How to Use Pin<T> in Rust

Use Pin<T> to prevent a value from being moved after it has been pinned, which is required for self-referential types and async runtimes. Wrap the value in Pin::new or Pin::new_unchecked and access it via Pin::get or Pin::get_mut to ensure the memory address remains stable.

use std::pin::Pin;

struct SelfReferential {
    data: String,
    ptr: *const String,
}

fn main() {
    let mut s = SelfReferential {
        data: String::from("hello"),
        ptr: std::ptr::null(),
    };
    let pin: Pin<&mut SelfReferential> = Pin::new(&mut s);
    unsafe {
        let ptr = Pin::get_unchecked_mut(pin);
        ptr.ptr = &ptr.data as *const String;
    }
}

Pin is a tool that tells Rust, "Do not move this data in memory." It matters because some advanced code, like async functions, relies on data staying at the exact same address to work correctly. Think of it like planting a flag in the ground; once the flag is there, you cannot pick it up and move it elsewhere without breaking the map.