Rust Pin Type

When working with async programming in Rust, you'll eventually encounter a somewhat challenging concept called Pin. While it might seem intimidating at first, understanding Pin is crucial for writing correct and efficient async code.

What is Pin and Why Do We Need It?

In Rust's async world, the Pin type solves a specific problem related to self-referential structures that can arise when working with futures. But before diving into Pin, let's understand the problem it aims to solve.

The Problem: Self-Referential Structures

Consider this simplified example of what might happen inside a future:

rust

struct SelfReferential {
    data: String,
    // This pointer points to a location within `data` above
    pointer: *const u8,
}

This structure contains a pointer that references memory owned by another field within the same struct. In normal Rust code, this pattern is extremely rare and generally discouraged. However, in the async/await ecosystem, the compiler transforms your code in ways that can create these self-referential structures behind the scenes.

The problem arises when these structures are moved in memory. When a struct is moved, all its fields are copied to a new memory location, but any self-references (pointers to its own fields) would still point to the old location - creating dangling pointers and undefined behavior!

Enter Pin: Memory Location Stability

This is where Pin comes in. Pin<P<T>> (where P is a pointer type like &mut or Box) ensures that the value T will not be moved from its memory location once it's "pinned." This guarantee allows self-referential structures to work safely.

How Pin Works

The Pin type works by wrapping a pointer (like &mut T or Box<T>) and restricting how that pointer can be used:

rust

use std::pin::Pin;

// A pinned mutable reference
let mut data = String::from("Hello");
let pinned_reference: Pin<&mut String> = Pin::new(&mut data);

// A pinned box
let boxed_data = Box::new(String::from("World"));
let pinned_box: Pin<Box<String>> = Pin::new(boxed_data);

Once a value is pinned, you cannot get a regular &mut T to it (which would allow moving it) unless the type T implements a trait called Unpin.

The Unpin Trait

Unpin is an auto-trait that most Rust types implement by default. It indicates that a type is safe to move even when pinned. Types that are not Unpin (like many future types) are the ones that need the guarantees that Pin provides.

rust

use std::marker::Unpin;

// Most types implement Unpin by default
struct Regular(u32);
// This is true automatically
let _: bool = Regular(42) implements Unpin;

// Future types generally don't implement Unpin
async fn example() {
    // This future might contain self-references after being transformed
}
// The returned future type doesn't implement Unpin

Using Pin in Practice

Let's look at practical examples of working with Pin:

Example 1: Creating a Pinned Future

rust

use std::pin::Pin;
use std::future::Future;
use std::task::{Context, Poll};

// A simple future that we'll pin
struct MyFuture {
    counter: u32,
}

impl Future for MyFuture {
    type Output = u32;

    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<u32> {
        let me = self.get_mut(); // Safe because MyFuture implements Unpin
        me.counter += 1;
        
        if me.counter >= 5 {
            Poll::Ready(me.counter)
        } else {
            Poll::Pending
        }
    }
}

// Using our future
async fn use_future() {
    let mut future = MyFuture { counter: 0 };
    
    // Pin the future to its current memory location
    let mut pinned = Pin::new(&mut future);
    
    // Now we can poll it
    let waker = /* create a waker somehow */;
    let mut context = Context::from_waker(&waker);
    
    loop {
        match pinned.as_mut().poll(&mut context) {
            Poll::Ready(result) => {
                println!("Future completed with result: {}", result);
                break;
            }
            Poll::Pending => {
                println!("Future not ready yet");
                // In real code, we'd yield here and try again later
            }
        }
    }
}

// Output when run:
// Future not ready yet
// Future not ready yet
// Future not ready yet
// Future not ready yet
// Future completed with result: 5

Example 2: Working with !Unpin Types

For types that don't implement Unpin, we need to be more careful:

rust

use std::pin::Pin;
use std::marker::PhantomPinned;

