Using enum

LinkedList.rs

use std::iter::FromIterator;

enum List { Nil, Cons{value: i32, next: Box<List>}}

fn main() {
    let v = vec![1, 5, 3, 8, 12, 56, 1230, 2, 1];

    //let root: List = v.iter().cloned().collect();
    let root = List::from_iter(v);

    println!("root: {}",
             root.to_string());
}

impl Into<Option<(i32, Box<List>)>> for List {
    fn into(self) -> Option<(i32, Box<List>)> {
        match self { List::Nil => None, List::Cons{ value, next} => Some((value, next)) }
    }
}

impl<'a> FromIterator<&'a i32> for List {
    fn from_iter<I: IntoIterator<Item = &'a i32>>(iterator: I) -> Self {
     iterator.into_iter().cloned().collect()
}}

impl FromIterator<i32> for List {
    fn from_iter<I: IntoIterator<Item = i32>>(iterator: I) -> Self {
        let mut root: List = List::Nil;
        let mut prev: *mut List = &mut root;

        for i in iterator {
            let curr = List::Cons{
                value: i,
                next: Box::new(List::Nil),
            };

            unsafe {
                (*prev) = curr;
                if let &mut List::Cons{ ref mut next, ..} = &mut *prev {
                  prev = next.as_mut();
                }
            }
        }

        root
    }
}

struct ListIter<'a> (&'a List);

impl<'a> Iterator for ListIter<'a> {
    type Item = i32;

    fn next(&mut self) -> Option<i32> {
        if let List::Cons{value,ref next} = *self.0 {
            self.0 = &*next;
            Some(value)
        } else {
          None
        }
    }
}

impl<'a> IntoIterator for &'a List {
    type Item = i32;
    type IntoIter = ListIter<'a>;

    fn into_iter(self) -> Self::IntoIter {
        ListIter(self)
    }
}


impl std::fmt::Display for List {
    fn fmt(&self, fmt: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
        let mut first = true;
        try!(write!(fmt, "["));
        for value in self {
            if !first {
                try!(write!(fmt, ", "));
            }
            first = false;
            try!(write!(fmt, "{}", value));
        }
        try!(write!(fmt, "]"));
        Ok(())
    }
}