Pin and PhantomPinned

pin.rs

use core::pin::Pin;
use core::marker::PhantomPinned;

#[derive(Debug)]
struct Unm(i32, PhantomPinned); // !Unpin
//struct Unm(i32); // Unpin

fn foo() -> Pin<Box<Unm>> {
    // PhantomPinned is !Unpin so Pin does not implement get_mut (disallowing mem::swap/replece and then move) nor into_inner (move out Unm of Box)
    Box::pin(Unm(1, PhantomPinned))
    // without PhantomPinned it is Unpin so Pin does allow for get_mut and into_inner making it mu
    //Box::pin(Unm(1))
}

fn mv(u: Unm) {
    drop(dbg![u])
}

// generic types don't implement Unpin, so Pin does not impl get_mut or into_inner
fn p<T>(mut pin: Pin<&mut T>) {
    // E: the trait `Unpin` is not implemented for `T`
    //let m: &mut T = pin.as_mut().get_mut();
}

fn main() {
    {
        let mut f = foo();
        
        //let u: &mut Unm = f.as_mut().get_mut();
        //let mut tmp: Unm = Unm(2);
        //std::mem::swap(&mut tmp, u);
        //mv(tmp); // moved u
    }
    {
        let mut f = foo();
    
        //let tmp = *Pin::into_inner(f);
        //mv(tmp);
    }
}