// A self-referential struct that should not be moved
struct SelfReferential {
    data: String,
    self_ptr: *const String,
    // This marker indicates this type shouldn't implement Unpin
    _marker: PhantomPinned,
}

impl SelfReferential {
    // Create a new pinned instance
    fn new(data: String) -> Pin<Box<Self>> {
        // Create the struct with a placeholder pointer
        let mut boxed = Box::new(SelfReferential {
            data,
            self_ptr: std::ptr::null(),
            _marker: PhantomPinned,
        });
        
        // Set up the self-reference - ONLY SAFE BECAUSE IT'S ALREADY BOXED
        // AND WILL NEVER MOVE AGAIN AFTER PINNING
        let self_ptr: *const String = &boxed.data;
        boxed.self_ptr = self_ptr;
        
        // Pin the box
        Pin::new(boxed)
    }
    
    // Safe method to get the data
    fn get_data(self: Pin<&Self>) -> &str {
        &self.data
    }
    
    // Safe method to demonstrate the self-reference
    fn get_self_ptr(self: Pin<&Self>) -> &str {
        unsafe {
            // Dereferencing raw pointers requires unsafe
            &(*self.self_ptr)
        }
    }
}

fn main() {
    let pinned = SelfReferential::new(String::from("Hello, self-reference!"));
    
    println!("Direct access: {}", pinned.as_ref().get_data());
    println!("Via self-ptr: {}", pinned.as_ref().get_self_ptr());
    
    // These values will be the same!
}

// Output:
// Direct access: Hello, self-reference!
// Via self-ptr: Hello, self-reference!

Pin and Async/Await

In async/await code, the compiler uses Pin behind the scenes. When you write:

rust

async fn process() {
    let s = String::from("Hello");
    // Do something with s
    delay_for(1).await;
    // Use s again
}

The compiler transforms this into a state machine that needs to store the value of s across .await points. This can create self-referential structures, which is why futures are typically not Unpin.

The good news is that you rarely need to work with Pin directly in most async Rust code because:

1. The async/await syntax handles it for you
2. Executor libraries like tokio take care of pinning futures

Common Pin Patterns

Here are some patterns you'll see with Pin:

1. The pin_mut! Macro

The futures crate provides a convenient macro for pinning values on the stack:

rust

use futures::pin_mut;

async fn example() {
    let future = async { /* ... */ };
    pin_mut!(future);  // Now `future` is pinned
    
    // Use pinned future
    future.await;
}

2. Box::pin

For heap allocation and pinning in one step:

rust

use std::pin::Pin;

let future = Box::pin(async {
    // This future is now pinned and boxed
});

3. Pinning In Place with pin-project Crate

For complex structs containing futures, the pin-project crate is invaluable:

rust

use pin_project::pin_project;

#[pin_project]
struct MyStruct {
    #[pin]
    future: SomeFuture,
    regular_field: String,
}

impl Future for MyStruct {
    type Output = ();
    
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        let this = self.project();
        
        // `this.future` is now Pin<&mut SomeFuture>
        // `this.regular_field` is &mut String
        match this.future.poll(cx) {
            Poll::Ready(_) => Poll::Ready(()),
            Poll::Pending => Poll::Pending,
        }
    }
}

Visualizing Pin

Let's understand Pin with a diagram:

Summary

The Pin type in Rust serves a crucial role in async programming by preventing certain values from being moved in memory. This is essential for self-referential structures that can emerge in the compiler's transformation of async code.

Key takeaways:

• Pin<P<T>> ensures the value T won't be moved from its location
• Most types implement Unpin by default, meaning they can be safely moved even when pinned
• Futures typically don't implement Unpin because they might contain self-references
• You rarely need to work with Pin directly thanks to language features and libraries

Exercises

1. Create a simple future type and implement the Future trait for it, using Pin correctly in the poll method.
2. Experiment with the pin-project crate by creating a struct with both pinned and unpinned fields.
3. Try to create a self-referential struct similar to the SelfReferential example above, but with different data types.
4. Implement a function that takes a Pin<&mut T> parameter and explore what you can and cannot do with it.

Additional Resources

• Rust Pin Documentation
• Pinning in the Rust Async Book
• Understanding Pin series on the Rust blog
• pin-project crate
• Tokio documentation on pinning