-leaf-futures is the kind of futures we as <em>users</em> of a runtime write
ourselves using the <code>async</code> keyword to create a <strong>task</strong> which can be run on the
executor.</p>
<p>The bulk of an async program will consist of non-leaf-futures, which are a kind
of pause-able computation. This is an important distinction since these futures represents a <em>set of operations</em>. Often, such a task will <code>await</code> a leaf future
as one of many operations to complete the task.</p>
<pre><code class="language-rust ignore noplaypen edition2018">// Non-leaf-future
let non_leaf = async {
let mut stream = TcpStream::connect(&quot;127.0.0.1:3000&quot;).await.unwrap();// &lt;- yield
println!(&quot;connected!&quot;);
let result = stream.write(b&quot;hello world\n&quot;).await; // &lt;- yield
println!(&quot;message sent!&quot;);
...
};
</code></pre>
<p>The key to these tasks is that they're able to yield control to the runtime's
scheduler and then resume execution again where it left off at a later point.</p>
<p>In contrast to leaf futures, these kind of futures does not themselves represent
an I/O resource. When we poll these futures we either run some code or we yield
to the scheduler while waiting for some resource to signal us that it's ready so
we can resume where we left off.</p>
<h2><a class="header" href="#runtimes" id="runtimes">Runtimes</a></h2>
<p>Languages like C#, JavaScript, Java, GO and many others comes with a runtime
for handling concurrency. So if you come from one of those languages this will
seem a bit strange to you.</p>
<p>Rust is different from these languages in the sense that Rust doesn't come with
a runtime for handling concurrency, so you need to use a library which provide
this for you.</p>
<p>Quite a bit of complexity attributed to Futures is actually complexity rooted
in runtimes. Creating an efficient runtime is hard.</p>
<p>Learning how to use one correctly requires quite a bit of effort as well, but
you'll see that there are several similarities between these kind of runtimes so
learning one makes learning the next much easier.</p>
<p>The difference between Rust and other languages is that you have to make an
active choice when it comes to picking a runtime. Most often, in other languages
you'll just use the one provided for you.</p>
<p><strong>An async runtime can be divided into two parts:</strong></p>
<ol>
<li>The Executor</li>
<li>The Reactor</li>
</ol>
<p>When Rusts Futures were designed there was a desire to separate the job of
notifying a <code>Future</code> that it can do more work, and actually doing the work
on the <code>Future</code>.</p>
<p>You can think of the former as the reactor's job, and the latter as the
executors job. These two parts of a runtime interact with each other using the <code>Waker</code> type.</p>
<p>The two most popular runtimes for Futures as of writing this is:</p>
<ul>
<li><a href="https://github.com/async-rs/async-std">async-std</a></li>
<li><a href="https://github.com/tokio-rs/tokio">Tokio</a></li>
</ul>
<h3><a class="header" href="#what-rusts-standard-library-takes-care-of" id="what-rusts-standard-library-takes-care-of">What Rust's standard library takes care of</a></h3>
<ol>
<li>A common interface representing an operation which will be completed in the
future through the <code>Future</code> trait.</li>
<li>An ergonomic way of creating tasks which can be suspended and resumed through
the <code>async</code> and <code>await</code> keywords.</li>
<li>A defined interface wake up a suspended task through the <code>Waker</code> type.</li>
</ol>
<p>That's really what Rusts standard library does. As you see there is no definition
of non-blocking I/O, how these tasks are created or how they're run.</p>
<h2><a class="header" href="#io-vs-cpu-intensive-tasks" id="io-vs-cpu-intensive-tasks">I/O vs CPU intensive tasks</a></h2>
<p>As you know now, what you normally write are called non-leaf futures. Let's
take a look at this async block using pseudo-rust as example:</p>
<pre><code class="language-rust ignore">let non_leaf = async {
let mut stream = TcpStream::connect(&quot;127.0.0.1:3000&quot;).await.unwrap(); // &lt;-- yield
// request a large dataset
let result = stream.write(get_dataset_request).await.unwrap(); // &lt;-- yield
// wait for the dataset
let mut response = vec![];
stream.read(&amp;mut response).await.unwrap(); // &lt;-- yield
// do some CPU-intensive analysis on the dataset
let report = analyzer::analyze_data(response).unwrap();
// send the results back
stream.write(report).await.unwrap(); // &lt;-- yield
};
</code></pre>
<p>Now, as you'll see when we go through how Futures work, the code we write between
the yield points are run on the same thread as our executor.</p>
<p>That means that while our <code>analyzer</code> is working on the dataset, the executor
is busy doing calculations instead of handling new requests.</p>
<p>Fortunately there are a few ways to handle this, and it's not difficult, but it's
something you must be aware of:</p>
<ol>
<li>
<p>We could create a new leaf future which sends our task to another thread and
resolves when the task is finished. We could <code>await</code> this leaf-future like any
other future.</p>
</li>
<li>
<p>The runtime could have some kind of supervisor that monitors how much time
different tasks take, and move the executor itself to a different thread so it can
continue to run even though our <code>analyzer</code> task is blocking the original executor thread.</p>
</li>
<li>
<p>You can create a reactor yourself which is compatible with the runtime which
does the analysis any way you see fit, and returns a Future which can be awaited.</p>
</li>
</ol>
<p>Now, #1 is the usual way of handling this, but some executors implement #2 as well.
The problem with #2 is that if you switch runtime you need to make sure that it
supports this kind of supervision as well or else you will end up blocking the
executor.</p>
<p>#3 is more of theoretical importance, normally you'd be happy by sending the task
to the thread-pool most runtimes provide.</p>
<p>Most executors have a way to accomplish #1 using methods like <code>spawn_blocking</code>.</p>
<p>These methods send the task to a thread-pool created by the runtime where you
can either perform CPU-intensive tasks or &quot;blocking&quot; tasks which is not supported
by the runtime.</p>
<p>Now, armed with this knowledge you are already on a good way for understanding
Futures, but we're not gonna stop yet, there is lots of details to cover.</p>
<p>Take a break or a cup of coffe and get ready as we go for a deep dive in the next chapters.</p>
<h2><a class="header" href="#bonus-section" id="bonus-section">Bonus section</a></h2>
<p>If you find the concepts of concurrency and async programming confusing in
general, I know where you're coming from and I have written some resources to
try to give a high level overview that will make it easier to learn Rusts
Futures afterwards:</p>
<ul>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/1_concurrent_vs_parallel.html">Async Basics - The difference between concurrency and parallelism</a></li>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/2_async_history.html">Async Basics - Async history</a></li>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/5_strategies_for_handling_io.html">Async Basics - Strategies for handling I/O</a></li>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/6_epoll_kqueue_iocp.html">Async Basics - Epoll, Kqueue and IOCP</a></li>
</ul>
<p>Learning these concepts by studying futures is making it much harder than
it needs to be, so go on and read these chapters if you feel a bit unsure.</p>
<p>I'll be right here when you're back.</p>
<p>However, if you feel that you have the basics covered, then let's get moving!</p>
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<h1><a class="header" href="#waker-and-context" id="waker-and-context">Waker and Context</a></h1>
<blockquote>
<p><strong>Overview:</strong></p>
<ul>
<li>Understand how the Waker object is constructed</li>
<li>Learn how the runtime know when a leaf-future can resume</li>
<li>Learn the basics of dynamic dispatch and trait objects</li>
</ul>
<p>The <code>Waker</code> type is described as part of <a href="https://github.com/rust-lang/rfcs/blob/master/text/2592-futures.md#waking-up">RFC#2592</a>.</p>
</blockquote>
<h2><a class="header" href="#the-waker" id="the-waker">The Waker</a></h2>
<p>The <code>Waker</code> type allows for a loose coupling between the reactor-part and the executor-part of a runtime.</p>
<p>By having a wake up mechanism that is <em>not</em> tied to the thing that executes
the future, runtime-implementors can come up with interesting new wake-up
mechanisms. An example of this can be spawning a thread to do some work that
eventually notifies the future, completely independent of the current runtime.</p>
<p>Without a waker, the executor would be the <em>only</em> way to notify a running
task, whereas with the waker, we get a loose coupling where it's easy to
extend the ecosystem with new leaf-level tasks.</p>
<blockquote>
<p>If you want to read more about the reasoning behind the <code>Waker</code> type I can
recommend <a href="https://boats.gitlab.io/blog/post/wakers-i/">Withoutboats articles series about them</a>.</p>
</blockquote>
<h2><a class="header" href="#the-context-type" id="the-context-type">The Context type</a></h2>
<p>As the docs state as of now this type only wrapps a <code>Waker</code>, but it gives some
flexibility for future evolutions of the API in Rust. The context can for example hold
task-local storage and provide space for debugging hooks in later iterations.</p>
<h2><a class="header" href="#understanding-the-waker" id="understanding-the-waker">Understanding the <code>Waker</code></a></h2>
<p>One of the most confusing things we encounter when implementing our own <code>Future</code>s
is how we implement a <code>Waker</code> . Creating a <code>Waker</code> involves creating a <code>vtable</code>
which allows us to use dynamic dispatch to call methods on a <em>type erased</em> trait
object we construct our selves.</p>
<blockquote>
<p>If you want to know more about dynamic dispatch in Rust I can recommend an
article written by Adam Schwalm called <a href="https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/">Exploring Dynamic Dispatch in Rust</a>.</p>
</blockquote>
<p>Let's explain this a bit more in detail.</p>
<h2><a class="header" href="#fat-pointers-in-rust" id="fat-pointers-in-rust">Fat pointers in Rust</a></h2>
<p>To get a better understanding of how we implement the <code>Waker</code> in Rust, we need
to take a step back and talk about some fundamentals. Let's start by taking a
look at the size of some different pointer types in Rust.</p>
<p>Run the following code <em>(You'll have to press &quot;play&quot; to see the output)</em>:</p>
<pre><pre class="playpen"><code class="language-rust"># use std::mem::size_of;
trait SomeTrait { }
fn main() {
println!(&quot;======== The size of different pointers in Rust: ========&quot;);
println!(&quot;&amp;dyn Trait:-----{}&quot;, size_of::&lt;&amp;dyn SomeTrait&gt;());
println!(&quot;&amp;[&amp;dyn Trait]:--{}&quot;, size_of::&lt;&amp;[&amp;dyn SomeTrait]&gt;());
println!(&quot;Box&lt;Trait&gt;:-----{}&quot;, size_of::&lt;Box&lt;SomeTrait&gt;&gt;());
println!(&quot;&amp;i32:-----------{}&quot;, size_of::&lt;&amp;i32&gt;());
println!(&quot;&amp;[i32]:---------{}&quot;, size_of::&lt;&amp;[i32]&gt;());
println!(&quot;Box&lt;i32&gt;:-------{}&quot;, size_of::&lt;Box&lt;i32&gt;&gt;());
println!(&quot;&amp;Box&lt;i32&gt;:------{}&quot;, size_of::&lt;&amp;Box&lt;i32&gt;&gt;());
println!(&quot;[&amp;dyn Trait;4]:-{}&quot;, size_of::&lt;[&amp;dyn SomeTrait; 4]&gt;());
println!(&quot;[i32;4]:--------{}&quot;, size_of::&lt;[i32; 4]&gt;());
}
</code></pre></pre>
<p>As you see from the output after running this, the sizes of the references varies.
Many are 8 bytes (which is a pointer size on 64 bit systems), but some are 16
bytes.</p>
<p>The 16 byte sized pointers are called &quot;fat pointers&quot; since they carry extra
information.</p>
<p><strong>Example <code>&amp;[i32]</code> :</strong></p>
<ul>
<li>The first 8 bytes is the actual pointer to the first element in the array (or part of an array the slice refers to)</li>
<li>The second 8 bytes is the length of the slice.</li>
</ul>
<p><strong>Example <code>&amp;dyn SomeTrait</code>:</strong></p>
<p>This is the type of fat pointer we'll concern ourselves about going forward.
<code>&amp;dyn SomeTrait</code> is a reference to a trait, or what Rust calls a <em>trait object</em>.</p>
<p>The layout for a pointer to a <em>trait object</em> looks like this:</p>
<ul>
<li>The first 8 bytes points to the <code>data</code> for the trait object</li>
<li>The second 8 bytes points to the <code>vtable</code> for the trait object</li>
</ul>
<p>The reason for this is to allow us to refer to an object we know nothing about
except that it implements the methods defined by our trait. To accomplish this
we use <em>dynamic dispatch</em>.</p>
<p>Let's explain this in code instead of words by implementing our own trait
object from these parts:</p>
<pre><pre class="playpen"><code class="language-rust">// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
trait Test {
fn add(&amp;self) -&gt; i32;
fn sub(&amp;self) -&gt; i32;
fn mul(&amp;self) -&gt; i32;
}
// This will represent our home brewn fat pointer to a trait object
#[repr(C)]
struct FatPointer&lt;'a&gt; {
/// A reference is a pointer to an instantiated `Data` instance
data: &amp;'a mut Data,
/// Since we need to pass in literal values like length and alignment it's
/// easiest for us to convert pointers to usize-integers instead of the other way around.
vtable: *const usize,
}
// This is the data in our trait object. It's just two numbers we want to operate on.
struct Data {
a: i32,
b: i32,
}
// ====== function definitions ======
fn add(s: &amp;Data) -&gt; i32 {
s.a + s.b
}
fn sub(s: &amp;Data) -&gt; i32 {
s.a - s.b
}
fn mul(s: &amp;Data) -&gt; i32 {
s.a * s.b
}
fn main() {
let mut data = Data {a: 3, b: 2};
// vtable is like special purpose array of pointer-length types with a fixed
// format where the three first values has a special meaning like the
// length of the array is encoded in the array itself as the second value.
let vtable = vec![
0, // pointer to `Drop` (which we're not implementing here)
6, // length of vtable
8, // alignment
// we need to make sure we add these in the same order as defined in the Trait.
