For with first and last element in Rust

main.rs

trait Boundarized {
    fn boundarize(self) -> Tripled<Self>;
}

struct Tripled<I> {
    iter: I,
    next: I,
}

impl<T> Boundarized for Iterator<Item = T> {
    fn triple_split(self) -> Tripled<T> {
        return Tripled { iter: self, next: None };
    }
}

impl<I> Iterator for Tripled<I> where I: Iterator {
    type Item = (usize, <I as Iterator>::Item);

    fn next(&mut self) -> Option<(ForPosition, <I as Iterator>::Item)> {
    }
}


enum ForPosition {
    First,
    Middle, // Такой элемент, который не является ни первым, ни последним
    Last,
    FirstLast, // Массив состоит из одного элемента, который является одновременно первым и последним
}

fn main() {
    let mas = vec![1, 2, 3, 4, 5];

    for (pos, &i) in &mas.boundarize() {
        match pos {
            ForPosition::First => {
                println!("The first element is {}", i);
            },
            ForPosition::Middle => {
                println!("The middle element is {}", i);
            },
            ForPosition::Last => {
                println!("The last element is {}", i);
            },
            ForPosition::FirstLast => {
                println!("The first and last element is {}", i);
            },
        }
    }
}