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Ownership Rules


Ownership Rules

Rust's ownership model ensures memory safety without needing a garbage collector. Ownership rules are crucial for writing safe and efficient Rust code. Sometimes, beginners may write code that violates these rules. In this challenge, you will fix a piece of Rust code that has ownership rule violations.

Ownership Rules

Here are some important ownership rules to remember:

  1. Each value in Rust has a variable that's called its owner.
  2. There can only be one owner at a time.
  3. When the owner goes out of scope, the value will be dropped.
  4. You can have either one mutable reference or any number of immutable references, but not both simultaneously.

Your task

Before starting to solve the challenge, try to run the code and see what error you'll get. This will help you understand the problem better.

The compile error tells us that we can't borrow s as mutable because it is also borrowed as immutable. If we borrow it as mutable, the value will change the the compiler can not guarantee that the immutable reference hasn't changed.

You are given a function calculate_and_modify that violates Rust's ownership rules. Your task is to identify and fix the ownership rule violations in this function.

  • Fix the code to adhere to Rust's ownership rules.
  • The code should print the reference s2 after without any ownership rule violations.
  • The code should run the push_str method to mutate the string.
  • The code should return the modified string and its length.


  • You only need to change the order of the function calls, just make sure you don't have invalid references.