add as usize, // function pointer - try changing the order of `add`
sub as usize, // function pointer - and `sub` to see what happens
mul as usize, // function pointer
];
let fat_pointer = FatPointer { data: &amp;mut data, vtable: vtable.as_ptr()};
let test = unsafe { std::mem::transmute::&lt;FatPointer, &amp;dyn Test&gt;(fat_pointer) };
// And voalá, it's now a trait object we can call methods on
println!(&quot;Add: 3 + 2 = {}&quot;, test.add());
println!(&quot;Sub: 3 - 2 = {}&quot;, test.sub());
println!(&quot;Mul: 3 * 2 = {}&quot;, test.mul());
}
</code></pre></pre>
<p>Later on, when we implement our own <code>Waker</code> we'll actually set up a <code>vtable</code>
like we do here. The way we create it is slightly different, but now that you know
how regular trait objects work you will probably recognize what we're doing which
makes it much less mysterious.</p>
<h2><a class="header" href="#bonus-section" id="bonus-section">Bonus section</a></h2>
<p>You might wonder why the <code>Waker</code> was implemented like this and not just as a
normal trait?</p>
<p>The reason is flexibility. Implementing the Waker the way we do here gives a lot
of flexibility of choosing what memory management scheme to use.</p>
<p>The &quot;normal&quot; way is by using an <code>Arc</code> to use reference count keep track of when
a Waker object can be dropped. However, this is not the only way, you could also
use purely global functions and state, or any other way you wish.</p>
<p>This leaves a lot of options on the table for runtime implementors.</p>
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<h1><a class="header" href="#generators-and-asyncawait" id="generators-and-asyncawait">Generators and async/await</a></h1>
<blockquote>
<p><strong>Overview:</strong></p>
<ul>
<li>Understand how the async/await syntax works under the hood</li>
<li>See first hand why we need <code>Pin</code></li>
<li>Understand what makes Rusts async model very memory efficient</li>
</ul>
<p>The motivation for <code>Generator</code>s can be found in <a href="https://github.com/rust-lang/rfcs/blob/master/text/2033-experimental-coroutines.md">RFC#2033</a>. It's very
well written and I can recommend reading through it (it talks as much about
async/await as it does about generators).</p>
</blockquote>
<h2><a class="header" href="#why-learn-about-generators" id="why-learn-about-generators">Why learn about generators?</a></h2>
<p>Generators/yield and async/await are so similar that once you understand one
you should be able to understand the other.</p>
<p>It's much easier for me to provide runnable and short examples using Generators
instead of Futures which require us to introduce a lot of concepts now that
we'll cover later just to show an example.</p>
<p>Async/await works like generators but instead of returning a generator it returns
a special object implementing the Future trait.</p>
<p>A small bonus is that you'll have a pretty good introduction to both Generators
and Async/Await by the end of this chapter.</p>
<p>Basically, there were three main options discussed when designing how Rust would
handle concurrency:</p>
<ol>
<li>Stackful coroutines, better known as green threads.</li>
<li>Using combinators.</li>
<li>Stackless coroutines, better known as generators.</li>
</ol>
<p>We covered <a href="0_background_information.html#green-threads">green threads in the background information</a>
so we won't repeat that here. We'll concentrate on the variants of stackless
coroutines which Rust uses today.</p>
<h3><a class="header" href="#combinators" id="combinators">Combinators</a></h3>
<p><code>Futures 0.1</code> used combinators. If you've worked with Promises in JavaScript,
you already know combinators. In Rust they look like this:</p>
<pre><code class="language-rust noplaypen ignore">let future = Connection::connect(conn_str).and_then(|conn| {
conn.query(&quot;somerequest&quot;).map(|row|{
SomeStruct::from(row)
}).collect::&lt;Vec&lt;SomeStruct&gt;&gt;()
});
let rows: Result&lt;Vec&lt;SomeStruct&gt;, SomeLibraryError&gt; = block_on(future);
</code></pre>
<p><strong>There are mainly three downsides I'll focus on using this technique:</strong></p>
<ol>
<li>The error messages produced could be extremely long and arcane</li>
<li>Not optimal memory usage</li>
<li>Did not allow to borrow across combinator steps.</li>
</ol>
<p>Point #3, is actually a major drawback with <code>Futures 0.1</code>.</p>
<p>Not allowing borrows across suspension points ends up being very
un-ergonomic and to accomplish some tasks it requires extra allocations or
copying which is inefficient.</p>
<p>The reason for the higher than optimal memory usage is that this is basically
a callback-based approach, where each closure stores all the data it needs
for computation. This means that as we chain these, the memory required to store
the needed state increases with each added step.</p>
<h3><a class="header" href="#stackless-coroutinesgenerators" id="stackless-coroutinesgenerators">Stackless coroutines/generators</a></h3>
<p>This is the model used in Rust today. It has a few notable advantages:</p>
<ol>
<li>It's easy to convert normal Rust code to a stackless coroutine using using
async/await as keywords (it can even be done using a macro).</li>
<li>No need for context switching and saving/restoring CPU state</li>
<li>No need to handle dynamic stack allocation</li>
<li>Very memory efficient</li>
<li>Allows us to borrow across suspension points</li>
</ol>
<p>The last point is in contrast to <code>Futures 0.1</code>. With async/await we can do this:</p>
<pre><code class="language-rust ignore">async fn myfn() {
let text = String::from(&quot;Hello world&quot;);
let borrowed = &amp;text[0..5];
somefuture.await;
println!(&quot;{}&quot;, borrowed);
}
</code></pre>
<p>Async in Rust is implemented using Generators. So to understand how async really
works we need to understand generators first. Generators in Rust are implemented
as state machines.</p>
<p>The memory footprint of a chain of computations is defined by <em>the largest footprint
that a single step requires</em>.</p>
<p>That means that adding steps to a chain of computations might not require any
increased memory at all and it's one of the reasons why Futures and Async in
Rust has very little overhead.</p>
<h2><a class="header" href="#how-generators-work" id="how-generators-work">How generators work</a></h2>
<p>In Nightly Rust today you can use the <code>yield</code> keyword. Basically using this
keyword in a closure, converts it to a generator. A closure could look like this
before we had a concept of <code>Pin</code>:</p>
<pre><code class="language-rust noplaypen ignore">#![feature(generators, generator_trait)]
use std::ops::{Generator, GeneratorState};
fn main() {
let a: i32 = 4;
let mut gen = move || {
println!(&quot;Hello&quot;);
yield a * 2;
println!(&quot;world!&quot;);
};
if let GeneratorState::Yielded(n) = gen.resume() {
println!(&quot;Got value {}&quot;, n);
}
if let GeneratorState::Complete(()) = gen.resume() {
()
};
}
</code></pre>
<p>Early on, before there was a consensus about the design of <code>Pin</code>, this
compiled to something looking similar to this:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let mut gen = GeneratorA::start(4);
if let GeneratorState::Yielded(n) = gen.resume() {
println!(&quot;Got value {}&quot;, n);
}
if let GeneratorState::Complete(()) = gen.resume() {
()
};
}
// If you've ever wondered why the parameters are called Y and R the naming from
// the original rfc most likely holds the answer
enum GeneratorState&lt;Y, R&gt; {
Yielded(Y), // originally called `Yield(Y)`
Complete(R), // originally called `Return(R)`
}
trait Generator {
type Yield;
type Return;
fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
}
enum GeneratorA {
Enter(i32),
Yield1(i32),
Exit,
}
impl GeneratorA {
fn start(a1: i32) -&gt; Self {
GeneratorA::Enter(a1)
}
}
impl Generator for GeneratorA {
type Yield = i32;
type Return = ();
fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
// lets us get ownership over current state
match std::mem::replace(self, GeneratorA::Exit) {
GeneratorA::Enter(a1) =&gt; {
/*----code before yield----*/
println!(&quot;Hello&quot;);
let a = a1 * 2;
*self = GeneratorA::Yield1(a);
GeneratorState::Yielded(a)
}
GeneratorA::Yield1(_) =&gt; {
/*-----code after yield-----*/
println!(&quot;world!&quot;);
*self = GeneratorA::Exit;
GeneratorState::Complete(())
}
GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
}
}
}
</code></pre></pre>
<blockquote>
<p>The <code>yield</code> keyword was discussed first in <a href="https://github.com/rust-lang/rfcs/pull/1823">RFC#1823</a> and in <a href="https://github.com/rust-lang/rfcs/pull/1832">RFC#1832</a>.</p>
</blockquote>
<p>Now that you know that the <code>yield</code> keyword in reality rewrites your code to become a state machine,
you'll also know the basics of how <code>await</code> works. It's very similar.</p>
<p>Now, there are some limitations in our naive state machine above. What happens when you have a
<code>borrow</code> across a <code>yield</code> point?</p>
<p>We could forbid that, but <strong>one of the major design goals for the async/await syntax has been
to allow this</strong>. These kinds of borrows were not possible using <code>Futures 0.1</code> so we can't let this
limitation just slip and call it a day yet.</p>
<p>Instead of discussing it in theory, let's look at some code.</p>
<blockquote>
<p>We'll use the optimized version of the state machines which is used in Rust today. For a more
in depth explanation see <a href="https://tmandry.gitlab.io/blog/posts/optimizing-await-1/">Tyler Mandry's excellent article: How Rust optimizes async/await</a></p>
</blockquote>
<pre><code class="language-rust noplaypen ignore">let mut generator = move || {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
yield borrowed.len();
println!(&quot;{} world!&quot;, borrowed);
};
</code></pre>
<p>We'll be hand-coding some versions of a state-machines representing a state
machine for the generator defined above.</p>
<p>We step through each step &quot;manually&quot; in every example, so it looks pretty
unfamiliar. We could add some syntactic sugar like implementing the <code>Iterator</code>
trait for our generators which would let us do this:</p>
<pre><code class="language-rust ignore">while let Some(val) = generator.next() {
println!(&quot;{}&quot;, val);
}
</code></pre>
<p>It's a pretty trivial change to make, but this chapter is already getting long.
Just keep this in the back of your head as we move forward.</p>
<p>Now what does our rewritten state machine look like with this example?</p>
<pre><pre class="playpen"><code class="language-rust compile_fail">
# #![allow(unused_variables)]
#fn main() {
# enum GeneratorState&lt;Y, R&gt; {
# Yielded(Y),
# Complete(R),
# }
#
# trait Generator {
# type Yield;
# type Return;
# fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
# }
enum GeneratorA {
Enter,
Yield1 {
to_borrow: String,
borrowed: &amp;String, // uh, what lifetime should this have?
},
Exit,
}
# impl GeneratorA {
# fn start() -&gt; Self {
# GeneratorA::Enter
# }
# }
impl Generator for GeneratorA {
type Yield = usize;
type Return = ();
fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
// lets us get ownership over current state
match std::mem::replace(self, GeneratorA::Exit) {
GeneratorA::Enter =&gt; {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow; // &lt;--- NB!
let res = borrowed.len();
*self = GeneratorA::Yield1 {to_borrow, borrowed};
GeneratorState::Yielded(res)
}
GeneratorA::Yield1 {to_borrow, borrowed} =&gt; {
println!(&quot;Hello {}&quot;, borrowed);
*self = GeneratorA::Exit;
GeneratorState::Complete(())
}
GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
}
}
}
#}</code></pre></pre>
<p>If you try to compile this you'll get an error (just try it yourself by pressing play).</p>
<p>What is the lifetime of <code>&amp;String</code>. It's not the same as the lifetime of <code>Self</code>. It's not <code>static</code>.
Turns out that it's not possible for us in Rusts syntax to describe this lifetime, which means, that
to make this work, we'll have to let the compiler know that <em>we</em> control this correctly ourselves.</p>
<p>That means turning to unsafe.</p>
<p>Let's try to write an implementation that will compiler using <code>unsafe</code>. As you'll
see we end up in a <em>self referential struct</em>. A struct which holds references
into itself.</p>
<p>As you'll notice, this compiles just fine!</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum GeneratorState&lt;Y, R&gt; {
Yielded(Y),
Complete(R),
}
trait Generator {
type Yield;
type Return;
fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
}
enum GeneratorA {
Enter,
Yield1 {
to_borrow: String,
borrowed: *const String, // NB! This is now a raw pointer!
},
Exit,
}
impl GeneratorA {
fn start() -&gt; Self {
GeneratorA::Enter
}
}
impl Generator for GeneratorA {
type Yield = usize;
type Return = ();
fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
match self {
GeneratorA::Enter =&gt; {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
let res = borrowed.len();
*self = GeneratorA::Yield1 {to_borrow, borrowed: std::ptr::null()};
// NB! And we set the pointer to reference the to_borrow string here
if let GeneratorA::Yield1 {to_borrow, borrowed} = self {
*borrowed = to_borrow;
}
GeneratorState::Yielded(res)
}
GeneratorA::Yield1 {borrowed, ..} =&gt; {
let borrowed: &amp;String = unsafe {&amp;**borrowed};
println!(&quot;{} world&quot;, borrowed);
*self = GeneratorA::Exit;
GeneratorState::Complete(())
}
GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
}
}
}
#}</code></pre></pre>
<p>Remember that our example is the generator we crated which looked like this:</p>
<pre><code class="language-rust noplaypen ignore">let mut gen = move || {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
yield borrowed.len();
println!(&quot;{} world!&quot;, borrowed);
};
</code></pre>
<p>Below is an example of how we could run this state-machine and as you see it
does what we'd expect. But there is still one huge problem with this:</p>
<pre><pre class="playpen"><code class="language-rust">pub fn main() {
let mut gen = GeneratorA::start();
let mut gen2 = GeneratorA::start();
if let GeneratorState::Yielded(n) = gen.resume() {
println!(&quot;Got value {}&quot;, n);
}
if let GeneratorState::Yielded(n) = gen2.resume() {
println!(&quot;Got value {}&quot;, n);
}
if let GeneratorState::Complete(()) = gen.resume() {
()
};
}
# enum GeneratorState&lt;Y, R&gt; {
# Yielded(Y),
# Complete(R),
# }
#
# trait Generator {
# type Yield;
# type Return;
# fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
# }
#
# enum GeneratorA {
# Enter,
# Yield1 {
# to_borrow: String,
# borrowed: *const String,
# },
# Exit,
# }
#
# impl GeneratorA {
# fn start() -&gt; Self {
# GeneratorA::Enter
# }
# }
# impl Generator for GeneratorA {
# type Yield = usize;
# type Return = ();
# fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
# match self {
# GeneratorA::Enter =&gt; {
# let to_borrow = String::from(&quot;Hello&quot;);
# let borrowed = &amp;to_borrow;
# let res = borrowed.len();
# *self = GeneratorA::Yield1 {to_borrow, borrowed: std::ptr::null()};
#
# // We set the self-reference here
# if let GeneratorA::Yield1 {to_borrow, borrowed} = self {
# *borrowed = to_borrow;
# }
#
# GeneratorState::Yielded(res)
# }
#
# GeneratorA::Yield1 {borrowed, ..} =&gt; {
# let borrowed: &amp;String = unsafe {&amp;**borrowed};
# println!(&quot;{} world&quot;, borrowed);
# *self = GeneratorA::Exit;
# GeneratorState::Complete(())
# }
# GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
# }
# }
# }
</code></pre></pre>
<p>The problem is that in safe Rust we can still do this:</p>
<p><em>Run the code and compare the results. Do you see the problem?</em></p>
<pre><pre class="playpen"><code class="language-rust should_panic"># #![feature(never_type)] // Force nightly compiler to be used in playground
# // by betting on it's true that this type is named after it's stabilization date...
pub fn main() {
let mut gen = GeneratorA::start();
let mut gen2 = GeneratorA::start();
if let GeneratorState::Yielded(n) = gen.resume() {
println!(&quot;Got value {}&quot;, n);
}
std::mem::swap(&amp;mut gen, &amp;mut gen2); // &lt;--- Big problem!
if let GeneratorState::Yielded(n) = gen2.resume() {
println!(&quot;Got value {}&quot;, n);
}
// This would now start gen2 since we swapped them.
if let GeneratorState::Complete(()) = gen.resume() {
()
};
}
# enum GeneratorState&lt;Y, R&gt; {
# Yielded(Y),
# Complete(R),
# }
#
# trait Generator {
# type Yield;
# type Return;
# fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
# }
#
# enum GeneratorA {
# Enter,
# Yield1 {
# to_borrow: String,
# borrowed: *const String,
# },
# Exit,
# }
#
# impl GeneratorA {
# fn start() -&gt; Self {
# GeneratorA::Enter
# }
# }
# impl Generator for GeneratorA {
# type Yield = usize;
# type Return = ();
# fn resume(&amp;mut self) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
# match self {
# GeneratorA::Enter =&gt; {
# let to_borrow = String::from(&quot;Hello&quot;);
# let borrowed = &amp;to_borrow;
# let res = borrowed.len();
# *self = GeneratorA::Yield1 {to_borrow, borrowed: std::ptr::null()};
#
# // We set the self-reference here
# if let GeneratorA::Yield1 {to_borrow, borrowed} = self {
# *borrowed = to_borrow;
# }
#
# GeneratorState::Yielded(res)
# }
#
# GeneratorA::Yield1 {borrowed, ..} =&gt; {
# let borrowed: &amp;String = unsafe {&amp;**borrowed};
# println!(&quot;{} world&quot;, borrowed);
# *self = GeneratorA::Exit;
# GeneratorState::Complete(())
# }
# GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
# }
# }
# }
</code></pre></pre>
<p>Wait? What happened to &quot;Hello&quot;? And why did our code segfault?</p>
<p>Turns out that while the example above compiles just fine, we expose consumers
of this this API to both possible undefined behavior and other memory errors
while using just safe Rust. This is a big problem!</p>
<blockquote>
<p>I've actually forced the code above to use the nightly version of the compiler.
