Newb solving AoC in rust

Sorry if this is a stupid question, but I'm struggling with what I think is ownership in rust.

I'm completely new to rust, but have done low level languages before so I know the concept of pointers pretty well. However, this rust ownership I can't quite wrap my head around.

I'm trying to refactor my solution to AoC Day 4 Part 2 using a struct reference instead of a stand-alone vector.

The error I'm getting, and can't figure out is in the process function at line

cards.iter_mut().for_each(|card | {

The error is

cannot borrow cards as mutable more than once at a time second mutable borrow occurs here

There is a lot of parsing in my code, so I stuck that in a spoiler below.

The relevant code is:

    
#[derive(Debug)]
struct Card {
    id: u32,
    score: u32,
    copies: u32,
}

fn process(input: &str) -> u32 {
    let mut cards: Vec = parse_cards(input);

    cards.iter_mut().for_each(|card| {
        let copy_from = card.id as usize + 1;
        let copy_to: usize = copy_from + card.score as usize - 1;

        if card.score == 0 || copy_from > cards.len() {
            return;
        }

        for card_copy in cards[copy_from - 1..copy_to].iter() {
            let id = card_copy.id as usize - 1;
            let add = cards[card.id as usize - 1].copies;
            cards[id].copies += add;
        }
    });

    return cards.iter().map(|c| c.copies).sum();
}


  

Other code: