Pin examples

self_referential.rs

extern crate pin_project_lite; // 0.2.16project;

use pin_project_lite::pin_project;
use std::marker::PhantomPinned;
use std::pin::{Pin, pin};
use std::ptr::NonNull;

pin_project! {
    #[derive(Debug)]
    pub struct A {
        #[pin]
        b: B,
        #[pin]
        c: C,
        _pin: PhantomPinned,
    }
}

#[derive(Debug)]
pub struct B {
    c: NonNull<C>,
    data: i32,
}

impl B {
    pub fn set_c(&mut self, c: Pin<&mut C>) {
        self.c = NonNull::from(c.get_mut());
    }

    pub fn c(&self) -> &C {
        unsafe { &*self.c.as_ptr() }
    }
}

#[derive(Debug)]
pub struct C {
    b: NonNull<B>,
    data: String,
}

impl C {
    pub fn set_b(&mut self, b: Pin<&mut B>) {
        self.b = NonNull::from(b.get_mut());
    }

    pub fn b(&self) -> &B {
        unsafe { &*self.b.as_ptr() }
    }
}

impl A {
    pub fn new() -> Pin<Box<Self>> {
        let mut a = Box::pin(A {
            b: B {
                c: NonNull::dangling(),
                data: 2,
            },
            c: C {
                b: NonNull::dangling(),
                data: "hello".to_owned(),
            },
            _pin: PhantomPinned,
        });
        a.b.c = NonNull::from(&a.c);
        a.c.b = NonNull::from(&a.b);
        a
    }
}

fn main() {
    {
        let a = A::new();
        println!("A: {a:?}");
        println!("A's B's C: {:?}", a.b.c().data);
        println!("A's C's B: {:?}", a.c.b().data);
        let data = a.b.c().b().data;
        drop(a);
        println!("A's B's C's B: {:?}", data);
    }
    {
        let mut a = pin!(A {
            b: B {
                c: NonNull::dangling(),
                data: 2,
            },
            c: C {
                b: NonNull::dangling(),
                data: "hello".to_owned(),
            },
            _pin: PhantomPinned,
        });
        let mut this = a.as_mut().project();
        this.b.set_c(this.c.as_mut());
        this.c.set_b(this.b.as_mut());
        
        let data = a.b.c().b().data;
        println!("A's B's C's B: {:?}", data);
    }
}