Chain two iterators while lazily constructing the second one

Question

I'd like a method like Iterator::chain() that only computes the argument iterator when it's needed. In the following code, expensive_function should never be called:

use std::{thread, time};

fn expensive_function() -> Vec<u64> {
    thread::sleep(time::Duration::from_secs(5));
    vec![4, 5, 6]
}

pub fn main() {
    let nums = [1, 2, 3];
    for &i in nums.iter().chain(expensive_function().iter()) {
        if i > 2 {
            break;
        } else {
            println!("{}", i);
        }
    }
}

Answer 1

One possible approach: delegate the expensive computation to an iterator adaptor.

let nums = [1, 2, 3];
for i in nums.iter()
    .cloned()
    .chain([()].into_iter().flat_map(|_| expensive_function()))
{
    if i > 2 {
        break;
    } else {
        println!("{}", i);
    }
}

Playground

The passed iterator is the result of flat-mapping a dummy unit value () to the list of values, which is lazy. Since the iterator needs to own the respective outcome of that computation, I chose to copy the number from the array.