Rust: Iterator that repeats the first and last element of another Iterator?

Question

Is there an Iterator, either in std or a maintained crate, that repeats the first and last element of another Iterator given Item is Clone?

Example:

let iter = [1, 2, 3].into_iter();

assert!(iter.repeat_first_and_last().eq([1, 1, 2, 3, 3]));

Answer 1

I ended up rolling my own.

use std::{mem::take, iter::Fuse};

use tap::Tap;

struct Padded<T: Iterator> {
    inner: Fuse<T>,
    repeat_first: bool,
    repeat_last: bool,
    next: Option<T::Item>,
}

impl<T: Iterator> Padded<T>
where
    T::Item: Clone,
{
    fn new(inner: impl IntoIterator<IntoIter = T>) -> Self
    where
        T::Item: Clone,
    {
        let mut inner = inner.into_iter().fuse();
        let next = inner.next();
        
        Self { inner, next, repeat_first: true, repeat_last: true, }
    }
}

impl<T: Iterator> Iterator for Padded<T>
where
    T::Item: Clone,
{
    type Item = T::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.take().tap(|next| {
            if take(&mut self.repeat_first) {
                self.next = next.clone();
            } else {
                self.next = self.inner.next().or_else(|| {
                    next.as_ref().filter(|_| take(&mut self.repeat_last)).cloned()
                })
            }
        })
    }
}

fn main() {
    Padded::new([1, 2, 3]).for_each(|n| print!("{n}"));
}