If you run <a href="https://play.rust-lang.org/?version=stable&amp;mode=debug&amp;edition=2018&amp;gist=5cbe9897c0e23a502afd2740c7e78b98">the example above on the playground</a>,
you'll see that it runs without panicking on the current stable (1.42.0) but
panics on the current nightly (1.44.0). Scary!</p>
</blockquote>
<p>We'll explain exactly what happened here using a slightly simpler example in the next
chapter and we'll fix our generator using <code>Pin</code> so don't worry, you'll see exactly
what goes wrong and see how <code>Pin</code> can help us deal with self-referential types safely in a
second.</p>
<p>Before we go and explain the problem in detail, let's finish off this chapter
by looking at how generators and the async keyword is related.</p>
<h2><a class="header" href="#async-and-generators" id="async-and-generators">Async and generators</a></h2>
<p>Futures in Rust are implemented as state machines much the same way Generators
are state machines.</p>
<p>You might have noticed the similarities in the syntax used in async blocks and
the syntax used in generators:</p>
<pre><code class="language-rust ignore">let mut gen = move || {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
yield borrowed.len();
println!(&quot;{} world!&quot;, borrowed);
};
</code></pre>
<p>Compare that with a similar example using async blocks:</p>
<pre><code class="language-rust ignore">let mut fut = async {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
SomeResource::some_task().await;
println!(&quot;{} world!&quot;, borrowed);
};
</code></pre>
<p>The difference is that Futures has different states than what a <code>Generator</code> would
have.</p>
<p>An async block will return a <code>Future</code> instead of a <code>Generator</code>, however, the way
a Future works and the way a Generator work internally is similar.</p>
<p>Instead of calling <code>Generator::resume</code> we call <code>Future::poll</code>, and instead of
returning <code>Yielded</code> or <code>Complete</code> it returns <code>Pending</code> or <code>Ready</code>. Each <code>await</code>
point in a future is like a <code>yield</code> point in a generator.</p>
<p>Do you see how they're connected now?</p>
<p>Thats why knowing how generators work and the challenges they pose also teaches
you how futures work and the challenges we need to tackle when working with them.</p>
<p>The same goes for the challenges of borrowing across yield/await points.</p>
<h2><a class="header" href="#bonus-section---self-referential-generators-in-rust-today" id="bonus-section---self-referential-generators-in-rust-today">Bonus section - self referential generators in Rust today</a></h2>
<p>Thanks to <a href="https://github.com/rust-lang/rust/pull/45337/files">PR#45337</a> you can actually run code like the one in our
example in Rust today using the <code>static</code> keyword on nightly. Try it for
yourself:</p>
<blockquote>
<p>Beware that the API is changing rapidly. As I was writing this book, generators
had an API change adding support for a &quot;resume&quot; argument to get passed into the
generator closure.</p>
<p>Follow the progress on the <a href="https://github.com/rust-lang/rust/issues/43122">tracking issue #4312</a> for <a href="https://github.com/rust-lang/rfcs/blob/master/text/2033-experimental-coroutines.md">RFC#033</a>.</p>
</blockquote>
<pre><pre class="playpen"><code class="language-rust">#![feature(generators, generator_trait)]
use std::ops::{Generator, GeneratorState};
pub fn main() {
let gen1 = static || {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
yield borrowed.len();
println!(&quot;{} world!&quot;, borrowed);
};
let gen2 = static || {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
yield borrowed.len();
println!(&quot;{} world!&quot;, borrowed);
};
let mut pinned1 = Box::pin(gen1);
let mut pinned2 = Box::pin(gen2);
if let GeneratorState::Yielded(n) = pinned1.as_mut().resume(()) {
println!(&quot;Gen1 got value {}&quot;, n);
}
if let GeneratorState::Yielded(n) = pinned2.as_mut().resume(()) {
println!(&quot;Gen2 got value {}&quot;, n);
};
let _ = pinned1.as_mut().resume(());
let _ = pinned2.as_mut().resume(());
}
</code></pre></pre>
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<h1><a class="header" href="#pin" id="pin">Pin</a></h1>
<blockquote>
<p><strong>Overview</strong></p>
<ol>
<li>Learn how to use <code>Pin</code> and why it's required when implementing your own <code>Future</code></li>
<li>Understand how to make self-referential types safe to use in Rust</li>
<li>Learn how borrowing across <code>await</code> points is accomplished</li>
<li>Get a set of practical rules to help you work with <code>Pin</code></li>
</ol>
<p><code>Pin</code> was suggested in <a href="https://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md">RFC#2349</a></p>
</blockquote>
<p>Let's jump strait to it. Pinning is one of those subjects which is hard to wrap
your head around in the start, but once you unlock a mental model for it
it gets significantly easier to reason about.</p>
<h2><a class="header" href="#definitions" id="definitions">Definitions</a></h2>
<p>Pin wraps a pointer. A reference to an object is a pointer. Pin gives some
guarantees about the <em>pointee</em> (the data it points to) which we'll explore further
in this chapter.</p>
<p>Pin consists of the <code>Pin</code> type and the <code>Unpin</code> marker. Pin's purpose in life is
to govern the rules that need to apply for types which implement <code>!Unpin</code>.</p>
<p>Yep, you're right, that's double negation right there. <code>!Unpin</code> means
&quot;not-un-pin&quot;.</p>
<blockquote>
<p><em>This naming scheme is one of Rusts safety features where it deliberately
tests if you're too tired to safely implement a type with this marker. If
you're starting to get confused, or even angry, by <code>!Unpin</code> it's a good sign
that it's time to lay down the work and start over tomorrow with a fresh mind.</em></p>
</blockquote>
<p>On a more serious note, I feel obliged to mention that there are valid reasons
for the names that were chosen. Naming is not easy, and I considered renaming
<code>Unpin</code> and <code>!Unpin</code> in this book to make them easier to reason about.</p>
<p>However, an experienced member of the Rust community convinced me that that there
is just too many nuances and edge-cases to consider which is easily overlooked when
naively giving these markers different names, and I'm convinced that we'll
just have to get used to them and use them as is.</p>
<p>If you want to you can read a bit of the discussion from the
<a href="https://internals.rust-lang.org/t/naming-pin-anchor-move/6864/12">internals thread</a>.</p>
<h2><a class="header" href="#pinning-and-self-referential-structs" id="pinning-and-self-referential-structs">Pinning and self-referential structs</a></h2>
<p>Let's start where we left off in the last chapter by making the problem we
saw using a self-references in our generator a lot simpler by making
some self-referential structs that are easier to reason about than our
state machines:</p>
<p>For now our example will look like this:</p>
<pre><code class="language-rust ignore">use std::pin::Pin;
#[derive(Debug)]
struct Test {
a: String,
b: *const String,
}
impl Test {
fn new(txt: &amp;str) -&gt; Self {
Test {
a: String::from(txt),
b: std::ptr::null(),
}
}
fn init(&amp;mut self) {
let self_ref: *const String = &amp;self.a;
self.b = self_ref;
}
fn a(&amp;self) -&gt; &amp;str {
&amp;self.a
}
fn b(&amp;self) -&gt; &amp;String {
unsafe {&amp;*(self.b)}
}
}
</code></pre>
<p>Let's walk through this example since we'll be using it the rest of this chapter.</p>
<p>We have a self-referential struct <code>Test</code>. <code>Test</code> needs an <code>init</code> method to be
created which is strange but we'll need that to keep this example as short as
possible.</p>
<p><code>Test</code> provides two methods to get a reference to the value of the fields
<code>a</code> and <code>b</code>. Since <code>b</code> is a reference to <code>a</code> we store it as a pointer since
the borrowing rules of Rust doesn't allow us to define this lifetime.</p>
<p>Now, let's use this example to explain the problem we encounter in detail. As
you see, this works as expected:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
test1.init();
let mut test2 = Test::new(&quot;test2&quot;);
test2.init();
println!(&quot;a: {}, b: {}&quot;, test1.a(), test1.b());
println!(&quot;a: {}, b: {}&quot;, test2.a(), test2.b());
}
# use std::pin::Pin;
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# }
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# let a = String::from(txt);
# Test {
# a,
# b: std::ptr::null(),
# }
# }
#
# // We need an `init` method to actually set our self-reference
# fn init(&amp;mut self) {
# let self_ref: *const String = &amp;self.a;
# self.b = self_ref;
# }
#
# fn a(&amp;self) -&gt; &amp;str {
# &amp;self.a
# }
#
# fn b(&amp;self) -&gt; &amp;String {
# unsafe {&amp;*(self.b)}
# }
# }
</code></pre></pre>
<p>In our main method we first instantiate two instances of <code>Test</code> and print out
the value of the fields on <code>test1</code>. We get what we'd expect:</p>
<pre><code class="language-rust ignore">a: test1, b: test1
a: test2, b: test2
</code></pre>
<p>Let's see what happens if we swap the data stored at the memory location <code>test1</code> with the
data stored at the memory location <code>test2</code> and vice a versa.</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
test1.init();
let mut test2 = Test::new(&quot;test2&quot;);
test2.init();
println!(&quot;a: {}, b: {}&quot;, test1.a(), test1.b());
std::mem::swap(&amp;mut test1, &amp;mut test2);
println!(&quot;a: {}, b: {}&quot;, test2.a(), test2.b());
}
# use std::pin::Pin;
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# }
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# let a = String::from(txt);
# Test {
# a,
# b: std::ptr::null(),
# }
# }
#
# fn init(&amp;mut self) {
# let self_ref: *const String = &amp;self.a;
# self.b = self_ref;
# }
#
# fn a(&amp;self) -&gt; &amp;str {
# &amp;self.a
# }
#
# fn b(&amp;self) -&gt; &amp;String {
# unsafe {&amp;*(self.b)}
# }
# }
</code></pre></pre>
<p>Naively, we could think that what we should get a debug print of <code>test1</code> two
times like this</p>
<pre><code class="language-rust ignore">a: test1, b: test1
a: test1, b: test1
</code></pre>
<p>But instead we get:</p>
<pre><code class="language-rust ignore">a: test1, b: test1
a: test1, b: test2
</code></pre>
<p>The pointer to <code>test2.b</code> still points to the old location which is inside <code>test1</code>
now. The struct is not self-referential anymore, it holds a pointer to a field
in a different object. That means we can't rely on the lifetime of <code>test2.b</code> to
be tied to the lifetime of <code>test2</code> anymore.</p>
<p>If your still not convinced, this should at least convince you:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
test1.init();
let mut test2 = Test::new(&quot;test2&quot;);
test2.init();
println!(&quot;a: {}, b: {}&quot;, test1.a(), test1.b());
std::mem::swap(&amp;mut test1, &amp;mut test2);
test1.a = &quot;I've totally changed now!&quot;.to_string();
println!(&quot;a: {}, b: {}&quot;, test2.a(), test2.b());
}
# use std::pin::Pin;
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# }
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# let a = String::from(txt);
# Test {
# a,
# b: std::ptr::null(),
# }
# }
#
# fn init(&amp;mut self) {
# let self_ref: *const String = &amp;self.a;
# self.b = self_ref;
# }
#
# fn a(&amp;self) -&gt; &amp;str {
# &amp;self.a
# }
#
# fn b(&amp;self) -&gt; &amp;String {
# unsafe {&amp;*(self.b)}
# }
# }
</code></pre></pre>
<p>That shouldn't happen. There is no serious error yet, but as you can imagine
it's easy to create serious bugs using this code.</p>
<p>I created a diagram to help visualize what's going on:</p>
<p><strong>Fig 1: Before and after swap</strong>
<img src="./assets/swap_problem.jpg" alt="swap_problem" /></p>
<p>As you can see this results in unwanted behavior. It's easy to get this to
segfault, show UB and fail in other spectacular ways as well.</p>
<h2><a class="header" href="#pinning-to-the-stack" id="pinning-to-the-stack">Pinning to the stack</a></h2>
<p>Now, we can solve this problem by using <code>Pin</code> instead. Let's take a look at what
our example would look like then:</p>
<pre><code class="language-rust ignore">use std::pin::Pin;
use std::marker::PhantomPinned;
#[derive(Debug)]
struct Test {
a: String,
b: *const String,
_marker: PhantomPinned,
}
impl Test {
fn new(txt: &amp;str) -&gt; Self {
let a = String::from(txt);
Test {
a: String::from(txt),
b: std::ptr::null(),
_marker: PhantomPinned, // This makes our type `!Unpin`
}
}
fn init&lt;'a&gt;(self: Pin&lt;&amp;'a mut Self&gt;) {
let self_ptr: *const String = &amp;self.a;
let this = unsafe { self.get_unchecked_mut() };
