Understanding Pin-safe Types in Rust

Introduction

Rust is famous for giving you memory safety without a garbage collector. But when you start doing lower-level work — self-referential structs, async state machines, or FFI — you run into a powerful but mysterious feature: Pin.

In this article, we’ll answer the following:

• What does it mean for a type to be pin-safe?
• Why would a type need to be pinned in the first place?
• How do you build one safely — without fighting the borrow checker?

We’ll walk through simple examples first, then build up to a self-referential type.

What is a Pin-safe Type?

A pin-safe type is a type that can be safely used with Rust’s Pin API.

You create a Pin-safe type when:

• You need to guarantee that a value won’t move in memory after being created.
• You want to allow self-referencing inside a struct (e.g., a field pointing to another field).
• You’re building async state machines, generators, or intrusive data structures.

Self-Referential Structs: The Core Problem

Let’s look at a classic case. Say you have a struct like this:

struct Example {
    a: String,
    b: *const String, // b points to a
}

This is a self-referential struct: b stores a pointer to another field inside the same struct.

Seems harmless?

Here’s the catch: Rust moves values around freely — into function calls, collections, etc. If you set up a pointer inside a struct and then move the struct, your pointer is now invalid. This opens the door to use-after-free bugs.

Rust’s borrow checker normally prevents you from doing this. But sometimes you do need this — and that’s where Pin comes in.

Pin to the Rescue

This is perfect for self-referential types — it guarantees their memory address won’t change.

But you have to build your type carefully to uphold this contract.

A Pin-safe Self-Referential Type

Now let’s build a Pin-safe type step-by-step.

Step 1: Define the structure

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

struct SelfRef {
    data: String,
    data_ref: *const String, // raw pointer, not a safe Rust reference
    _pin: PhantomPinned,     // opt-out of Unpin
}

• data: holds some content.
• data_ref: stores a pointer to that data.
• PhantomPinned: tells Rust this type is not safe to move after being pinned.

Step 2: Pin and initialize

impl SelfRef {
    fn new(data: String) -> Pin<Box<SelfRef>> {
        let mut s = Box::pin(SelfRef {
            data,
            data_ref: std::ptr::null(),
            _pin: PhantomPinned,
        });

        let data_ref = &s.data as *const String;

        unsafe {
            let mut_ref = Pin::as_mut(&mut s);
            Pin::get_unchecked_mut(mut_ref).data_ref = data_ref;
        }

        s
    }

    fn get(&self) -> &String {
        unsafe { &*self.data_ref }
    }
}

Step 3: Use it

fn main() {
    let s = SelfRef::new("Hello, world!".into());
    println!("Data ref points to: {}", s.get());
}

Key Points

• You must pin the struct before setting the self-reference.
• Box::pin allocates it on the heap and returns a pinned pointer.
• PhantomPinned disables auto-Unpin so it can’t be accidentally moved.
• unsafe is required to set the internal pointer — you must guarantee it’s only done after pinning.

Summary Table

Conclusion

Most types in Rust are move-safe and don’t need Pin. But when you’re working with:

• self-referential structs,
• low-level async primitives,
• foreign function interfaces (FFI),

…you may need to reach for Pin.

A Pin-safe type is your promise to the compiler that “this won’t move again — and I’ve made sure everything inside is OK with that.”