borrowed value does not live long enough reference in a struct

Question

I am new to Rust and stumbled upon a problem that I cannot wrap my head around. I have a struct Customer that contains a vector of account references. The compiler suggested I should use the lifetime specifier 'a which as far as I understand means, that the accounts cannot be destroyed before the customer.

pub struct Account {
    pub balance: f32,
}

pub struct Customer<'a> {
    accounts: Vec<&'a mut Account>,
}

Now I want to implement a function that creates a new account for my customer, I assumed it could work something like this:

impl<'a> Customer<'a> {
    pub fn create_account(&mut self, balance: f32) {
        let mut account = Account{balance};
        self.accounts.push(&mut account);
    }
}

However I get an error:

error[E0597]: `account` does not live long enough
  --> src\banking\mod.rs:16:28
   |
8  | impl<'a> Customer<'a> {
   |      -- lifetime `'a` defined here
...
16 |         self.accounts.push(&mut account);
   |         -------------------^^^^^^^^^^^^-
   |         |                  |
   |         |                  borrowed value does not live long enough
   |         argument requires that `account` is borrowed for `'a`
17 |     }
   |     - `account` dropped here while still borrowed

I assume the reason is that the account variable gets destroyed at the end of the create_account function. However, how can I create a new account and assign it to the customer?

Answer 1

Your customers currently mutably borrow their accounts. Someone has to own that data. Since there's no other structures in the code that I can see, I assume you meant to have the customers own the data. If so, just get rid of the &mut.

pub struct Customer {
  accounts: Vec<Account>,
}

...

impl<'a> Customer<'a> {
  pub fn create_account(&mut self, balance: f32) {
    let account = Account { balance };
    self.accounts.push(account);
  }
}

As a bonus point, Customer no longer has lifetime arguments, it simply exists independently.

Answer 2

You are correctly identifying the cause of the compiler error. For how to fix it, you need to consider your design. Because your Customer type has a vector of references to Accounts, structurally you are implying that the accounts are owned by something else. For example, you could have a Bank type that actually stores and owns the Accounts, and then the Customer can have a reference. Alternatively, you could change your Customer type to own the Accounts, rather than references to Accounts.