Persistent List in rust

List.rs

use std::{iter::FromIterator, rc::Rc};

#[derive(Eq, PartialEq, Debug, Ord, PartialOrd, Clone)]
struct Node<T> {
    elem: T,
    next: Link<T>,
}

#[derive(Eq, PartialEq, Debug, Ord, PartialOrd, Clone)]
pub struct List<T> {
    head: Link<T>,
}

type Link<T> = Option<Rc<Node<T>>>;

pub struct Iter<'a, T> {
    next: Option<&'a Node<T>>,
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None }
    }

    pub fn append(&self, elem: T) -> Self {
        List {
            head: Some(Rc::new(Node {
                elem: elem,
                next: self.head.clone(),
            })),
        }
    }

    pub fn head(&self) -> Option<&T> {
        self.head.as_ref().map(|node| &node.elem)
    }

    pub fn tail(&self) -> Self {
        List {
            head: self.head.as_ref().and_then(|node| node.next.clone()),
        }
    }

    pub fn iter(&self) -> Iter<'_, T> {
        Iter {
            next: self.head.as_ref().map(|node| &**node),
        }
    }
}

// allow Iterator functions for List Iter 
impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.map(|node| {
            self.next = node.next.as_ref().map(|node| &**node);
            &node.elem
        })
    }
}

// Collect from Iterator to List 
impl<T: Copy> FromIterator<T> for List<T> {
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
        // cheating, meh. 
        // Could make a DoubleEndedIterator TC instance and call iter.rev() instead
        // but much slower than vec.reverse
        let mut vec = Vec::new();
        iter.into_iter().for_each(|e| vec.push(e));
        vec.reverse();
        vec.iter().fold(List::new(), |acc, elem| acc.append(*elem))
    }
}

//Destructor
impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut link = self.head.take();
        while let Some(rc_node) = link {
            if let Ok(mut node) = Rc::try_unwrap(rc_node) {
                link = node.next.take();
            } else {
                break;
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::List;
    #[test]
    fn append_tail() {
        let empty_list = List::new();

        assert_eq!(empty_list.head(), None);

        let list = empty_list.append(1).append(2).append(3);
        assert_eq!(list.head(), Some(&3));
        assert_eq!(list.tail().head(), Some(&2));
        assert_eq!(list.tail().tail().head(), Some(&1));
        assert_eq!(list.tail().tail().tail().head(), None);
        assert_eq!(list.tail().tail().tail().tail().head(), None);
    }

    #[test]
    fn rc_tail() {
        let list = List::new().append(1).append(2);
        assert_eq!(list.append(0).tail(), list.append(9).tail());
    }

    #[test]
    fn iter() {
        let list = List::new().append(1).append(2).append(3);

        let mut iter = list.iter();
        assert_eq!(iter.next(), Some(&3));
        assert_eq!(iter.next(), Some(&2));
        assert_eq!(iter.next(), Some(&1));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn map() {
        let list = List::new().append(1).append(2).append(3);
        let list_inc = List::new().append(2).append(3).append(4);
        assert_eq!(list.iter().map(|i| i + 1).collect::<List<_>>(), list_inc);
    }

    #[test]
    fn find() {
        let list = List::new().append(1).append(2).append(3);
        assert_eq!(
            list.iter().find(|i| *i == &2),
            Some(&2)
        );
    }

    #[test]
    fn fold() {
        let list = List::new().append(1).append(2).append(3);
        assert_eq!(list.iter().fold(0, |acc, elem| acc + elem), 6)
    }
}

List0.rs

//Could have used the enum below but head can end up on the stack rather than heap
pub enum List<T> {
    Nil,
    Cons(T, Rc<List<T>>)
}