Why can't I store a value and a reference to that value in the same struct?

If you've been exploring Rust's powerful type system and ownership model, you might have stumbled upon an odd limitation: you cannot store a value and a reference to that value in the same struct. This constraint often trips up new Rustaceans, especially those coming from other languages where circular references or "self-referencing" structures are more straightforward.

In this article, we’ll explore why this limitation exists, some common pitfalls, and how you can work around it in idiomatic Rust.

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The Problem

Let's start with a basic example. Imagine you want a struct that stores a String and a reference to that String:

struct MyStruct<'a> {
    value: String,
    reference: &'a String,
}

fn main() {
    let my_struct = MyStruct {
        value: String::from("Hello, Rust!"),
        reference: &my_struct.value, // This doesn't work!
    };
}

At first glance, this seems reasonable. But the Rust compiler immediately shuts it down:

error[E0505]: cannot borrow `my_struct.value` as immutable because it is also borrowed as mutable

What’s going on here?

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Why This Doesn't Work

The core issue lies in Rust's ownership and borrowing rules. To understand the problem, let’s break it down:

1. Memory Layout
   
   A struct in Rust lays out its fields in memory contiguously. If a struct contains both a value and a reference to that value, Rust cannot determine a safe memory layout for the reference because it points to a part of the struct itself. This creates a self-referential structure, which Rust doesn't allow because of the potential for undefined behavior.

2. Borrowing Rules
   
   Rust enforces strict borrowing rules to ensure memory safety. When you try to create MyStruct, the value field is owned by the struct. To create a reference to it (&my_struct.value), you’re essentially borrowing from my_struct while still initializing it—a circular dependency.

3. Lifetimes and Safety
   
   Lifetimes in Rust are used to guarantee that references are valid as long as they're needed. If you store a reference to value in the same struct, Rust cannot ensure the reference remains valid because the struct owns value. If the struct moves or gets dropped, the reference becomes invalid.

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How to Work Around It

Although Rust doesn't allow storing a value and a reference to that value directly in the same struct, there are several ways to achieve similar functionality:

1. Use Option and Late Initialization

You can use an Option to delay setting the reference until after the struct is created:

struct MyStruct<'a> {
    value: String,
    reference: Option<&'a String>,
}

fn main() {
    let mut my_struct = MyStruct {
        value: String::from("Hello, Rust!"),
        reference: None, // Initially, there's no reference
    };
    my_struct.reference = Some(&my_struct.value); // Set the reference later
    println!("{}", my_struct.reference.unwrap());
}

This works because the reference is set after the struct is fully initialized.

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2. Use Rc or Arc

You can use Rc (Reference Counted) or Arc (Atomic Reference Counted) to create shared ownership:

use std::rc::Rc;

struct MyStruct {
    value: Rc<String>,
    reference: Rc<String>,
}

fn main() {
    let value = Rc::new(String::from("Hello, Rust!"));
    let my_struct = MyStruct {
        value: Rc::clone(&value),
        reference: Rc::clone(&value),
    };
    println!("{}", my_struct.reference);
}

With Rc, both fields share ownership of the same String. However, this approach doesn't preserve the semantics of "a reference to a field" but achieves a similar result.

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3. Use an Index or Key

Another idiomatic approach is to separate the value storage and reference using an index or key:

use std::collections::HashMap;

struct MyStruct<'a> {
    value: String,
    reference: &'a str,
}

fn main() {
    let mut storage = HashMap::new();
    storage.insert("key", "Hello, Rust!");

    let my_struct = MyStruct {
        value: String::from("Owned Value"),
        reference: storage.get("key").unwrap(),
    };
    println!("{}", my_struct.reference);
}

Here, the reference is managed externally, avoiding self-referential issues.

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4. Use Cell or RefCell

For interior mutability, you can use Cell or RefCell:

use std::cell::RefCell;

struct MyStruct {
    value: RefCell<String>,
}

fn main() {
    let my_struct = MyStruct {
        value: RefCell::new(String::from("Hello, Rust!")),
    };
    let reference = my_struct.value.borrow();
    println!("{}", reference);
}

This approach allows runtime borrow checking, but you must ensure correctness yourself.

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Why Rust Says "No"

Rust's safety guarantees rely on compile-time checks. Allowing self-referential structs could lead to dangling references, use-after-free errors, or memory corruption. Instead, Rust encourages developers to rethink their designs and use idiomatic patterns that achieve the same goals while maintaining safety.

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Conclusion

While Rust's restriction on storing a value and a reference to that value in the same struct can be frustrating, it’s there for good reason. The alternatives—like Option, Rc, or external references—may require rethinking your design but ensure your program is robust and memory-safe. Embracing these patterns helps you write better Rust code while gaining a deeper understanding of its ownership model.

Have you encountered this limitation in your Rust journey? Share your thoughts and experiences in the comments below!