#![feature(pin_ergonomics)]
#![expect(incomplete_features)]

use std::marker::PhantomPinned;
use std::pin::{Pin, pin};

struct Thing<T>(T);
impl<T> Unpin for Thing<T> {}

// Pins the value, calls the callback with it, then returns back the value.
// This function violates the Pin guaragntee.
fn wrong_pin<T>(value: T, callback: impl FnOnce(Pin<&mut T>)) -> T {
    let mut pinned_thing: Pin<&mut Thing<Option<T>>> = pin!(Thing(Some(value)));
    // pinned_option has type Pin<&mut Option<T>>
    let &pin mut Thing(ref pin mut pinned_option) = pinned_thing;
    let pinned_value: Pin<&mut T> = pinned_option.as_pin_mut().unwrap();
    callback(pinned_value);
    // Allowed because Thing implements Unpin
    let mut_thing: &mut Thing<Option<T>> = &mut *pinned_thing;
    mut_thing.0.take().unwrap()
}

struct NotUnpin(#[expect(dead_code)] i32, PhantomPinned);

fn main() {
    let value = NotUnpin(1_i32, PhantomPinned);
    let returned_value: NotUnpin = wrong_pin(value, |pinned_value: Pin<&mut NotUnpin>| {
        println!("Pinned at {pinned_value:p}");
    });
    println!("Moved to {:p}", &returned_value);
}