fn get_middle<T>(elements: Vec<T>) -> T {
    elements[elements.len() / 2]
}

get_middle(vec![1, 2, 3])

get_middle(vec![1, 2, 3])
// as well as...
get_middle(0..3)

fn get_middle<T, U>(elements: U) -> Option<T>
where
    U: IntoIterator<Item = T>,
    U::IntoIter: ExactSizeIterator,
{
    let mut it = elements.into_iter();
    let len = it.len();

    it.nth(len / 2)
}

fn main() {
    assert_eq!(get_middle(vec![1, 2, 3]), Some(2));
    assert_eq!(get_middle(0..3), Some(1));
}