In Rust, What is the Real Benefit of Pin?

Rust, the beloved systems programming language, has given us many gifts, and Pin is one of them. But what exactly is Pin, and why should you care? In this article, we’ll dive into the world of Pin and explore its real benefits in Rust programming.

The Problem of Self-Referential Structs

Before we dive into Pin, let’s talk about the problem it solves. In Rust, you can’t have a struct that owns a reference to itself. This is because Rust’s ownership system ensures that a value can’t own itself, which makes sense, as it would create a cycle of ownership.

 #[derive(Debug)]
 struct Recursive {
     value: i32,
     next: Option>,
 }
 
 impl Recursive {
     fn new(value: i32) -> Self {
         Recursive { value, next: None }
     }
 
     fn set_next(&mut self, next: Recursive) {
         self.next = Some(Box::new(next));
     }
 }
 
 let mut rec = Recursive::new(1);
 let rec2 = Recursive::new(2);
 rec.set_next(rec2); // error: cannot move out of `rec2` because it is borrowed

As you can see, we can’t move `rec2` into `rec` because `rec` still holds a reference to itself. This is where Pin comes in.

What is Pin?

Pin is a type in Rust that guarantees the pinned value won’t be moved until it’s dropped. This allows us to create self-referential structs, which are structs that own a reference to themselves.

 use std::pin::Pin;
 
 #[derive(Debug)]
 struct Recursive {
     value: i32,
     next: Option<'static Pin>,
 }
 
 impl Recursive {
     fn new(value: i32) -> Pin> {
         Pin::new(Box::new(Recursive { value, next: None }))
     }
 
     fn set_next(self: Pin<'static Box>, next: Pin>) {
         self.get_mut().next = Some(next);
     }
 }
 
 let rec = Recursive::new(1);
 let rec2 = Recursive::new(2);
 rec.set_next(rec2);

With Pin, we can create self-referential structs that own references to themselves. But that’s not all Pin can do.

The Real Benefit of Pin

Pin’s real benefit lies in its ability to create async-aware types. In Rust, async functions return a type that implements the Future trait. This type represents a value that may not be available yet, and the Future trait provides a way to poll the value and get its result.

Async-Aware Types

When working with async code, you often need to store the future returned by an async function in a struct. This struct needs to be able to store the future and poll it when required. Pin comes into play here by providing a way to create async-aware types that can store futures.

 use std::pin::Pin;
 use std::future::Future;
 
 #[derive(Debug)]
 struct AsyncStruct {
     future: Pin>>,
 }
 
 impl AsyncStruct {
     fn new(future: impl Future
 + 'static) -> Self {
         AsyncStruct {
             future: Box::pin(future),
         }
     }
 
     fn poll(&mut self) -> Poll {
         self.future.as_mut().poll()
     }
 }

With Pin, we can create async-aware types that can store futures and poll them when required. This is especially useful when working with async code that needs to store the future returned by an async function.

Pin and Unpin

Pin is not the only trait related to pinning in Rust. There’s another trait called Unpin, which is the opposite of Pin. Unpin guarantees that the unpinned value can be moved safely.

 use std::pin::Pin;
 use std::marker::Unpin;
 
 struct UnpinnedStruct {
     value: i32,
 }
 
 impl Unpin for UnpinnedStruct {}
 
 let mut unpinned = UnpinnedStruct { value: 1 };
 let pinned = Pin::new(&mut unpinned);
 // The following line would move the unpinned struct if it was pinned
 let pinned_ref = pinned.get_mut();

As you can see, Unpin is used to mark types that can be moved safely, even if they’re pinned.

When to Use Pin

So, when should you use Pin? The answer is simple: whenever you need to create self-referential structs or async-aware types.

• Self-referential structs: Pin allows you to create structs that own a reference to themselves. This is useful when working with linked lists or trees where each node needs to reference its children or parent.
• Async-aware types: Pin provides a way to create async-aware types that can store futures and poll them when required. This is especially useful when working with async code that needs to store the future returned by an async function.

In summary, Pin is a powerful tool in Rust that allows you to create self-referential structs and async-aware types. Its real benefit lies in its ability to create async-aware types that can store futures and poll them when required.

Conclusion

In conclusion, Pin is an essential tool in Rust that provides a way to create self-referential structs and async-aware types. By understanding the benefits of Pin, you can write more efficient and effective Rust code that takes advantage of the language’s powerful features. So, the next time you’re working with async code or self-referential structs, remember to Pin it!

Pin	Unpin
Guarantees the pinned value won’t be moved until it’s dropped	Guarantees the unpinned value can be moved safely

We hope you enjoyed this article on Pin in Rust. Remember, Pin is an essential tool in Rust that provides a way to create self-referential structs and async-aware types. By understanding the benefits of Pin, you can write more efficient and effective Rust code that takes advantage of the language’s powerful features.

This article was optimized for the keyword “In Rust, what is the real benefit of Pin?” and is meant to provide clear and direct instructions on how to use Pin in Rust programming.

References

Rust Documentation – Pin: https://doc.rust-lang.org/std/pin/index.html

Rust Documentation – Unpin: https://doc.rust-lang.org/std/marker/trait.Unpin.html

Frequently Asked Question

Rust enthusiasts, gather ’round! Let’s dive into the world of Pin and uncover its real benefits.