this.b = self_ptr;
}
fn a&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a str {
&amp;self.get_ref().a
}
fn b&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a String {
unsafe { &amp;*(self.b) }
}
}
</code></pre>
<p>Now, what we've done here is pinning an object to the stack. That will always be
<code>unsafe</code> if our type implements <code>!Unpin</code>.</p>
<p>We use the same tricks here, including requiring an <code>init</code>. If we want to fix that
and let users avoid <code>unsafe</code> we need to pin our data on the heap instead which
we'll show in a second.</p>
<p>Let's see what happens if we run our example now:</p>
<pre><pre class="playpen"><code class="language-rust">pub fn main() {
// test1 is safe to move before we initialize it
let mut test1 = Test::new(&quot;test1&quot;);
// Notice how we shadow `test1` to prevent it from being accessed again
let mut test1 = unsafe { Pin::new_unchecked(&amp;mut test1) };
Test::init(test1.as_mut());
let mut test2 = Test::new(&quot;test2&quot;);
let mut test2 = unsafe { Pin::new_unchecked(&amp;mut test2) };
Test::init(test2.as_mut());
println!(&quot;a: {}, b: {}&quot;, Test::a(test1.as_ref()), Test::b(test1.as_ref()));
println!(&quot;a: {}, b: {}&quot;, Test::a(test2.as_ref()), Test::b(test2.as_ref()));
}
# use std::pin::Pin;
# use std::marker::PhantomPinned;
#
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# _marker: PhantomPinned,
# }
#
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# let a = String::from(txt);
# Test {
# a,
# b: std::ptr::null(),
# // This makes our type `!Unpin`
# _marker: PhantomPinned,
# }
# }
# fn init&lt;'a&gt;(self: Pin&lt;&amp;'a mut Self&gt;) {
# let self_ptr: *const String = &amp;self.a;
# let this = unsafe { self.get_unchecked_mut() };
# this.b = self_ptr;
# }
#
# fn a&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a str {
# &amp;self.get_ref().a
# }
#
# fn b&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a String {
# unsafe { &amp;*(self.b) }
# }
# }
</code></pre></pre>
<p>Now, if we try to pull the same trick which got us in to trouble the last time
you'll get a compilation error.</p>
<pre><pre class="playpen"><code class="language-rust compile_fail">pub fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
let mut test1 = unsafe { Pin::new_unchecked(&amp;mut test1) };
Test::init(test1.as_mut());
let mut test2 = Test::new(&quot;test2&quot;);
let mut test2 = unsafe { Pin::new_unchecked(&amp;mut test2) };
Test::init(test2.as_mut());
println!(&quot;a: {}, b: {}&quot;, Test::a(test1.as_ref()), Test::b(test1.as_ref()));
std::mem::swap(test1.get_mut(), test2.get_mut());
println!(&quot;a: {}, b: {}&quot;, Test::a(test2.as_ref()), Test::b(test2.as_ref()));
}
# use std::pin::Pin;
# use std::marker::PhantomPinned;
#
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# _marker: PhantomPinned,
# }
#
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# Test {
# a: let a = String::from(txt),
# b: std::ptr::null(),
# _marker: PhantomPinned, // This makes our type `!Unpin`
# }
# }
# fn init&lt;'a&gt;(self: Pin&lt;&amp;'a mut Self&gt;) {
# let self_ptr: *const String = &amp;self.a;
# let this = unsafe { self.get_unchecked_mut() };
# this.b = self_ptr;
# }
#
# fn a&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a str {
# &amp;self.get_ref().a
# }
#
# fn b&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a String {
# unsafe { &amp;*(self.b) }
# }
# }
</code></pre></pre>
<p>As you see from the error you get by running the code the type system prevents
us from swapping the pinned pointers.</p>
<blockquote>
<p>It's important to note that stack pinning will always depend on the current
stack frame we're in, so we can't create a self referential object in one
stack frame and return it since any pointers we take to &quot;self&quot; is invalidated.</p>
<p>It also puts a lot of responsibility in your hands if you pin an object to the
stack. A mistake that is easy to make is, forgetting to shadow the original variable
since you could drop the <code>Pin</code> and access the old value after it's initialized
like this:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
let mut test1_pin = unsafe { Pin::new_unchecked(&amp;mut test1) };
Test::init(test1_pin.as_mut());
drop(test1_pin);
let mut test2 = Test::new(&quot;test2&quot;);
mem::swap(&amp;mut test1, &amp;mut test2);
println!(&quot;Not self referential anymore: {:?}&quot;, test1.b);
}
# use std::pin::Pin;
# use std::marker::PhantomPinned;
# use std::mem;
#
# #[derive(Debug)]
# struct Test {
# a: String,
# b: *const String,
# _marker: PhantomPinned,
# }
#
#
# impl Test {
# fn new(txt: &amp;str) -&gt; Self {
# Test {
# a: String::from(txt),
# b: std::ptr::null(),
# _marker: PhantomPinned, // This makes our type `!Unpin`
# }
# }
# fn init&lt;'a&gt;(self: Pin&lt;&amp;'a mut Self&gt;) {
# let self_ptr: *const String = &amp;self.a;
# let this = unsafe { self.get_unchecked_mut() };
# this.b = self_ptr;
# }
#
# fn a&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a str {
# &amp;self.get_ref().a
# }
#
# fn b&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a String {
# unsafe { &amp;*(self.b) }
# }
# }
</code></pre></pre>
</blockquote>
<h2><a class="header" href="#pinning-to-the-heap" id="pinning-to-the-heap">Pinning to the heap</a></h2>
<p>For completeness let's remove some unsafe and the need for an <code>init</code> method
at the cost of a heap allocation. Pinning to the heap is safe so the user
doesn't need to implement any unsafe code:</p>
<pre><pre class="playpen"><code class="language-rust edition2018">use std::pin::Pin;
use std::marker::PhantomPinned;
#[derive(Debug)]
struct Test {
a: String,
b: *const String,
_marker: PhantomPinned,
}
impl Test {
fn new(txt: &amp;str) -&gt; Pin&lt;Box&lt;Self&gt;&gt; {
let t = Test {
a: String::from(txt),
b: std::ptr::null(),
_marker: PhantomPinned,
};
let mut boxed = Box::pin(t);
let self_ptr: *const String = &amp;boxed.as_ref().a;
unsafe { boxed.as_mut().get_unchecked_mut().b = self_ptr };
boxed
}
fn a&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a str {
&amp;self.get_ref().a
}
fn b&lt;'a&gt;(self: Pin&lt;&amp;'a Self&gt;) -&gt; &amp;'a String {
unsafe { &amp;*(self.b) }
}
}
pub fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
let mut test2 = Test::new(&quot;test2&quot;);
println!(&quot;a: {}, b: {}&quot;,test1.as_ref().a(), test1.as_ref().b());
println!(&quot;a: {}, b: {}&quot;,test2.as_ref().a(), test2.as_ref().b());
}
</code></pre></pre>
<p>The fact that it's safe to pin heap allocated data even if it is <code>!Unpin</code>
makes sense. Once the data is allocated on the heap it will have a stable address.</p>
<p>There is no need for us as users of the API to take special care and ensure
that the self-referential pointer stays valid.</p>
<p>There are ways to safely give some guarantees on stack pinning as well, but right
now you need to use a crate like <a href="https://docs.rs/pin-project/">pin_project</a> to do that.</p>
<h2><a class="header" href="#practical-rules-for-pinning" id="practical-rules-for-pinning">Practical rules for Pinning</a></h2>
<ol>
<li>
<p>If <code>T: Unpin</code> (which is the default), then <code>Pin&lt;'a, T&gt;</code> is entirely
equivalent to <code>&amp;'a mut T</code>. in other words: <code>Unpin</code> means it's OK for this type
to be moved even when pinned, so <code>Pin</code> will have no effect on such a type.</p>
</li>
<li>
<p>Getting a <code>&amp;mut T</code> to a pinned T requires unsafe if <code>T: !Unpin</code>. In
other words: requiring a pinned pointer to a type which is <code>!Unpin</code> prevents
the <em>user</em> of that API from moving that value unless it choses to write <code>unsafe</code>
code.</p>
</li>
<li>
<p>Pinning does nothing special with memory allocation like putting it into some
&quot;read only&quot; memory or anything fancy. It only uses the type system to prevent
certain operations on this value.</p>
</li>
<li>
<p>Most standard library types implement <code>Unpin</code>. The same goes for most
&quot;normal&quot; types you encounter in Rust. <code>Future</code>s and <code>Generator</code>s are two
exceptions.</p>
</li>
<li>
<p>The main use case for <code>Pin</code> is to allow self referential types, the whole
justification for stabilizing them was to allow that.</p>
</li>
<li>
<p>The implementation behind objects that are <code>!Unpin</code> is most likely unsafe.
Moving such a type after it has been pinned can cause the universe to crash. As of the time of writing
this book, creating and reading fields of a self referential struct still requires <code>unsafe</code>
(the only way to do it is to create a struct containing raw pointers to itself).</p>
</li>
<li>
<p>You can add a <code>!Unpin</code> bound on a type on nightly with a feature flag, or
by adding <code>std::marker::PhantomPinned</code> to your type on stable.</p>
</li>
<li>
<p>You can either pin an object to the stack or to the heap.</p>
</li>
<li>
<p>Pinning a <code>!Unpin</code> object to the stack requires <code>unsafe</code></p>
</li>
<li>
<p>Pinning a <code>!Unpin</code> object to the heap does not require <code>unsafe</code>. There is a shortcut for doing this using <code>Box::pin</code>.</p>
</li>
</ol>
<blockquote>
<p>Unsafe code does not mean it's literally &quot;unsafe&quot;, it only relieves the
guarantees you normally get from the compiler. An <code>unsafe</code> implementation can
be perfectly safe to do, but you have no safety net.</p>
</blockquote>
<h3><a class="header" href="#projectionstructural-pinning" id="projectionstructural-pinning">Projection/structural pinning</a></h3>
<p>In short, projection is a programming language term. <code>mystruct.field1</code> is a
projection. Structural pinning is using <code>Pin</code> on fields. This has several
caveats and is not something you'll normally see so I refer to the documentation
for that.</p>
<h3><a class="header" href="#pin-and-drop" id="pin-and-drop">Pin and Drop</a></h3>
<p>The <code>Pin</code> guarantee exists from the moment the value is pinned until it's dropped.
In the <code>Drop</code> implementation you take a mutable reference to <code>self</code>, which means
extra care must be taken when implementing <code>Drop</code> for pinned types.</p>
<h2><a class="header" href="#putting-it-all-together" id="putting-it-all-together">Putting it all together</a></h2>
<p>This is exactly what we'll do when we implement our own <code>Future</code>, so stay tuned,
we're soon finished.</p>
<h2><a class="header" href="#bonus-section-fixing-our-self-referential-generator-and-learning-more-about-pin" id="bonus-section-fixing-our-self-referential-generator-and-learning-more-about-pin">Bonus section: Fixing our self-referential generator and learning more about Pin</a></h2>
<p>But now, let's prevent this problem using <code>Pin</code>. I've commented along the way to
make it easier to spot and understand the changes we need to make.</p>
<pre><pre class="playpen"><code class="language-rust">#![feature(optin_builtin_traits, negative_impls)] // needed to implement `!Unpin`
use std::pin::Pin;
pub fn main() {
let gen1 = GeneratorA::start();
let gen2 = GeneratorA::start();
// Before we pin the data, this is safe to do
// std::mem::swap(&amp;mut gen, &amp;mut gen2);
// constructing a `Pin::new()` on a type which does not implement `Unpin` is
// unsafe. An object pinned to heap can be constructed while staying in safe
// Rust so we can use that to avoid unsafe. You can also use crates like
// `pin_utils` to pin to the stack safely, just remember that they use
// unsafe under the hood so it's like using an already-reviewed unsafe
// implementation.
let mut pinned1 = Box::pin(gen1);
let mut pinned2 = Box::pin(gen2);
// Uncomment these if you think it's safe to pin the values to the stack instead
// (it is in this case). Remember to comment out the two previous lines first.
//let mut pinned1 = unsafe { Pin::new_unchecked(&amp;mut gen1) };
//let mut pinned2 = unsafe { Pin::new_unchecked(&amp;mut gen2) };
if let GeneratorState::Yielded(n) = pinned1.as_mut().resume() {
println!(&quot;Gen1 got value {}&quot;, n);
}
if let GeneratorState::Yielded(n) = pinned2.as_mut().resume() {
println!(&quot;Gen2 got value {}&quot;, n);
};
// This won't work:
// std::mem::swap(&amp;mut gen, &amp;mut gen2);
// This will work but will just swap the pointers so nothing bad happens here:
// std::mem::swap(&amp;mut pinned1, &amp;mut pinned2);
let _ = pinned1.as_mut().resume();
let _ = pinned2.as_mut().resume();
}
enum GeneratorState&lt;Y, R&gt; {
Yielded(Y),
Complete(R),
}
trait Generator {
type Yield;
type Return;
fn resume(self: Pin&lt;&amp;mut Self&gt;) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt;;
}
enum GeneratorA {
Enter,
Yield1 {
to_borrow: String,
borrowed: *const String,
},
Exit,
}
impl GeneratorA {
fn start() -&gt; Self {
GeneratorA::Enter
}
}
// This tells us that this object is not safe to move after pinning.
