Rust macro to create lists

List.rs

#[macro_export]
macro_rules! list {
    ($val: expr => $($tail: expr)=>+) => {
        {
            let mut node = ListNode::new($val);
            node.next = Some(Box::new($crate::list!($($tail)=>+)));
            node
        }
    };
    ($val: expr) => {
        ListNode::new($val)
    };
    () => {
        ListNode::default()
    };
}

#[derive(Default, Debug)]
struct ListNode<T> {
    pub(crate) next: Option<Box<ListNode<T>>>,
    pub val: T,
}

impl<T> ListNode<T> {
    fn new(value: T) -> Self {
        ListNode {
            next: None,
            val: value,
        }
    }
}

#[test]
pub fn empty() {
    let list: ListNode<i32> = list![];
    assert_eq!(list.val, 0);
}

#[test]
pub fn single() {
    let list = list![1];
    assert_eq!(list.val, 1);
    assert_eq!(list.next.is_none(), true);
}

#[test]
pub fn many(){
    let list = list![1 => 2];
    assert_eq!(list.val, 1);
    assert_eq!(list.next.is_some(), true);
    match(list.next){
        Some(ref next) => {
            assert_eq!(next.val, 2);
            assert_eq!(next.next.is_none(), true);
        },
        None => {
            assert!(false);
        }
    }
}