How to resolve "error[E0499]: cannot borrow ... as mutable more than once at a time" in this case

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3

After looking at your code closer, I see why the compiler led you astray: you are trying to create a self-referential struct.

When you have a lifetime <'a> on a struct, that lifetime denotes references to values stored outside of the struct. If you try to store a reference that points inside the struct rather than outside, you will run into a compiler error when the compiler notices you lied to it.

Use the key instead of a reference:

pub struct IndexerCustomer {
    index_code: BTreeMap<u32, Customer>,
    index_name: BTreeMap<String, u32>,
    index_city: BTreeMap<String, u32>,
}

impl IndexerCustomer {
    pub fn insert(&mut self, customer: Customer) {
        let customer_opt = match self.index_code.entry(customer.code) {
            Entry::Occupied(_) => None,
            Entry::Vacant(e) => Some(&*e.insert(customer)),
        };
        match customer_opt {
            Some(customer) => {
                self.index_name.insert(customer.name.clone(), customer.code);
                self.index_city.insert(customer.city.clone(), customer.code);
            }
            None => {}
        }
    }
}

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