// In this case, only we as implementors &quot;feel&quot; this, however, if someone is
// relying on our Pinned data this will prevent them from moving it. You need
// to enable the feature flag `#![feature(optin_builtin_traits)]` and use the
// nightly compiler to implement `!Unpin`. Normally, you would use
// `std::marker::PhantomPinned` to indicate that the struct is `!Unpin`.
impl !Unpin for GeneratorA { }
impl Generator for GeneratorA {
type Yield = usize;
type Return = ();
fn resume(self: Pin&lt;&amp;mut Self&gt;) -&gt; GeneratorState&lt;Self::Yield, Self::Return&gt; {
// lets us get ownership over current state
let this = unsafe { self.get_unchecked_mut() };
match this {
GeneratorA::Enter =&gt; {
let to_borrow = String::from(&quot;Hello&quot;);
let borrowed = &amp;to_borrow;
let res = borrowed.len();
*this = GeneratorA::Yield1 {to_borrow, borrowed: std::ptr::null()};
// Trick to actually get a self reference. We can't reference
// the `String` earlier since these references will point to the
// location in this stack frame which will not be valid anymore
// when this function returns.
if let GeneratorA::Yield1 {to_borrow, borrowed} = this {
*borrowed = to_borrow;
}
GeneratorState::Yielded(res)
}
GeneratorA::Yield1 {borrowed, ..} =&gt; {
let borrowed: &amp;String = unsafe {&amp;**borrowed};
println!(&quot;{} world&quot;, borrowed);
*this = GeneratorA::Exit;
GeneratorState::Complete(())
}
GeneratorA::Exit =&gt; panic!(&quot;Can't advance an exited generator!&quot;),
}
}
}
</code></pre></pre>
<p>Now, as you see, the consumer of this API must either:</p>
<ol>
<li>Box the value and thereby allocating it on the heap</li>
<li>Use <code>unsafe</code> and pin the value to the stack. The user knows that if they move
the value afterwards it will violate the guarantee they promise to uphold when
they did their unsafe implementation.</li>
</ol>
<p>Hopefully, after this you'll have an idea of what happens when you use the
<code>yield</code> or <code>await</code> keywords inside an async function, and why we need <code>Pin</code> if
we want to be able to safely borrow across <code>yield/await</code> points.</p>
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<h1><a class="header" href="#our-finished-code" id="our-finished-code">Our finished code</a></h1>
<p>Here is the whole example. You can edit it right here in your browser and
run it yourself. Have fun!</p>
<pre><pre class="playpen"><code class="language-rust editable edition2018">fn main() {
let start = Instant::now();
let reactor = Reactor::new();
let fut1 = async {
let val = Task::new(reactor.clone(), 1, 1).await;
println!(&quot;Got {} at time: {:.2}.&quot;, val, start.elapsed().as_secs_f32());
};
let fut2 = async {
let val = Task::new(reactor.clone(), 2, 2).await;
println!(&quot;Got {} at time: {:.2}.&quot;, val, start.elapsed().as_secs_f32());
};
let mainfut = async {
fut1.await;
fut2.await;
};
block_on(mainfut);
reactor.lock().map(|mut r| r.close()).unwrap();
}
use std::{
future::Future, sync::{ mpsc::{channel, Sender}, Arc, Mutex, Condvar},
task::{Context, Poll, RawWaker, RawWakerVTable, Waker}, mem, pin::Pin,
thread::{self, JoinHandle}, time::{Duration, Instant}, collections::HashMap
};
// ============================= EXECUTOR ====================================
#[derive(Default)]
struct Parker(Mutex&lt;bool&gt;, Condvar);
impl Parker {
fn park(&amp;self) {
let mut resumable = self.0.lock().unwrap();
while !*resumable {
resumable = self.1.wait(resumable).unwrap();
}
*resumable = false;
}
fn unpark(&amp;self) {
*self.0.lock().unwrap() = true;
self.1.notify_one();
}
}
fn block_on&lt;F: Future&gt;(mut future: F) -&gt; F::Output {
let parker = Arc::new(Parker::default());
let mywaker = Arc::new(MyWaker { parker: parker.clone() });
let waker = mywaker_into_waker(Arc::into_raw(mywaker));
let mut cx = Context::from_waker(&amp;waker);
// SAFETY: we shadow `future` so it can't be accessed again.
let mut future = unsafe { Pin::new_unchecked(&amp;mut future) };
loop {
match Future::poll(future.as_mut(), &amp;mut cx) {
Poll::Ready(val) =&gt; break val,
Poll::Pending =&gt; parker.park(),
};
}
}
// ====================== FUTURE IMPLEMENTATION ==============================
#[derive(Clone)]
struct MyWaker {
parker: Arc&lt;Parker&gt;,
}
#[derive(Clone)]
pub struct Task {
id: usize,
reactor: Arc&lt;Mutex&lt;Box&lt;Reactor&gt;&gt;&gt;,
data: u64,
}
fn mywaker_wake(s: &amp;MyWaker) {
let waker_arc = unsafe { Arc::from_raw(s) };
waker_arc.parker.unpark();
}
fn mywaker_clone(s: &amp;MyWaker) -&gt; RawWaker {
let arc = unsafe { Arc::from_raw(s) };
std::mem::forget(arc.clone()); // increase ref count
RawWaker::new(Arc::into_raw(arc) as *const (), &amp;VTABLE)
}
const VTABLE: RawWakerVTable = unsafe {
RawWakerVTable::new(
|s| mywaker_clone(&amp;*(s as *const MyWaker)), // clone
|s| mywaker_wake(&amp;*(s as *const MyWaker)), // wake
|s| mywaker_wake(*(s as *const &amp;MyWaker)), // wake by ref
|s| drop(Arc::from_raw(s as *const MyWaker)), // decrease refcount
)
};
fn mywaker_into_waker(s: *const MyWaker) -&gt; Waker {
let raw_waker = RawWaker::new(s as *const (), &amp;VTABLE);
unsafe { Waker::from_raw(raw_waker) }
}
impl Task {
fn new(reactor: Arc&lt;Mutex&lt;Box&lt;Reactor&gt;&gt;&gt;, data: u64, id: usize) -&gt; Self {
Task { id, reactor, data }
}
}
impl Future for Task {
type Output = usize;
fn poll(self: Pin&lt;&amp;mut Self&gt;, cx: &amp;mut Context&lt;'_&gt;) -&gt; Poll&lt;Self::Output&gt; {
let mut r = self.reactor.lock().unwrap();
if r.is_ready(self.id) {
*r.tasks.get_mut(&amp;self.id).unwrap() = TaskState::Finished;
Poll::Ready(self.id)
} else if r.tasks.contains_key(&amp;self.id) {
r.tasks.insert(self.id, TaskState::NotReady(cx.waker().clone()));
Poll::Pending
} else {
r.register(self.data, cx.waker().clone(), self.id);
Poll::Pending
}
}
}
// =============================== REACTOR ===================================
enum TaskState {
Ready,
NotReady(Waker),
Finished,
}
struct Reactor {
dispatcher: Sender&lt;Event&gt;,
handle: Option&lt;JoinHandle&lt;()&gt;&gt;,
tasks: HashMap&lt;usize, TaskState&gt;,
}
#[derive(Debug)]
enum Event {
Close,
Timeout(u64, usize),
}
impl Reactor {
fn new() -&gt; Arc&lt;Mutex&lt;Box&lt;Self&gt;&gt;&gt; {
let (tx, rx) = channel::&lt;Event&gt;();
let reactor = Arc::new(Mutex::new(Box::new(Reactor {
dispatcher: tx,
handle: None,
tasks: HashMap::new(),
})));
let reactor_clone = Arc::downgrade(&amp;reactor);
let handle = thread::spawn(move || {
let mut handles = vec![];
for event in rx {
let reactor = reactor_clone.clone();
match event {
Event::Close =&gt; break,
Event::Timeout(duration, id) =&gt; {
let event_handle = thread::spawn(move || {
thread::sleep(Duration::from_secs(duration));
let reactor = reactor.upgrade().unwrap();
reactor.lock().map(|mut r| r.wake(id)).unwrap();
});
handles.push(event_handle);
}
}
}
handles.into_iter().for_each(|handle| handle.join().unwrap());
});
reactor.lock().map(|mut r| r.handle = Some(handle)).unwrap();
reactor
}
fn wake(&amp;mut self, id: usize) {
let state = self.tasks.get_mut(&amp;id).unwrap();
match mem::replace(state, TaskState::Ready) {
TaskState::NotReady(waker) =&gt; waker.wake(),
TaskState::Finished =&gt; panic!(&quot;Called 'wake' twice on task: {}&quot;, id),
_ =&gt; unreachable!()
}
}
fn register(&amp;mut self, duration: u64, waker: Waker, id: usize) {
if self.tasks.insert(id, TaskState::NotReady(waker)).is_some() {
panic!(&quot;Tried to insert a task with id: '{}', twice!&quot;, id);
}
self.dispatcher.send(Event::Timeout(duration, id)).unwrap();
}
fn close(&amp;mut self) {
self.dispatcher.send(Event::Close).unwrap();
}
fn is_ready(&amp;self, id: usize) -&gt; bool {
self.tasks.get(&amp;id).map(|state| match state {
TaskState::Ready =&gt; true,
_ =&gt; false,
}).unwrap_or(false)
}
}
impl Drop for Reactor {
fn drop(&amp;mut self) {
self.handle.take().map(|h| h.join().unwrap()).unwrap();
}
}
</code></pre></pre>
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window.onunload = function () { };
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function playpen_text(playpen) {
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let editor = window.ace.edit(code_block);
return editor.getValue();
} else {
return code_block.textContent;
}
}
(function codeSnippets() {
// Hide Rust code lines prepended with a specific character
var hiding_character = "#";
function fetch_with_timeout(url, options, timeout = 6000) {
return Promise.race([
fetch(url, options),
new Promise((_, reject) => setTimeout(() => reject(new Error('timeout')), timeout))
]);
}
var playpens = Array.from(document.querySelectorAll(".playpen"));
if (playpens.length > 0) {
fetch_with_timeout("https://play.rust-lang.org/meta/crates", {
headers: {
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},
method: 'POST',
mode: 'cors',
})
.then(response => response.json())
.then(response => {
// get list of crates available in the rust playground
let playground_crates = response.crates.map(item => item["id"]);
playpens.forEach(block => handle_crate_list_update(block, playground_crates));
});
}
function handle_crate_list_update(playpen_block, playground_crates) {
// update the play buttons after receiving the response
update_play_button(playpen_block, playground_crates);
// and install on change listener to dynamically update ACE editors
if (window.ace) {
let code_block = playpen_block.querySelector("code");
if (code_block.classList.contains("editable")) {
let editor = window.ace.edit(code_block);
editor.addEventListener("change", function (e) {
update_play_button(playpen_block, playground_crates);
});
}
}
}
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// used crates vs ones available on http://play.rust-lang.org
function update_play_button(pre_block, playground_crates) {
var play_button = pre_block.querySelector(".play-button");
// skip if code is `no_run`
if (pre_block.querySelector('code').classList.contains("no_run")) {
play_button.classList.add("hidden");
return;
}
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var txt = playpen_text(pre_block);
var re = /extern\s+crate\s+([a-zA-Z_0-9]+)\s*;/g;
var snippet_crates = [];
var item;
while (item = re.exec(txt)) {
snippet_crates.push(item[1]);
}
// check if all used crates are available on play.rust-lang.org
var all_available = snippet_crates.every(function (elem) {
return playground_crates.indexOf(elem) > -1;
});
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edition: edition
};
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params.version = "nightly";
}
result_block.innerText = "Running...";
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},
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body: JSON.stringify(params)
})
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<h1><a class="header" href="#conclusion-and-exercises" id="conclusion-and-exercises">Conclusion and exercises</a></h1>
<p>Congratulations. Good job! If you got this far you must have stayed with me
all the way. I hope you enjoyed the ride!</p>
<p>Remember that you call always leave feedback, suggest improvements or ask questions
in the <a href="https://github.com/cfsamson/books-futures-explained/issues">issue_tracker</a> for this book.
I'll try my best to respond to each one of them.</p>
<p>I'll leave you with some suggestions for exercises if you want to explore a little further below.</p>
<p>Until next time!</p>
<h2><a class="header" href="#reader-exercises" id="reader-exercises">Reader exercises</a></h2>
<p>So our implementation has taken some obvious shortcuts and could use some improvement.
