give me an example when we have to use Pin in Rust

One example of when we have to use Pin in Rust is when working with asynchronous code that involves mutable data. When using the async/await syntax, we may need to ensure that the data being modified by a future remains valid and does not move in memory before the future completes. In such cases, we can use Pin to "pin" the data in place and prevent it from being moved. This is important because moving the data could invalidate any references to it, which could lead to undefined behavior or memory safety issues.

Here is an example of using Pin with an asynchronous future that modifies mutable data:

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

struct AsyncData {
    value: i32,
}

impl AsyncData {
    async fn increment(&mut self) {
        self.value += 1;
    }
}

struct IncrementFuture<'a> {
    data: Pin<&'a mut AsyncData>,
}

impl<'a> Future for IncrementFuture<'a> {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.data.as_mut().increment(); // Modify the data
        Poll::Ready(())
    }
}

fn main() {
    let mut data = AsyncData { value: 0 };
    let mut future = IncrementFuture { data: Pin::new(&mut data) };
    let _ = futures::executor::block_on(future); // Run the future
}

In this example, we define a struct AsyncData that holds a mutable value. We also define a future IncrementFuture that takes a Pin<&mut AsyncData> reference to the data and increments its value. To ensure that the data is not moved during the future's execution, we use Pin::new to create a pinned reference to the data. Finally, we run the future using the block_on function from the futures crate.