What is pin in Rust?

In Rust, the Pin type is a concept in the standard library used for handling objects that can only be safely accessed via references. These objects are commonly referred to as 'unmovable' objects. The Pin<P> type encapsulates a pointer P and provides a guarantee that the data it encapsulates remains fixed in memory. This guarantee is crucial for asynchronous programming and scenarios where objects must be non-copyable or unmovable.

Unmovable Objects

In Rust, most types are movable, meaning their values can be relocated in memory (e.g., via assignment operations). However, certain objects cannot be moved, such as when the type contains pointers to its own fields. If such an object is moved, these internal pointers may reference invalid memory locations, leading to undefined behavior.

Use Cases for Pin

One of the most common use cases for Pin is in asynchronous programming. In asynchronous tasks (Futures), the task may be partially executed across multiple invocations, requiring the data structure to remain fixed in memory. By using Pin, we can create an asynchronous task with a fixed position, ensuring the task's execution environment remains consistent and stable during asynchronous operations.

Example

Suppose we have a struct that contains self-referential fields, where one field directly points to another field within the struct. Such a struct cannot be safely moved because moving it would cause the self-reference to point to incorrect locations.

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

struct SelfReferential {
    data: String,
    pointer: *const String,
    _pin: PhantomPinned, // This field indicates the struct cannot be moved
}

impl SelfReferential {
    fn new(data: String) -> Pin<Box<SelfReferential>> {
        let res = SelfReferential {
            data,
            pointer: std::ptr::null(),
            _pin: PhantomPinned,
        };
        let boxed = Box::pin(res);
        let mut_ref: Pin<&mut SelfReferential> = boxed.as_mut();
        let pointer: *const String = &mut_ref.data;
        unsafe {
            let mut_ref = Pin::get_unchecked_mut(mut_ref);
            mut_ref.pointer = pointer;
        }
        boxed
    }
}

fn main() {
    let sr = SelfReferential::new("hello".to_string());
    let sr_ref = unsafe { &*sr.pointer };
    println!("data: {}", sr_ref);
}

In this example, since the SelfReferential struct contains a pointer to its internal data, it uses PhantomPinned and Pin to ensure the struct cannot be moved, thereby maintaining the validity of internal pointers. By doing so, we can safely create and use self-referential types or other types that require fixed memory locations.