Actually digging into the code and try things yourself is a good way to learn. Here are
some good exercises if you want to explore more:</p>
<h3><a class="header" href="#avoid-wrapping-the-whole-reactor-in-a-mutex-and-pass-it-around" id="avoid-wrapping-the-whole-reactor-in-a-mutex-and-pass-it-around">Avoid wrapping the whole <code>Reactor</code> in a mutex and pass it around</a></h3>
<p>First of all, protecting the whole <code>Reactor</code> and passing it around is overkill. We're only
interested in synchronizing some parts of the information it contains. Try to refactor that
out and only synchronize access to what's really needed.</p>
<p>I'd encourage you to have a look at how async_std solves this with a global <a href="https://github.com/async-rs/async-std/blob/b446cd023084a157b8a531cff65b8df37750be58/src/rt/mod.rs#L15-L23">runtime</a> which includes the <a href="https://github.com/async-rs/async-std/blob/b446cd023084a157b8a531cff65b8df37750be58/src/rt/runtime.rs#L39-L70">reactor</a>
and <a href="https://github.com/tokio-rs/tokio/blob/19a87e090ed528001e0363a30f6165304a710d49/tokio/src/runtime/context.rs#L2-L12">how tokio's rutime</a> solves the same
thing in a slightly different way to get some inspiration.</p>
<ul>
<li>Do you want to pass around a reference to this information using an <code>Arc</code>?</li>
<li>Do you want to make a global <code>Reactor</code> so it can be accessed from anywhere?</li>
</ul>
<h3><a class="header" href="#building-a-better-exectuor" id="building-a-better-exectuor">Building a better exectuor</a></h3>
<p>Right now, we can only run one and one future. Most runtimes has a <code>spawn</code>
function which let's you start off a future and <code>await</code> it later so you
can run multiple futures concurrently.</p>
<p>As I suggested in the start of this book, visiting <a href="https://stjepang.github.io/2020/01/31/build-your-own-executor.html">@stjepan'sblog series about implementing your own executors</a>
is the place I would start and take it from there.</p>
<h2><a class="header" href="#further-reading" id="further-reading">Further reading</a></h2>
<p>There are many great resources. In addition to the RFCs and articles I've already
linked to in the book, here are some of my suggestions:</p>
<p><a href="https://rust-lang.github.io/async-book/01_getting_started/01_chapter.html">The official Asyc book</a></p>
<p><a href="https://book.async.rs/">The async_std book</a></p>
<p><a href="https://aturon.github.io/blog/2016/09/07/futures-design/">Aron Turon: Designing futures for Rust</a></p>
<p><a href="https://www.infoq.com/presentations/rust-2019/">Steve Klabnik's presentation: Rust's journey to Async/Await</a></p>
<p><a href="https://tokio.rs/blog/2019-10-scheduler/">The Tokio Blog</a></p>
<p><a href="https://stjepang.github.io/">Stjepan's blog with a series where he implements an Executor</a></p>
<p><a href="https://youtu.be/DkMwYxfSYNQ">Jon Gjengset's video on The Why, What and How of Pinning in Rust</a></p>
<p><a href="https://boats.gitlab.io/blog/post/2018-01-25-async-i-self-referential-structs/">Withoutboats blog series about async/await</a></p>
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ul#searchresults span.teaser {
display: block;
clear: both;
margin: 5px 0 0 20px;
font-size: 0.8em;
}
ul#searchresults span.teaser em {
font-weight: bold;
font-style: normal;
}
/* Sidebar */
.sidebar {
position: fixed;
left: 0;
top: 0;
bottom: 0;
width: var(--sidebar-width);
font-size: 0.875em;
box-sizing: border-box;
-webkit-overflow-scrolling: touch;
overscroll-behavior-y: contain;
background-color: var(--sidebar-bg);
color: var(--sidebar-fg);
}
.sidebar-resizing {
-moz-user-select: none;
-webkit-user-select: none;
-ms-user-select: none;
user-select: none;
}
.js:not(.sidebar-resizing) .sidebar {
transition: transform 0.3s; /* Animation: slide away */
}
.sidebar code {
line-height: 2em;
}
.sidebar .sidebar-scrollbox {
overflow-y: auto;
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
padding: 10px 10px;
}
.sidebar .sidebar-resize-handle {
position: absolute;
cursor: col-resize;
width: 0;
right: 0;
top: 0;
bottom: 0;
}
.js .sidebar .sidebar-resize-handle {
cursor: col-resize;
width: 5px;
}
.sidebar-hidden .sidebar {
transform: translateX(calc(0px - var(--sidebar-width)));
}
.sidebar::-webkit-scrollbar {
background: var(--sidebar-bg);
}
.sidebar::-webkit-scrollbar-thumb {
background: var(--scrollbar);
}
.sidebar-visible .page-wrapper {
transform: translateX(var(--sidebar-width));
}
@media only screen and (min-width: 620px) {
.sidebar-visible .page-wrapper {
transform: none;
margin-left: var(--sidebar-width);
}
}
.chapter {
list-style: none outside none;
padding-left: 0;
line-height: 2.2em;
}
.chapter li {
color: var(--sidebar-non-existant);
}
.chapter li a {
display: block;
padding: 0;
text-decoration: none;
color: var(--sidebar-fg);
}
.chapter li a:hover {
color: var(--sidebar-active);
}
.chapter li .active {
color: var(--sidebar-active);
}
.spacer {
width: 100%;
height: 3px;
margin: 5px 0px;
}
.chapter .spacer {
background-color: var(--sidebar-spacer);
}
@media (-moz-touch-enabled: 1), (pointer: coarse) {
.chapter li a { padding: 5px 0; }
.spacer { margin: 10px 0; }
}
.section {
list-style: none outside none;
padding-left: 20px;
line-height: 1.9em;
}
/* Theme Menu Popup */
.theme-popup {
position: absolute;
left: 10px;
top: 50px;
z-index: 1000;
border-radius: 4px;
font-size: 0.7em;
color: var(--fg);
background: var(--theme-popup-bg);
border: 1px solid var(--theme-popup-border);
margin: 0;
padding: 0;
list-style: none;
display: none;
}
.theme-popup .default {
color: var(--icons);
}
.theme-popup .theme {
width: 100%;
border: 0;
margin: 0;
padding: 2px 10px;
line-height: 25px;
white-space: nowrap;
text-align: left;
cursor: pointer;
color: inherit;
background: inherit;
font-size: inherit;
}
.theme-popup .theme:hover {
background-color: var(--theme-hover);
}
.theme-popup .theme:hover:first-child,
.theme-popup .theme:hover:last-child {
border-top-left-radius: inherit;
border-top-right-radius: inherit;
}

143
book/css/general.css
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@@ -1,143 +0,0 @@
/* Base styles and content styles */
@import 'variables.css';
html {
font-family: "Open Sans", sans-serif;
color: var(--fg);
background-color: var(--bg);
text-size-adjust: none;
}
body {
margin: 0;
font-size: 1rem;
overflow-x: hidden;
}
code {
font-family: "Source Code Pro", Consolas, "Ubuntu Mono", Menlo, "DejaVu Sans Mono", monospace, monospace;
font-size: 0.875em; /* please adjust the ace font size accordingly in editor.js */
}
.left { float: left; }
.right { float: right; }
.hidden { display: none; }
.play-button.hidden { display: none; }
h2, h3 { margin-top: 2.5em; }
h4, h5 { margin-top: 2em; }
.header + .header h3,
.header + .header h4,
.header + .header h5 {
margin-top: 1em;
}
h1 a.header:target::before,
h2 a.header:target::before,
h3 a.header:target::before,
h4 a.header:target::before {
display: inline-block;
content: "»";
margin-left: -30px;
width: 30px;
}
.page {
outline: 0;
padding: 0 var(--page-padding);
}
.page-wrapper {
box-sizing: border-box;
}
.js:not(.sidebar-resizing) .page-wrapper {
transition: margin-left 0.3s ease, transform 0.3s ease; /* Animation: slide away */
}
.content {
overflow-y: auto;
padding: 0 15px;
padding-bottom: 50px;
}
.content main {
margin-left: auto;
margin-right: auto;
max-width: var(--content-max-width);
}
.content a { text-decoration: none; }
.content a:hover { text-decoration: underline; }
.content img { max-width: 100%; }
.content .header:link,
.content .header:visited {
color: var(--fg);
}
.content .header:link,
.content .header:visited:hover {
text-decoration: none;
}
table {
margin: 0 auto;
border-collapse: collapse;
}
table td {
padding: 3px 20px;
border: 1px var(--table-border-color) solid;
}
table thead {
background: var(--table-header-bg);
}
table thead td {
font-weight: 700;
border: none;
}
table thead tr {
border: 1px var(--table-header-bg) solid;
}
/* Alternate background colors for rows */
table tbody tr:nth-child(2n) {
background: var(--table-alternate-bg);
}
blockquote {
margin: 20px 0;
padding: 0 20px;
color: var(--fg);
background-color: var(--quote-bg);
border-top: .1em solid var(--quote-border);
border-bottom: .1em solid var(--quote-border);
}
:not(.footnote-definition) + .footnote-definition,
.footnote-definition + :not(.footnote-definition) {
margin-top: 2em;
}
.footnote-definition {
font-size: 0.9em;
margin: 0.5em 0;
}
.footnote-definition p {
display: inline;
}
.tooltiptext {
position: absolute;
visibility: hidden;
color: #fff;
background-color: #333;
transform: translateX(-50%); /* Center by moving tooltip 50% of its width left */
left: -8px; /* Half of the width of the icon */
top: -35px;
font-size: 0.8em;
text-align: center;
border-radius: 6px;
padding: 5px 8px;
margin: 5px;
z-index: 1000;
}
.tooltipped .tooltiptext {
visibility: visible;
}

54
book/css/print.css
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@@ -1,54 +0,0 @@
#sidebar,
#menu-bar,
.nav-chapters,
.mobile-nav-chapters {
display: none;
}
#page-wrapper.page-wrapper {
transform: none;
margin-left: 0px;
overflow-y: initial;
}
#content {
max-width: none;
margin: 0;
padding: 0;
}
.page {
overflow-y: initial;
}
code {
background-color: #666666;
border-radius: 5px;
/* Force background to be printed in Chrome */
-webkit-print-color-adjust: exact;
}
pre > .buttons {
z-index: 2;
}
a, a:visited, a:active, a:hover {
color: #4183c4;
text-decoration: none;
}
h1, h2, h3, h4, h5, h6 {
page-break-inside: avoid;
page-break-after: avoid;
}
pre, code {
page-break-inside: avoid;
white-space: pre-wrap;
}
.fa {
display: none !important;
}

210
book/css/variables.css
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@@ -1,210 +0,0 @@
/* Globals */
:root {
--sidebar-width: 300px;
--page-padding: 15px;
--content-max-width: 750px;
}
/* Themes */
.ayu {
--bg: hsl(210, 25%, 8%);
--fg: #c5c5c5;
--sidebar-bg: #14191f;
--sidebar-fg: #c8c9db;
--sidebar-non-existant: #5c6773;
--sidebar-active: #ffb454;
--sidebar-spacer: #2d334f;
--scrollbar: var(--sidebar-fg);
--icons: #737480;
--icons-hover: #b7b9cc;
--links: #0096cf;
--inline-code-color: #ffb454;
--theme-popup-bg: #14191f;
--theme-popup-border: #5c6773;
--theme-hover: #191f26;
--quote-bg: hsl(226, 15%, 17%);
--quote-border: hsl(226, 15%, 22%);
--table-border-color: hsl(210, 25%, 13%);
--table-header-bg: hsl(210, 25%, 28%);
--table-alternate-bg: hsl(210, 25%, 11%);
--searchbar-border-color: #848484;
--searchbar-bg: #424242;
--searchbar-fg: #fff;
--searchbar-shadow-color: #d4c89f;
--searchresults-header-fg: #666;
--searchresults-border-color: #888;
--searchresults-li-bg: #252932;
--search-mark-bg: #e3b171;
}
.coal {
--bg: hsl(200, 7%, 8%);
--fg: #98a3ad;
--sidebar-bg: #292c2f;
--sidebar-fg: #a1adb8;
--sidebar-non-existant: #505254;
--sidebar-active: #3473ad;
--sidebar-spacer: #393939;
--scrollbar: var(--sidebar-fg);
--icons: #43484d;
--icons-hover: #b3c0cc;
--links: #2b79a2;
--inline-code-color: #c5c8c6;;
--theme-popup-bg: #141617;
--theme-popup-border: #43484d;
--theme-hover: #1f2124;
--quote-bg: hsl(234, 21%, 18%);
--quote-border: hsl(234, 21%, 23%);
--table-border-color: hsl(200, 7%, 13%);
--table-header-bg: hsl(200, 7%, 28%);
--table-alternate-bg: hsl(200, 7%, 11%);
--searchbar-border-color: #aaa;
--searchbar-bg: #b7b7b7;
--searchbar-fg: #000;
--searchbar-shadow-color: #aaa;
--searchresults-header-fg: #666;
--searchresults-border-color: #98a3ad;
--searchresults-li-bg: #2b2b2f;
--search-mark-bg: #355c7d;
}
.light {
--bg: hsl(0, 0%, 100%);
--fg: #333333;
--sidebar-bg: #fafafa;
--sidebar-fg: #364149;
--sidebar-non-existant: #aaaaaa;
--sidebar-active: #008cff;
--sidebar-spacer: #f4f4f4;
--scrollbar: #cccccc;
--icons: #cccccc;
--icons-hover: #333333;
--links: #4183c4;
--inline-code-color: #6e6b5e;
--theme-popup-bg: #fafafa;
--theme-popup-border: #cccccc;
--theme-hover: #e6e6e6;
--quote-bg: hsl(197, 37%, 96%);
--quote-border: hsl(197, 37%, 91%);
--table-border-color: hsl(0, 0%, 95%);
--table-header-bg: hsl(0, 0%, 80%);
--table-alternate-bg: hsl(0, 0%, 97%);
--searchbar-border-color: #aaa;
--searchbar-bg: #fafafa;
--searchbar-fg: #000;
--searchbar-shadow-color: #aaa;
--searchresults-header-fg: #666;
--searchresults-border-color: #888;
--searchresults-li-bg: #e4f2fe;
--search-mark-bg: #a2cff5;
}
.navy {
--bg: hsl(226, 23%, 11%);
--fg: #bcbdd0;
--sidebar-bg: #282d3f;
--sidebar-fg: #c8c9db;
--sidebar-non-existant: #505274;
--sidebar-active: #2b79a2;
--sidebar-spacer: #2d334f;
--scrollbar: var(--sidebar-fg);
--icons: #737480;
--icons-hover: #b7b9cc;
--links: #2b79a2;
--inline-code-color: #c5c8c6;;
--theme-popup-bg: #161923;
--theme-popup-border: #737480;
--theme-hover: #282e40;
--quote-bg: hsl(226, 15%, 17%);
--quote-border: hsl(226, 15%, 22%);
--table-border-color: hsl(226, 23%, 16%);
--table-header-bg: hsl(226, 23%, 31%);
--table-alternate-bg: hsl(226, 23%, 14%);
--searchbar-border-color: #aaa;
--searchbar-bg: #aeaec6;
--searchbar-fg: #000;
--searchbar-shadow-color: #aaa;
--searchresults-header-fg: #5f5f71;
--searchresults-border-color: #5c5c68;
--searchresults-li-bg: #242430;
--search-mark-bg: #a2cff5;
}
.rust {
--bg: hsl(60, 9%, 87%);
--fg: #262625;
--sidebar-bg: #3b2e2a;
--sidebar-fg: #c8c9db;
--sidebar-non-existant: #505254;
--sidebar-active: #e69f67;
--sidebar-spacer: #45373a;
--scrollbar: var(--sidebar-fg);
--icons: #737480;
--icons-hover: #262625;
--links: #2b79a2;
--inline-code-color: #6e6b5e;
--theme-popup-bg: #e1e1db;
--theme-popup-border: #b38f6b;
--theme-hover: #99908a;
--quote-bg: hsl(60, 5%, 75%);
--quote-border: hsl(60, 5%, 70%);
--table-border-color: hsl(60, 9%, 82%);
--table-header-bg: #b3a497;
--table-alternate-bg: hsl(60, 9%, 84%);
--searchbar-border-color: #aaa;
--searchbar-bg: #fafafa;
--searchbar-fg: #000;
--searchbar-shadow-color: #aaa;
--searchresults-header-fg: #666;
--searchresults-border-color: #888;
--searchresults-li-bg: #dec2a2;
--search-mark-bg: #e69f67;
}

27
book/editor.js
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@@ -1,27 +0,0 @@
"use strict";
window.editors = [];
(function(editors) {
if (typeof(ace) === 'undefined' || !ace) {
return;
}
Array.from(document.querySelectorAll('.editable')).forEach(function(editable) {
let editor = ace.edit(editable);
editor.setOptions({
highlightActiveLine: false,
showPrintMargin: false,
showLineNumbers: false,
showGutter: false,
maxLines: Infinity,
fontSize: "0.875em" // please adjust the font size of the code in general.css
});
editor.$blockScrolling = Infinity;
editor.getSession().setMode("ace/mode/rust");
editor.originalCode = editor.getValue();
editors.push(editor);
});
})(window.editors);

10
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book/favicon.png
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Before

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82
book/highlight.css
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@@ -1,82 +0,0 @@
/* Base16 Atelier Dune Light - Theme */
/* by Bram de Haan (http://atelierbram.github.io/syntax-highlighting/atelier-schemes/dune) */
/* Original Base16 color scheme by Chris Kempson (https://github.com/chriskempson/base16) */
/* Atelier-Dune Comment */
.hljs-comment {
color: rgb(160, 160, 160);;
font-style: italic;
}
.hljs-quote {
color: #AAA;
}
/* Atelier-Dune Red */
.hljs-variable,
.hljs-template-variable,
.hljs-attribute,
.hljs-tag,
.hljs-name,
.hljs-regexp,
.hljs-link,
.hljs-name,
.hljs-selector-id,
.hljs-selector-class {
color: #d73737;
}
/* Atelier-Dune Orange */
.hljs-number,
.hljs-meta,
.hljs-built_in,
.hljs-builtin-name,
.hljs-literal,
.hljs-type,
.hljs-params {
color: #b65611;
}
/* Atelier-Dune Green */
.hljs-string,
.hljs-symbol,
.hljs-bullet {
color: #60ac39;
}
/* Atelier-Dune Blue */
.hljs-title,
.hljs-section {
color: #6684e1;
}
/* Atelier-Dune Purple */
.hljs-keyword,
.hljs-selector-tag {
color: #b854d4;
}
.hljs {
display: block;
overflow-x: auto;
background: #f1f1f1;
color: #6e6b5e;
padding: 0.5em;
}
.hljs-emphasis {
font-style: italic;
}
.hljs-strong {
font-weight: bold;
}
.hljs-addition {
color: #22863a;
background-color: #f0fff4;
}
.hljs-deletion {
color: #b31d28;
background-color: #ffeef0;
}

2
book/highlight.js
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288
book/index.html
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@@ -1,288 +0,0 @@
<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Introduction - Futures Explained in 200 Lines of Rust</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="This book aims to explain Futures in Rust using an example driven approach.">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="shortcut icon" href="favicon.png">
<link rel="stylesheet" href="css/variables.css">
<link rel="stylesheet" href="css/general.css">
<link rel="stylesheet" href="css/chrome.css">
<link rel="stylesheet" href="css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
<link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css">
<link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<link rel="stylesheet" href="ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body class="light">
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "";
var default_theme = "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
document.body.className = theme;
document.querySelector('html').className = theme + ' js';
