UnlimitedPeek.rs

struct UnlimitedPeekable<T, I: Iterator<Item = T>> {
    iter: I,
    read: Vec<T>,
}
impl<T, I: Iterator<Item = T>> UnlimitedPeekable<T, I> {
    fn peek_n(&mut self, n: usize) -> Option<&T> {
        // 1- indexed
        if n <= self.read.len() {
            self.read.get(n - 1)
        } else {
            while self.read.len() < n {
                self.read.push(self.iter.next()?);
            }
            self.read.get(n - 1)
        }
    }
}

impl<T, I: Iterator<Item = T>> Iterator for UnlimitedPeekable<T, I> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        if self.read.is_empty() {
            self.iter.next()
        } else {
            Some(self.read.remove(0))
        }
    }
}
trait UnlimitedPeek: Iterator + Sized {
    fn unlimited_peek(
        self,
    ) -> UnlimitedPeekable<Self::Item, Self> {
        UnlimitedPeekable {
            iter: self,
            read: vec![],
        }
    }
}
impl<I: Iterator> UnlimitedPeek for I {}

fn main() {
    let mut s = "hello".chars().unlimited_peek();
    dbg!(s.peek_n(3));
    dbg!(s.next());
}