How to mutate a child of my struct

help
1

Hi, just learning Rust this week, so possibly a dumb question: I am trying to write a method on my class to perform some mutation. I have the following setup:

struct Card {
    name: String,
    variant: CardVariant
}
enum CardVariant {
    Unit(Unit),
    Spell
}
enum Unit {
    position: Position
}

I was able to write a method to mutate the name of a Card without any trouble:

fn set_name(card: &mut Card, name: String) {
    card.name = name;
}

But now I am trying to write another method to mutate the child field, and I'm not allowed to:

fn set_position(card: &mut Card, position: Position) {
    match card.variant {
        CardVariant::Unit(u) => u.position = position,
        CardVariant::Spell => (...)
    }
}

This gives me a "cannot move out of reference" error for card.variant. My beginner understanding of Rust is mostly "you are allowed to mutate things if you use &mut", which seems to not be true here. What is the correct way for me to "look into" my child field and modify it? Thanks!

2

Fun point is you've already answered your question yourself. You are allowed to mutate things if you use &mut.

fn set_position(card: &mut Card, position: Position) {
    match &mut card.variant {
        CardVariant::Unit(u) => u.position = position,
        CardVariant::Spell => {...}
    }
}
3

Ah, thanks! I didn't know you had to keep doing &mut.

4

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