Pinning Demystified: The Rust Feature You Fear but Can’t Avoid

When I first heard the word Pin, I thought:

“Great. Another obscure Rust type that exists just to ruin my compile.” And I wasn’t entirely wrong. The first time I met Pin<T>, it was wrapped around some Future type deep inside an async function’s generated code. I stared at it, Googled it, and closed the tab in panic. But months later, when I started digging into how async/await actually works under the hood — and why Rust forbids certain movements — I realized that Pin isn’t evil. It’s one of Rust’s most quietly brilliant features, designed to solve a problem that almost every other language ignores (and then pays for later with segfaults and undefined behavior). The Real Problem: Moving Data That Shouldn’t Move Before we jump into Pin, let’s start with the problem it solves. In Rust, when you move a value, its memory location can change. That’s fine — usually. But what if something holds a pointer to that value’s internal memory? struct SelfRef {

   name: String,
   ptr: *const String,

}

impl SelfRef {

   fn new(name: String) -> Self {
       Self {
           ptr: std::ptr::null(),
           name,
       }
   }
   fn init(&mut self) {
       self.ptr = &self.name;
   }
   fn print(&self) {
       unsafe {
           println!("Name: {}, ptr -> {}", self.name, *self.ptr);
       }
   }

}

fn main() {

   let mut s = SelfRef::new("Rust".to_string());
   s.init();
   s.print();

} This works — until you move s. let mut s = SelfRef::new("Rust".to_string()); s.init(); let s2 = s; // Move happens here s2.print(); Boom. 💥 You’ve now got a dangling pointer. The ptr inside s points to memory that’s no longer valid, because s was moved elsewhere. And Rust’s borrow checker can’t prevent this — because it can’t track self-references that depend on stable memory addresses. This is where pinning comes in. Enter Pin: A Value That Can’t Move Pinning is Rust’s way of saying: “I’m placing this value somewhere in memory, and from now on, it’s not allowed to move.” The type looks like this: use std::pin::Pin;

let pinned_value = Pin::new(Box::new(42)); Now, pinned_value wraps a Box<i32>. The Box gives it a stable address in the heap, and Pin guarantees that you won’t accidentally move the inner data. If you try: let inner = *pinned_value; // ❌ not allowed Rust refuses, because it might move the data out of its pinned location. Why Async Rust Needs Pinning Let’s go deeper — because this is where things get real. When you write: async fn hello() {

   println!("Hello, world!");

} Rust transforms this into a state machine under the hood: enum HelloFuture {

   Start,
   Done,

} Each await point becomes a suspension point, and the future’s internal fields store intermediate state. But here’s the catch — those internal fields can hold references to other fields inside the same future struct. If the future moves while it’s running, those references would dangle. So Rust’s async system wraps every future with a Pin before polling it. That’s why async traits and executors use this signature: fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<T, E>>; The Pin<&mut Self> says: “This future cannot be moved after being pinned — so references inside it stay valid.” Architecture Flow Here’s the high-level architecture of how pinning interacts with async futures: ┌─────────────────────┐ │ Async Function │ │ (user code) │ └───────┬─────────────┘

       │
       ▼

┌─────────────────────┐ │ Compiler expands │ │ into state machine │ └───────┬─────────────┘

       │
       ▼

┌──────────────────────────┐ │ Future Struct Generated │ │ Contains internal refs │ └───────┬──────────────────┘

       │
       ▼

┌──────────────────────────┐ │ Executor pins Future │ │ (Pin<&mut Future>) │ └──────────────────────────┘ The Pin layer ensures that once the executor starts polling the future, its internal memory layout never changes. Code Flow Example Let’s simulate what happens in a simplified async runtime: use std::future::Future; use std::pin::Pin; use std::task::{Context, Poll, Waker}; use std::time::Duration; use std::thread;

struct Delay {

   done: bool,

} impl Future for Delay {

   type Output = ();
   fn poll(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<()> {
       if self.done {
           println!("Done!");
           Poll::Ready(())
       } else {
           self.as_mut().done = true;
           println!("Waiting...");
           thread::sleep(Duration::from_millis(100));
           Poll::Pending
       }
   }

} fn main() {

   let mut delay = Delay { done: false };
   let mut pinned = Box::pin(delay);
   let waker = futures::task::noop_waker();
   let mut cx = Context::from_waker(&waker);
   loop {
       match pinned.as_mut().poll(&mut cx) {
           Poll::Ready(_) => break,
           Poll::Pending => continue,
       }
   }

} What’s Happening

We box the Delay future (allocating it on the heap).
We pin it using Box::pin().
Then we repeatedly poll it via a dummy context.
Inside poll, we can safely modify its internal state without worrying about being moved.

Benchmark: Why Pinning Isn’t “Slow” Let’s benchmark to bust a myth — pinning is not slow. use std::pin::Pin; use std::time::Instant;

fn no_pin_sum(v: &mut Vec<i32>) -> i32 {

   v.iter().sum()

}

fn pin_sum(v: Pin<&mut Vec<i32>>) -> i32 {

   v.iter().sum()

}

fn main() {

   let mut data = vec![1; 1_000_000];
   let start = Instant::now();
   let s1 = no_pin_sum(&mut data);
   let t1 = start.elapsed();
   let mut data2 = vec![1; 1_000_000];
   let pinned = Pin::new(&mut data2);
   let start = Instant::now();
   let s2 = pin_sum(pinned);
   let t2 = start.elapsed();
   println!("no_pin: {:?}, pin: {:?}", t1, t2);
   assert_eq!(s1, s2);

} Output (on my M2 Mac): no_pin: 1.5ms, pin: 1.5ms No measurable slowdown — because Pin is zero-cost at runtime. It’s purely a compile-time guarantee. Key Takeaways | Concept | Description | | ------------------ | ------------------------------------------------------------------- | | Pin<T> | A wrapper that prevents data from being moved in memory. | | Why it matters | Self-referential structs and async futures depend on stable memory. | | Performance | Zero runtime cost — purely a type-level safety tool. | | Common in | Async/await, generators, and low-level async executors. | | Rule | Once pinned, don’t move. Only mutate through `Pin<&mut T>`. | The Emotional Truth When I finally “got” pinning, it felt like discovering why Rust’s async model is so honest. Other languages hide these details behind GC or heap allocation, pretending everything’s safe. Rust makes you face the truth: data safety isn’t magic — it’s architecture. Pin isn’t there to frustrate you. It’s there to make sure your code can sleep at night knowing its futures won’t move in their sleep. And that’s why, even though it scared me at first, I can’t imagine writing safe async code without it anymore. Final Thoughts If you’re still afraid of Pin, here’s my advice:

Play with small examples (like the Delay future above).
Don’t memorize — experiment.
Remember that Pin is not a runtime lock — it’s a guarantee to the compiler.

In the end, Pin is like Rust’s quiet bodyguard — unseen, often misunderstood, but always keeping your async dreams from falling apart.

Read the full article here: https://medium.com/@syntaxSavage/pinning-demystified-the-rust-feature-you-fear-but-cant-avoid-29dd379cb804