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<ol class="chapter"><li><a href="0_0_introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="1_0_background_information.html"><strong aria-hidden="true">2.</strong> Some background information</a></li><li><ol class="section"><li><a href="1_1_trait_objects.html"><strong aria-hidden="true">2.1.</strong> Trait objects and fat pointers</a></li><li><a href="1_2_generators_pin.html"><strong aria-hidden="true">2.2.</strong> Generators and Pin</a></li><li><a href="1_3_pin.html" class="active"><strong aria-hidden="true">2.3.</strong> Pin</a></li></ol></li><li><a href="2_0_future_example.html"><strong aria-hidden="true">3.</strong> The main example</a></li><li><a href="2_1_concurrent_futures.html"><strong aria-hidden="true">4.</strong> Bonus 1: concurrent futures</a></li></ol>
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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<h2><a class="header" href="#pin" id="pin">Pin</a></h2>
<blockquote>
<p><strong>Relevant for</strong></p>
<ol>
<li>To understand <code>Generators</code> and <code>Futures</code></li>
<li>Knowing how to use <code>Pin</code> is required when implementing your own <code>Future</code></li>
<li>To understand self-referential types in Rust</li>
<li>This is the way borrowing across <code>await</code> points is accomplished</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>Ping consists of the <code>Pin</code> type and the <code>Unpin</code> marker. Let's start off with some general rules:</p>
<ol>
<li>Pin does nothing special, it only prevents the user of an API to violate some assumtions you make when writing your (most likely) unsafe code.</li>
<li>Most standard library types implement <code>Unpin</code></li>
<li><code>Unpin</code> means it's OK for this type to be moved even when pinned.</li>
<li>If you <code>Box</code> a value, that boxed value automatcally implements <code>Unpin</code>.</li>
<li>The main use case for <code>Pin</code> is to allow self referential types</li>
<li>The implementation behind objects that doens't implement <code>Unpin</code> is most likely unsafe
<ol>
<li><code>Pin</code> prevents users from your code to break the assumtions you make when writing the <code>unsafe</code> implementation</li>
<li>It doesn't solve the fact that you'll have to write unsafe code to actually implement it</li>
</ol>
</li>
<li>You're not really meant to be implementing <code>!Unpin</code>, but you can on nightly with a feature flag</li>
</ol>
<blockquote>
<p>Unsafe code does not mean it's litterally &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>
<p>Let's take a look at an example:</p>
<pre><pre class="playpen"><code class="language-rust editable">use std::pin::Pin;
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); // try commenting out this line
println!(&quot;a: {}, b: {}&quot;, test2.a(), test2.b());
}
#[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>As you can see this results in unwanted behavior. The pointer to <code>b</code> stays the
same and points to the old value. It's easy to get this to segfault, and fail
in other spectacular ways as well.</p>
<p>Pin essentially prevents the <strong>user</strong> of your unsafe code
(even if that means yourself) move the value after it's pinned.</p>
<p>If we change the example to using <code>Pin</code> instead:</p>
<pre><pre class="playpen"><code class="language-rust editable">use std::pin::Pin;
use std::marker::PhantomPinned;
pub fn main() {
let mut test1 = Test::new(&quot;test1&quot;);
test1.init();
let mut test1_pin = unsafe { Pin::new_unchecked(&amp;mut test1) };
let mut test2 = Test::new(&quot;test2&quot;);
test2.init();
let mut test2_pin = unsafe { Pin::new_unchecked(&amp;mut test2) };
println!(
&quot;a: {}, b: {}&quot;,
Test::a(test1_pin.as_ref()),
Test::b(test1_pin.as_ref())
);
// Try to uncomment this and see what happens
// std::mem::swap(test1_pin.as_mut(), test2_pin.as_mut());
println!(
&quot;a: {}, b: {}&quot;,
Test::a(test2_pin.as_ref()),
Test::b(test2_pin.as_ref())
);
}
#[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(&amp;mut self) {
let self_ptr: *const String = &amp;self.a;
self.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, what we've done here is pinning a stack address. That will always be
<code>unsafe</code> if our type implements <code>!Unpin</code>, in other words. That our type is not
<code>Unpin</code> which is the norm.</p>
<p>We use some tricks here, including requiring an <code>init</code>. If we want to fix that
and let users avoid <code>unsafe</code> we need to place our data on the heap.</p>
<p>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>The next example solves some of our friction at the cost of a heap allocation.</p>
<pre><pre class="playpen"><code class="language-rust editbable">use std::pin::Pin;
use std::marker::PhantomPinned;
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());
// Try to uncomment this and see what happens
// std::mem::swap(&amp;mut test1, &amp;mut test2);
println!(&quot;a: {}, b: {}&quot;,test2.as_ref().a(), test2.as_ref().b());
}
#[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 a = String::from(txt);
let t = Test {
a,
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) }
}
}
</code></pre></pre>
<p>Seeing this we're ready to sum up with a few more points to remember about
pinning:</p>
<ol>
<li>Pinning only makes sense to do for types that are <code>!Unpin</code></li>
<li>Pinning a <code>!Unpin</code> pointer to the stack will requires <code>unsafe</code></li>
<li>Pinning a boxed value will not require <code>unsafe</code>, even if the type is <code>!Unpin</code></li>
<li>If T: Unpin (which is the default), then Pin&lt;'a, T&gt; is entirely equivalent to &amp;'a mut T.</li>
<li>Getting a <code>&amp;mut T</code> to a pinned pointer requires unsafe if <code>T: !Unpin</code></li>
<li>Pinning is really only useful when implementing self-referential types.<br />
For all intents and purposes you can think of <code>!Unpin</code> = self-referential-type</li>
</ol>
<p>The fact that boxing (heap allocating) a value that implements <code>!Unpin</code> is safe
makes sense. Once the data is allocated on the heap it will have a stable address. </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://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md">pin_utils</a>:<a href="https://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md">pin_utils</a> to do that.</p>
<h3><a class="header" href="#projectionstructural-pinning" id="projectionstructural-pinning">Projection/structural pinning</a></h3>
<p>In short, projection is using a field on your type. <code>mystruct.field1</code> is a
projection. Structural pinning is using <code>Pin</code> on struct 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>Futures</code> stay tuned,
we're soon finished.</p>
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