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="0_background_information.html"><strong aria-hidden="true">1.</strong> Background information</a></li><li><a href="1_futures_in_rust.html"><strong aria-hidden="true">2.</strong> Futures in Rust</a></li><li><a href="2_waker_context.html"><strong aria-hidden="true">3.</strong> Waker and Context</a></li><li><a href="3_generators_async_await.html"><strong aria-hidden="true">4.</strong> Generators and async/await</a></li><li><a href="4_pin.html"><strong aria-hidden="true">5.</strong> Pin</a></li><li><a href="6_future_example.html"><strong aria-hidden="true">6.</strong> Implementing Futures</a></li><li><a href="8_finished_example.html"><strong aria-hidden="true">7.</strong> Finished example (editable)</a></li><li class="affix"><a href="conclusion.html">Conclusion and exercises</a></li></ol>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar" class="menu-bar">
<div id="menu-bar-sticky-container">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
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</button>
</div>
<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
<div class="right-buttons">
<a href="print.html" title="Print this book" aria-label="Print this book">
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</a>
<a href="https://github.com/cfsamson/books-futures-explained" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
</div>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1><a class="header" href="#futures-explained-in-200-lines-of-rust" id="futures-explained-in-200-lines-of-rust">Futures Explained in 200 Lines of Rust</a></h1>
<p>This book aims to explain Futures in Rust using an example driven approach,
exploring why they're designed the way they are, and how they work. We'll also
take a look at some of the alternatives we have when dealing with concurrency
in programming.</p>
<p>Going into the level of detail I do in this book is not needed to use futures
or async/await in Rust. It's for the curious out there that want to know <em>how</em>
it all works.</p>
<h2><a class="header" href="#what-this-book-covers" id="what-this-book-covers">What this book covers</a></h2>
<p>This book will try to explain everything you might wonder about up until the
topic of different types of executors and runtimes. We'll just implement a very
simple runtime in this book introducing some concepts but it's enough to get
started.</p>
<p><a href="https://github.com/stjepang">Stjepan Glavina</a> has made an excellent series of
articles about async runtimes and executors, and if the rumors are right there
is more to come from him in the near future.</p>
<p>The way you should go about it is to read this book first, then continue
reading the <a href="https://stjepang.github.io/">articles from stejpang</a> to learn more
about runtimes and how they work, especially:</p>
<ol>
<li><a href="https://stjepang.github.io/2020/01/25/build-your-own-block-on.html">Build your own block_on()</a></li>
<li><a href="https://stjepang.github.io/2020/01/31/build-your-own-executor.html">Build your own executor</a></li>
</ol>
<p>I've limited myself to a 200 line main example (hence the title) to limit the
scope and introduce an example that can easily be explored further.</p>
<p>However, there is a lot to digest and it's not what I would call easy, but we'll
take everything step by step so get a cup of tea and relax.</p>
<p>I hope you enjoy the ride.</p>
<blockquote>
<p>This book is developed in the open, and contributions are welcome. You'll find
<a href="https://github.com/cfsamson/books-futures-explained">the repository for the book itself here</a>. The final example which
you can clone, fork or copy <a href="https://github.com/cfsamson/examples-futures">can be found here</a>. Any suggestions
or improvements can be filed as a PR or in the issue tracker for the book.</p>
<p>As always, all kinds of feedback is welcome.</p>
</blockquote>
<h2><a class="header" href="#reader-exercises-and-further-reading" id="reader-exercises-and-further-reading">Reader exercises and further reading</a></h2>
<p>In the last <a href="conclusion.html">chapter</a> I've taken the liberty to suggest some
small exercises if you want to explore a little further.</p>
<p>This book is also the fourth book I have written about concurrent programming
in Rust. If you like it, you might want to check out the others as well:</p>
<ul>
<li><a href="https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/">Green Threads Explained in 200 lines of rust</a></li>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/">The Node Experiment - Exploring Async Basics with Rust</a></li>
<li><a href="https://cfsamsonbooks.gitbook.io/epoll-kqueue-iocp-explained/">Epoll, Kqueue and IOCP Explained with Rust</a></li>
</ul>
<h2><a class="header" href="#credits-and-thanks" id="credits-and-thanks">Credits and thanks</a></h2>
<p>I'd like to take this chance to thank the people behind <code>mio</code>, <code>tokio</code>,
<code>async_std</code>, <code>futures</code>, <code>libc</code>, <code>crossbeam</code> which underpins so much of the
async ecosystem and and rarely gets enough praise in my eyes.</p>
<p>A special thanks to <a href="https://twitter.com/jonhoo">jonhoo</a> who was kind enough to
give me some valuable feedback on a very early draft of this book. He has not
read the finished product, but a big thanks is definitely due.</p>
<h2><a class="header" href="#translations" id="translations">Translations</a></h2>
<p><a href="https://stevenbai.top/rust/futures_explained_in_200_lines_of_rust/">This book has been translated to Chinese</a> by <a href="https://github.com/nkbai">nkbai</a>.</p>
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<h1><a class="header" href="#futures-explained-in-200-lines-of-rust" id="futures-explained-in-200-lines-of-rust">Futures Explained in 200 Lines of Rust</a></h1>
<p>This book aims to explain Futures in Rust using an example driven approach,
exploring why they're designed the way they are, and how they work. We'll also
take a look at some of the alternatives we have when dealing with concurrency
in programming.</p>
<p>Going into the level of detail I do in this book is not needed to use futures
or async/await in Rust. It's for the curious out there that want to know <em>how</em>
it all works.</p>
<h2><a class="header" href="#what-this-book-covers" id="what-this-book-covers">What this book covers</a></h2>
<p>This book will try to explain everything you might wonder about up until the
topic of different types of executors and runtimes. We'll just implement a very
simple runtime in this book introducing some concepts but it's enough to get
started.</p>
<p><a href="https://github.com/stjepang">Stjepan Glavina</a> has made an excellent series of
articles about async runtimes and executors, and if the rumors are right there
is more to come from him in the near future.</p>
<p>The way you should go about it is to read this book first, then continue
reading the <a href="https://stjepang.github.io/">articles from stejpang</a> to learn more
about runtimes and how they work, especially:</p>
<ol>
<li><a href="https://stjepang.github.io/2020/01/25/build-your-own-block-on.html">Build your own block_on()</a></li>
<li><a href="https://stjepang.github.io/2020/01/31/build-your-own-executor.html">Build your own executor</a></li>
</ol>
<p>I've limited myself to a 200 line main example (hence the title) to limit the
scope and introduce an example that can easily be explored further.</p>
<p>However, there is a lot to digest and it's not what I would call easy, but we'll
take everything step by step so get a cup of tea and relax.</p>
<p>I hope you enjoy the ride.</p>
<blockquote>
<p>This book is developed in the open, and contributions are welcome. You'll find
<a href="https://github.com/cfsamson/books-futures-explained">the repository for the book itself here</a>. The final example which
you can clone, fork or copy <a href="https://github.com/cfsamson/examples-futures">can be found here</a>. Any suggestions
or improvements can be filed as a PR or in the issue tracker for the book.</p>
<p>As always, all kinds of feedback is welcome.</p>
</blockquote>
<h2><a class="header" href="#reader-exercises-and-further-reading" id="reader-exercises-and-further-reading">Reader exercises and further reading</a></h2>
<p>In the last <a href="conclusion.html">chapter</a> I've taken the liberty to suggest some
small exercises if you want to explore a little further.</p>
<p>This book is also the fourth book I have written about concurrent programming
in Rust. If you like it, you might want to check out the others as well:</p>
<ul>
<li><a href="https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/">Green Threads Explained in 200 lines of rust</a></li>
<li><a href="https://cfsamson.github.io/book-exploring-async-basics/">The Node Experiment - Exploring Async Basics with Rust</a></li>
<li><a href="https://cfsamsonbooks.gitbook.io/epoll-kqueue-iocp-explained/">Epoll, Kqueue and IOCP Explained with Rust</a></li>
</ul>
<h2><a class="header" href="#credits-and-thanks" id="credits-and-thanks">Credits and thanks</a></h2>
<p>I'd like to take this chance to thank the people behind <code>mio</code>, <code>tokio</code>,
<code>async_std</code>, <code>futures</code>, <code>libc</code>, <code>crossbeam</code> which underpins so much of the
async ecosystem and and rarely gets enough praise in my eyes.</p>
<p>A special thanks to <a href="https://twitter.com/jonhoo">jonhoo</a> who was kind enough to
give me some valuable feedback on a very early draft of this book. He has not
read the finished product, but a big thanks is definitely due.</p>
<h2><a class="header" href="#translations" id="translations">Translations</a></h2>
<p><a href="https://stevenbai.top/rust/futures_explained_in_200_lines_of_rust/">This book has been translated to Chinese</a> by <a href="https://github.com/nkbai">nkbai</a>.</p>
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// Then use a sliding window with a constant number of words and count the
// sum of the values of the words within the window. Then use the window that got the
// maximum sum. If there are multiple maximas, then get the last one.
// Enclose the terms in <em>.
var stemmed_searchterms = searchterms.map(function(w) {
return elasticlunr.stemmer(w.toLowerCase());
});
var searchterm_weight = 40;
var weighted = []; // contains elements of ["word", weight, index_in_document]
// split in sentences, then words
var sentences = body.toLowerCase().split('. ');
var index = 0;
var value = 0;
var searchterm_found = false;
for (var sentenceindex in sentences) {
var words = sentences[sentenceindex].split(' ');
value = 8;
for (var wordindex in words) {
var word = words[wordindex];
if (word.length > 0) {
for (var searchtermindex in stemmed_searchterms) {
if (elasticlunr.stemmer(word).startsWith(stemmed_searchterms[searchtermindex])) {
value = searchterm_weight;
searchterm_found = true;
}
};
weighted.push([word, value, index]);
value = 2;
}
index += word.length;
index += 1; // ' ' or '.' if last word in sentence
};
index += 1; // because we split at a two-char boundary '. '
};
if (weighted.length == 0) {
return body;
}
var window_weight = [];
var window_size = Math.min(weighted.length, results_options.teaser_word_count);
var cur_sum = 0;
for (var wordindex = 0; wordindex < window_size; wordindex++) {
cur_sum += weighted[wordindex][1];
};
window_weight.push(cur_sum);
for (var wordindex = 0; wordindex < weighted.length - window_size; wordindex++) {
cur_sum -= weighted[wordindex][1];
cur_sum += weighted[wordindex + window_size][1];
window_weight.push(cur_sum);
};
if (searchterm_found) {
var max_sum = 0;
var max_sum_window_index = 0;
// backwards
for (var i = window_weight.length - 1; i >= 0; i--) {
if (window_weight[i] > max_sum) {
max_sum = window_weight[i];
max_sum_window_index = i;
}
};
} else {
max_sum_window_index = 0;
}
// add <em/> around searchterms
var teaser_split = [];
var index = weighted[max_sum_window_index][2];
for (var i = max_sum_window_index; i < max_sum_window_index+window_size; i++) {
var word = weighted[i];
if (index < word[2]) {
// missing text from index to start of `word`
teaser_split.push(body.substring(index, word[2]));
index = word[2];
}
if (word[1] == searchterm_weight) {
teaser_split.push("<em>")
}
index = word[2] + word[0].length;
teaser_split.push(body.substring(word[2], index));
if (word[1] == searchterm_weight) {
teaser_split.push("</em>")
}
};
return teaser_split.join('');
}
function init(config) {
results_options = config.results_options;
search_options = config.search_options;
searchbar_outer = config.searchbar_outer;
doc_urls = config.doc_urls;
searchindex = elasticlunr.Index.load(config.index);
// Set up events
searchicon.addEventListener('click', function(e) { searchIconClickHandler(); }, false);
searchbar.addEventListener('keyup', function(e) { searchbarKeyUpHandler(); }, false);
document.addEventListener('keydown', function(e) { globalKeyHandler(e); }, false);
// If the user uses the browser buttons, do the same as if a reload happened
window.onpopstate = function(e) { doSearchOrMarkFromUrl(); };
// Suppress "submit" events so the page doesn't reload when the user presses Enter
document.addEventListener('submit', function(e) { e.preventDefault(); }, false);
// If reloaded, do the search or mark again, depending on the current url parameters
doSearchOrMarkFromUrl();
}
function unfocusSearchbar() {
// hacky, but just focusing a div only works once
var tmp = document.createElement('input');
tmp.setAttribute('style', 'position: absolute; opacity: 0;');
searchicon.appendChild(tmp);
tmp.focus();
tmp.remove();
}
// On reload or browser history backwards/forwards events, parse the url and do search or mark
function doSearchOrMarkFromUrl() {
// Check current URL for search request
var url = parseURL(window.location.href);
if (url.params.hasOwnProperty(URL_SEARCH_PARAM)
&& url.params[URL_SEARCH_PARAM] != "") {
showSearch(true);
searchbar.value = decodeURIComponent(
(url.params[URL_SEARCH_PARAM]+'').replace(/\+/g, '%20'));
searchbarKeyUpHandler(); // -> doSearch()
} else {
showSearch(false);
}
if (url.params.hasOwnProperty(URL_MARK_PARAM)) {
var words = url.params[URL_MARK_PARAM].split(' ');
marker.mark(words, {
exclude: mark_exclude
});
var markers = document.querySelectorAll("mark");
function hide() {
for (var i = 0; i < markers.length; i++) {
markers[i].classList.add("fade-out");
window.setTimeout(function(e) { marker.unmark(); }, 300);
}
}
for (var i = 0; i < markers.length; i++) {
markers[i].addEventListener('click', hide);
}
}
}
// Eventhandler for keyevents on `document`
function globalKeyHandler(e) {
if (e.altKey || e.ctrlKey || e.metaKey || e.shiftKey || e.target.type === 'textarea') { return; }
if (e.keyCode === ESCAPE_KEYCODE) {
e.preventDefault();
searchbar.classList.remove("active");
setSearchUrlParameters("",
(searchbar.value.trim() !== "") ? "push" : "replace");
if (hasFocus()) {
unfocusSearchbar();
}
showSearch(false);
marker.unmark();
} else if (!hasFocus() && e.keyCode === SEARCH_HOTKEY_KEYCODE) {
e.preventDefault();
showSearch(true);
window.scrollTo(0, 0);
searchbar.select();
} else if (hasFocus() && e.keyCode === DOWN_KEYCODE) {
e.preventDefault();
unfocusSearchbar();
searchresults.firstElementChild.classList.add("focus");
} else if (!hasFocus() && (e.keyCode === DOWN_KEYCODE
|| e.keyCode === UP_KEYCODE
|| e.keyCode === SELECT_KEYCODE)) {
// not `:focus` because browser does annoying scrolling
var focused = searchresults.querySelector("li.focus");
if (!focused) return;
e.preventDefault();
if (e.keyCode === DOWN_KEYCODE) {
var next = focused.nextElementSibling;
if (next) {
focused.classList.remove("focus");
next.classList.add("focus");
}
} else if (e.keyCode === UP_KEYCODE) {
focused.classList.remove("focus");
var prev = focused.previousElementSibling;
if (prev) {
prev.classList.add("focus");
} else {
searchbar.select();
}
} else { // SELECT_KEYCODE
window.location.assign(focused.querySelector('a'));
}
}
}
function showSearch(yes) {
if (yes) {
search_wrap.classList.remove('hidden');
searchicon.setAttribute('aria-expanded', 'true');
} else {
search_wrap.classList.add('hidden');
searchicon.setAttribute('aria-expanded', 'false');
var results = searchresults.children;
for (var i = 0; i < results.length; i++) {
results[i].classList.remove("focus");
}
}
}
function showResults(yes) {
if (yes) {
searchresults_outer.classList.remove('hidden');
} else {
searchresults_outer.classList.add('hidden');
}
}
// Eventhandler for search icon
function searchIconClickHandler() {
if (search_wrap.classList.contains('hidden')) {
showSearch(true);
window.scrollTo(0, 0);
searchbar.select();
} else {
showSearch(false);
}
}
// Eventhandler for keyevents while the searchbar is focused
function searchbarKeyUpHandler() {
var searchterm = searchbar.value.trim();
if (searchterm != "") {
searchbar.classList.add("active");
doSearch(searchterm);
} else {
searchbar.classList.remove("active");
showResults(false);
removeChildren(searchresults);
}
setSearchUrlParameters(searchterm, "push_if_new_search_else_replace");
// Remove marks
marker.unmark();
}
// Update current url with ?URL_SEARCH_PARAM= parameter, remove ?URL_MARK_PARAM and #heading-anchor .
