---
title: "What are Rust's rules around mutable and immutable variables?"  
description: "What are Rust's rules around mutable and immutable variables?"  
author: "Steilla Mitchel"  
published: 2023-07-10  
updated: 2023-07-11  
canonical: https://www.mindstick.com/forum/159019/what-are-rust-s-rules-around-mutable-and-immutable-variables  
category: "rust"  
tags: ["variable", "rust"]  
reading_time: 2 minutes  

---

# What are Rust's rules around mutable and immutable variables?

What are [Rust](https://www.mindstick.com/forum/160170/explain-rust-s-ownership-borrowing-and-lifetimes-system)'s [rules](https://www.mindstick.com/articles/43901/blackjack-rules) around [mutable and immutable](https://www.mindstick.com/forum/161888/what-are-python-s-mutable-and-immutable-types) [variables](https://www.mindstick.com/articles/715/php-variables)?

## Replies

### Reply by Aryan Kumar

In Rust, variables are [immutable](https://www.mindstick.com/interview/2439/how-to-create-immutable-class) by default. This means that once a variable is assigned a value, it cannot be changed. If you need to be able to change the value of a variable, you can declare it as mutable by adding the `mut` keyword before the variable name.

Here is an example of a mutable variable:

Rust

```plaintext
let mut x = 5;
x = 10;
```

In this example, the variable `x` is mutable. This means that we can change the value of `x` from 5 to 10.

Here is an example of an immutable variable:

Rust

```plaintext
let y = 5;
// y = 10; // Error: cannot assign to immutable variable `y`
```

In this example, the variable `y` is immutable. This means that we cannot change the value of `y` from 5 to 10.

Rust's rules around mutable and immutable variables are designed to prevent data races. A data race is a situation where two or more threads are accessing the same data at the same time and one of the threads is trying to modify the data. Data races can cause unpredictable behavior and can even lead to crashes.

By making variables immutable by default, Rust helps to prevent data races. This is because immutable variables can only be read, not modified. This means that two or more threads can safely read the same immutable variable at the same time without causing a data race.

If you need to be able to change the value of a variable, you can declare it as mutable. However, you should be careful when using mutable variables, as they can lead to data races if not used correctly.

Here are some of the rules around mutable and immutable variables in Rust:

- Immutable variables can only be read, not modified.
- Mutable variables can be read and modified.
- You cannot have a mutable reference to an immutable variable.
- You cannot have an immutable reference to a mutable variable.
- You cannot have two mutable references to the same variable.

These rules are designed to prevent data races and to make Rust's ownership and borrowing system more predictable.


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Original Source: https://www.mindstick.com/forum/159019/what-are-rust-s-rules-around-mutable-and-immutable-variables

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