Using a peekable iterator in a function in Rust

1_working.rs

use std::iter::Peekable;

fn take_when_3(peeky: &mut Peekable<impl Iterator<Item = u8>>) -> Option<u8> {
    if let Some(3) = peeky.peek() {
        peeky.next()
    } else {
        None
    }
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let iter = vec![5, 3u8, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6].into_iter();
    let mut peeky = iter.peekable();

    println!("iter: {peeky:?}");
    println!("taking: {:?}", peeky.next());
    println!("iter: {peeky:?}");
    println!("take_when_3: {:?}", take_when_3(&mut peeky));
    println!("iter: {peeky:?}");
    println!("take_when_3: {:?}", take_when_3(&mut peeky));
    println!("iter: {peeky:?}");
    println!("taking: {:?}", peeky.next());
    println!("iter: {peeky:?}");

    Ok(())
}

// The peekable peeked state is persistant and the peeked item get returned next.
// Output:
//
// iter: Peekable { iter: IntoIter([5, 3, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6]), peeked: None }
// taking: Some(5)
// iter: Peekable { iter: IntoIter([3, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6]), peeked: None }
// take_when_3: Some(3)
// iter: Peekable { iter: IntoIter([1, 2, 3, 4, 5, 5, 1, 5, 6, 6]), peeked: None }
// take_when_3: None
// iter: Peekable { iter: IntoIter([2, 3, 4, 5, 5, 1, 5, 6, 6]), peeked: Some(Some(1)) }
// taking: Some(1)
// iter: Peekable { iter: IntoIter([2, 3, 4, 5, 5, 1, 5, 6, 6]), peeked: None }

2_broken.rs

fn take_when_3(iter: &mut dyn Iterator<Item = u8>) -> Option<u8> {
    let mut peeky = iter.peekable();
    if let Some(3) = peeky.peek() {
        peeky.next()
    } else {
        None
    }
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut iter = vec![5, 3u8, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6].into_iter();

    println!("iter: {iter:?}");
    println!("taking: {:?}", iter.next());
    println!("iter: {iter:?}");
    println!("take_when_3: {:?}", take_when_3(&mut iter));
    println!("iter: {iter:?}");
    println!("take_when_3: {:?}", take_when_3(&mut iter));
    println!("iter: {iter:?}");
    println!("taking: {:?}", iter.next());
    println!("iter: {iter:?}");

    Ok(())
}

// The peekable still takes away from the iterator.
// Output:
//
// iter: IntoIter([5, 3, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6])
// taking: Some(5)
// iter: IntoIter([3, 1, 2, 3, 4, 5, 5, 1, 5, 6, 6])
// take_when_3: Some(3)
// iter: IntoIter([1, 2, 3, 4, 5, 5, 1, 5, 6, 6])
// take_when_3: None
// iter: IntoIter([2, 3, 4, 5, 5, 1, 5, 6, 6])
// taking: Some(2)
// iter: IntoIter([3, 4, 5, 5, 1, 5, 6, 6])