Rust Pin

When Pin is Needed

1. async/await Futures

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

struct MyFuture {
    state: State,
}

impl Future for MyFuture {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // self is pinned, guaranteed not to move
        let this = self.get_mut();
        Poll::Ready(())
    }
}

2. Self-Referential Structures

use std::pin::Pin;

struct Node {
    value: i32,
    // Self-reference: pointer to field within same struct
    next: Option<Pin<Box<Node>>>,
}

Solution Patterns

Pattern 1: Pinning on Heap

use std::pin::Pin;

let future = async {
    // async block creates a Future
};

// Pin future on heap
let pinned: Pin<Box<dyn Future<Output = ()>>> = Box::pin(future);

// Now safe to poll

Pattern 2: Pinning with Pin::new_unchecked

use std::pin::Pin;

struct SelfReferential {
    data: String,
    ptr: *const String,  // Points to data field
}

impl SelfReferential {
    fn new(data: String) -> Pin<Box<Self>> {
        let mut boxed = Box::new(SelfReferential {
            data,
            ptr: std::ptr::null(),
        });

        let ptr = &boxed.data as *const String;
        boxed.ptr = ptr;

        // SAFETY: boxed is on heap and won't move
        unsafe { Pin::new_unchecked(boxed) }
    }

    fn data(&self) -> &str {
        // SAFETY: ptr still valid because we're pinned
        unsafe { &*self.ptr }
    }
}

Pattern 3: Pin Projection

use std::pin::Pin;

struct Wrapper<T> {
    inner: T,
    extra: String,
}

impl<T: Unpin> Wrapper<T> {
    // Safe projection: T is Unpin
    fn project(self: Pin<&mut Self>) -> Pin<&mut T> {
        Pin::new(&mut self.get_mut().inner)
    }
}

impl<T> Wrapper<T> {
    // Unsafe projection: must maintain invariants
    fn project_unchecked(self: Pin<&mut Self>) -> Pin<&mut T> {
        // SAFETY: if Self is pinned, inner field is also pinned
        unsafe {
            Pin::new_unchecked(&mut self.get_unchecked_mut().inner)
        }
    }
}

Pattern 4: Pinning in Async Context

use std::pin::Pin;
use futures::Future;

async fn process_data() {
    let mut state = String::new();

    // This reference is held across await
    let state_ref = &mut state;

    some_async_operation().await;

    // state_ref must remain valid
    state_ref.push_str("data");
}

// Compiler ensures state doesn't move by pinning the Future

Pin Types

Unpin Marker Trait

// Most types implement Unpin (safe to move)
struct MyType {
    data: Vec<u8>,
}
// Unpin auto-implemented

// Which types DON'T implement Unpin?
// - Futures (from async/await)
// - Generators
// - Manually marked with PhantomPinned

use std::marker::PhantomPinned;

struct NotUnpin {
    data: String,
    _pin: PhantomPinned,  // Opts out of Unpin
}

Workflow

Step 1: Determine if Pin Needed

Need Pin when:
  → async/await (Future trait)
  → Self-referential struct
  → Implementing custom Future
  → Working with generators

Don't need Pin when:
  → Synchronous code
  → No self-references
  → Stack-allocated temporaries
  → Type is Unpin

Step 2: Choose Pinning Strategy

Heap pinning:
  → Box::pin(value)
  → Safe, most common

Stack pinning:
  → pin!(value)  // macro in std
  → More complex, zero allocation

Unsafe pinning:
  → Pin::new_unchecked()
  → Require SAFETY comments

Step 3: Handle Projections

Projecting to field:
  → If T: Unpin → Safe with Pin::new
  → If !Unpin → Unsafe, need Pin::new_unchecked
  → Use pin-project crate for safety

Common Use Cases

Review Checklist

When working with Pin:

• Pin actually necessary (async or self-ref)
• Correct pinning strategy chosen (heap vs stack)
• Unsafe projections have SAFETY comments
• Type correctly implements/opts-out of Unpin
• No accidental moves after pinning
• Projection maintains structural pinning
• Drop implementation respects pinning
• Documentation explains why pinned

Verification Commands

# Check if type is Unpin
cargo expand

# Verify async state machine
cargo expand --lib my_async_fn

# Test with miri
cargo +nightly miri test

Common Pitfalls

1. Forgetting to Pin Future

Symptom: Compilation error about poll signature

// ❌ Bad: Future not pinned
fn poll_future(mut future: impl Future) {
    future.poll();  // Error: no poll method
}

// ✅ Good: Pin the Future
fn poll_future(mut future: Pin<&mut impl Future>) {
    future.as_mut().poll(cx);  // OK
}

2. Moving Pinned Value

Symptom: Undefined behavior

// ❌ Bad: moving after pinning
let pinned = Box::pin(value);
let moved = *pinned;  // Error: cannot move out of pinned

// ✅ Good: work with pinned reference
let pinned = Box::pin(value);
let pinned_ref: Pin<&mut Value> = pinned.as_mut();

3. Incorrect Projection

Symptom: Unsoundness in self-referential types

// ❌ Bad: unsafe projection without guarantee
impl<T> Wrapper<T> {
    fn bad_project(self: Pin<&mut Self>) -> &mut T {
        &mut self.get_mut().inner  // Unsound if T: !Unpin
    }
}

// ✅ Good: safe projection with Unpin bound
impl<T: Unpin> Wrapper<T> {
    fn safe_project(self: Pin<&mut Self>) -> Pin<&mut T> {
        Pin::new(&mut self.get_mut().inner)
    }
}

Related Skills

• rust-async - Async/await and Future trait
• rust-unsafe - Unsafe code for Pin::new_unchecked
• rust-ownership - Lifetime and borrowing
• rust-type-driven - PhantomPinned and marker types
• rust-performance - Zero-cost abstractions with Pin

Localized Reference

• Chinese version: SKILL_ZH.md - 完整中文版本，包含所有内容