// `action` can be one of "push", "replace", "push_if_new_search_else_replace"
// and replaces or pushes a new browser history item.
// "push_if_new_search_else_replace" pushes if there is no `?URL_SEARCH_PARAM=abc` yet.
function setSearchUrlParameters(searchterm, action) {
var url = parseURL(window.location.href);
var first_search = ! url.params.hasOwnProperty(URL_SEARCH_PARAM);
if (searchterm != "" || action == "push_if_new_search_else_replace") {
url.params[URL_SEARCH_PARAM] = searchterm;
delete url.params[URL_MARK_PARAM];
url.hash = "";
} else {
delete url.params[URL_SEARCH_PARAM];
}
// A new search will also add a new history item, so the user can go back
// to the page prior to searching. A updated search term will only replace
// the url.
if (action == "push" || (action == "push_if_new_search_else_replace" && first_search) ) {
history.pushState({}, document.title, renderURL(url));
} else if (action == "replace" || (action == "push_if_new_search_else_replace" && !first_search) ) {
history.replaceState({}, document.title, renderURL(url));
}
}
function doSearch(searchterm) {
// Don't search the same twice
if (current_searchterm == searchterm) { return; }
else { current_searchterm = searchterm; }
if (searchindex == null) { return; }
// Do the actual search
var results = searchindex.search(searchterm, search_options);
var resultcount = Math.min(results.length, results_options.limit_results);
// Display search metrics
searchresults_header.innerText = formatSearchMetric(resultcount, searchterm);
// Clear and insert results
var searchterms = searchterm.split(' ');
removeChildren(searchresults);
for(var i = 0; i < resultcount ; i++){
var resultElem = document.createElement('li');
resultElem.innerHTML = formatSearchResult(results[i], searchterms);
searchresults.appendChild(resultElem);
}
// Display results
showResults(true);
}
fetch(path_to_root + 'searchindex.json')
.then(response => response.json())
.then(json => init(json))
.catch(error => { // Try to load searchindex.js if fetch failed
var script = document.createElement('script');
script.src = path_to_root + 'searchindex.js';
script.onload = () => init(window.search);
document.head.appendChild(script);
});
// Exported functions
search.hasFocus = hasFocus;
})(window.search);

1
book/searchindex.js
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book/searchindex.json
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7
book/theme-dawn.js
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ace.define("ace/theme/dawn",["require","exports","module","ace/lib/dom"],function(e,t,n){t.isDark=!1,t.cssClass="ace-dawn",t.cssText=".ace-dawn .ace_gutter {background: #ebebeb;color: #333}.ace-dawn .ace_print-margin {width: 1px;background: #e8e8e8}.ace-dawn {background-color: #F9F9F9;color: #080808}.ace-dawn .ace_cursor {color: #000000}.ace-dawn .ace_marker-layer .ace_selection {background: rgba(39, 95, 255, 0.30)}.ace-dawn.ace_multiselect .ace_selection.ace_start {box-shadow: 0 0 3px 0px #F9F9F9;}.ace-dawn .ace_marker-layer .ace_step {background: rgb(255, 255, 0)}.ace-dawn .ace_marker-layer .ace_bracket {margin: -1px 0 0 -1px;border: 1px solid rgba(75, 75, 126, 0.50)}.ace-dawn .ace_marker-layer .ace_active-line {background: rgba(36, 99, 180, 0.12)}.ace-dawn .ace_gutter-active-line {background-color : #dcdcdc}.ace-dawn .ace_marker-layer .ace_selected-word {border: 1px solid rgba(39, 95, 255, 0.30)}.ace-dawn .ace_invisible {color: rgba(75, 75, 126, 0.50)}.ace-dawn .ace_keyword,.ace-dawn .ace_meta {color: #794938}.ace-dawn .ace_constant,.ace-dawn .ace_constant.ace_character,.ace-dawn .ace_constant.ace_character.ace_escape,.ace-dawn .ace_constant.ace_other {color: #811F24}.ace-dawn .ace_invalid.ace_illegal {text-decoration: underline;font-style: italic;color: #F8F8F8;background-color: #B52A1D}.ace-dawn .ace_invalid.ace_deprecated {text-decoration: underline;font-style: italic;color: #B52A1D}.ace-dawn .ace_support {color: #691C97}.ace-dawn .ace_support.ace_constant {color: #B4371F}.ace-dawn .ace_fold {background-color: #794938;border-color: #080808}.ace-dawn .ace_list,.ace-dawn .ace_markup.ace_list,.ace-dawn .ace_support.ace_function {color: #693A17}.ace-dawn .ace_storage {font-style: italic;color: #A71D5D}.ace-dawn .ace_string {color: #0B6125}.ace-dawn .ace_string.ace_regexp {color: #CF5628}.ace-dawn .ace_comment {font-style: italic;color: #5A525F}.ace-dawn .ace_heading,.ace-dawn .ace_markup.ace_heading {color: #19356D}.ace-dawn .ace_variable {color: #234A97}.ace-dawn .ace_indent-guide {background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAACCAYAAACZgbYnAAAAEklEQVQImWNgYGBgYLh/5+x/AAizA4hxNNsZAAAAAElFTkSuQmCC) right repeat-y}";var r=e("../lib/dom");r.importCssString(t.cssText,t.cssClass)}); (function() {
ace.require(["ace/theme/dawn"], function(m) {
if (typeof module == "object" && typeof exports == "object" && module) {
module.exports = m;
}
});
})();

7
book/theme-tomorrow_night.js
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ace.define("ace/theme/tomorrow_night",["require","exports","module","ace/lib/dom"],function(e,t,n){t.isDark=!0,t.cssClass="ace-tomorrow-night",t.cssText=".ace-tomorrow-night .ace_gutter {background: #25282c;color: #C5C8C6}.ace-tomorrow-night .ace_print-margin {width: 1px;background: #25282c}.ace-tomorrow-night {background-color: #1D1F21;color: #C5C8C6}.ace-tomorrow-night .ace_cursor {color: #AEAFAD}.ace-tomorrow-night .ace_marker-layer .ace_selection {background: #373B41}.ace-tomorrow-night.ace_multiselect .ace_selection.ace_start {box-shadow: 0 0 3px 0px #1D1F21;}.ace-tomorrow-night .ace_marker-layer .ace_step {background: rgb(102, 82, 0)}.ace-tomorrow-night .ace_marker-layer .ace_bracket {margin: -1px 0 0 -1px;border: 1px solid #4B4E55}.ace-tomorrow-night .ace_marker-layer .ace_active-line {background: #282A2E}.ace-tomorrow-night .ace_gutter-active-line {background-color: #282A2E}.ace-tomorrow-night .ace_marker-layer .ace_selected-word {border: 1px solid #373B41}.ace-tomorrow-night .ace_invisible {color: #4B4E55}.ace-tomorrow-night .ace_keyword,.ace-tomorrow-night .ace_meta,.ace-tomorrow-night .ace_storage,.ace-tomorrow-night .ace_storage.ace_type,.ace-tomorrow-night .ace_support.ace_type {color: #B294BB}.ace-tomorrow-night .ace_keyword.ace_operator {color: #8ABEB7}.ace-tomorrow-night .ace_constant.ace_character,.ace-tomorrow-night .ace_constant.ace_language,.ace-tomorrow-night .ace_constant.ace_numeric,.ace-tomorrow-night .ace_keyword.ace_other.ace_unit,.ace-tomorrow-night .ace_support.ace_constant,.ace-tomorrow-night .ace_variable.ace_parameter {color: #DE935F}.ace-tomorrow-night .ace_constant.ace_other {color: #CED1CF}.ace-tomorrow-night .ace_invalid {color: #CED2CF;background-color: #DF5F5F}.ace-tomorrow-night .ace_invalid.ace_deprecated {color: #CED2CF;background-color: #B798BF}.ace-tomorrow-night .ace_fold {background-color: #81A2BE;border-color: #C5C8C6}.ace-tomorrow-night .ace_entity.ace_name.ace_function,.ace-tomorrow-night .ace_support.ace_function,.ace-tomorrow-night .ace_variable {color: #81A2BE}.ace-tomorrow-night .ace_support.ace_class,.ace-tomorrow-night .ace_support.ace_type {color: #F0C674}.ace-tomorrow-night .ace_heading,.ace-tomorrow-night .ace_markup.ace_heading,.ace-tomorrow-night .ace_string {color: #B5BD68}.ace-tomorrow-night .ace_entity.ace_name.ace_tag,.ace-tomorrow-night .ace_entity.ace_other.ace_attribute-name,.ace-tomorrow-night .ace_meta.ace_tag,.ace-tomorrow-night .ace_string.ace_regexp,.ace-tomorrow-night .ace_variable {color: #CC6666}.ace-tomorrow-night .ace_comment {color: #969896}.ace-tomorrow-night .ace_indent-guide {background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAACCAYAAACZgbYnAAAAEklEQVQImWNgYGBgYHB3d/8PAAOIAdULw8qMAAAAAElFTkSuQmCC) right repeat-y}";var r=e("../lib/dom");r.importCssString(t.cssText,t.cssClass)}); (function() {
ace.require(["ace/theme/tomorrow_night"], function(m) {
if (typeof module == "object" && typeof exports == "object" && module) {
module.exports = m;
}
});
})();

104
book/tomorrow-night.css
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@@ -1,104 +0,0 @@
/* Tomorrow Night Theme */
/* http://jmblog.github.com/color-themes-for-google-code-highlightjs */
/* Original theme - https://github.com/chriskempson/tomorrow-theme */
/* http://jmblog.github.com/color-themes-for-google-code-highlightjs */
/* Tomorrow Comment */
.hljs-comment {
color: #969896;
}
/* Tomorrow Red */
.hljs-variable,
.hljs-attribute,
.hljs-tag,
.hljs-regexp,
.ruby .hljs-constant,
.xml .hljs-tag .hljs-title,
.xml .hljs-pi,
.xml .hljs-doctype,
.html .hljs-doctype,
.css .hljs-id,
.css .hljs-class,
.css .hljs-pseudo {
color: #cc6666;
}
/* Tomorrow Orange */
.hljs-number,
.hljs-preprocessor,
.hljs-pragma,
.hljs-built_in,
.hljs-literal,
.hljs-params,
.hljs-constant {
color: #de935f;
}
/* Tomorrow Yellow */
.ruby .hljs-class .hljs-title,
.css .hljs-rule .hljs-attribute {
color: #f0c674;
}
/* Tomorrow Green */
.hljs-string,
.hljs-value,
.hljs-inheritance,
.hljs-header,
.hljs-name,
.ruby .hljs-symbol,
.xml .hljs-cdata {
color: #b5bd68;
}
/* Tomorrow Aqua */
.hljs-title,
.css .hljs-hexcolor {
color: #8abeb7;
}
/* Tomorrow Blue */
.hljs-function,
.python .hljs-decorator,
.python .hljs-title,
.ruby .hljs-function .hljs-title,
.ruby .hljs-title .hljs-keyword,
.perl .hljs-sub,
.javascript .hljs-title,
.coffeescript .hljs-title {
color: #81a2be;
}
/* Tomorrow Purple */
.hljs-keyword,
.javascript .hljs-function {
color: #b294bb;
}
.hljs {
display: block;
overflow-x: auto;
background: #1d1f21;
color: #c5c8c6;
padding: 0.5em;
-webkit-text-size-adjust: none;
}
.coffeescript .javascript,
.javascript .xml,
.tex .hljs-formula,
.xml .javascript,
.xml .vbscript,
.xml .css,
.xml .hljs-cdata {
opacity: 0.5;
}
.hljs-addition {
color: #718c00;
}
.hljs-deletion {
color: #c82829;
}

6
examples/bonus_example/cargo.toml
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@@ -1,6 +0,0 @@
[package]
name = "futures_example"
version = "0.1.0"
authors = ["Carl Fredrik Samson <cfsamson@gmail.com>"]
edition = "2018"

0
examples/bonus_example/src/main.rs
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