It loops!

itloops.rs

use std::mem;
use std::ptr;

type Link<T> = *mut DoubleLinkNode<T>;

struct DoubleLinkNode<T> {
    prev: Link<T>,
    next: Link<T>,
    value: T
}

impl<T> DoubleLinkNode<T> where T: std::fmt::Debug {
    fn push_back(&mut self, value: T) {
        let mut juggling: Link<T> = self as *mut Self;

        loop {
            unsafe {
                if (*juggling).next.is_null() {
                    println!("found null, stop loop.");
                    break;
                }
                else {
                    let tmp_ptr: Link<T> = (*juggling).next;
                    println!("found not-null. move to next <- {:?}", (*tmp_ptr).value);
                    juggling = (*juggling).next;
                }
            }
        }

        unsafe {
            println!("assign <- {:?}", value);
            (*juggling).next = &mut DoubleLinkNode { prev: juggling as *mut DoubleLinkNode<T>, next:ptr::null_mut::<DoubleLinkNode<T>>(), value:value } as *mut DoubleLinkNode<T>;
        }
    }
}

fn main() {
    let mut l1: DoubleLinkNode<i32> = DoubleLinkNode { prev: ptr::null_mut::<DoubleLinkNode<i32>>(), next: ptr::null_mut::<DoubleLinkNode<i32>>(), value:1 };
    l1.push_back(2);
    l1.push_back(3);
    l1.push_back(4);
    l1.push_back(5);

    println!("{}", l1.value);
}