Variables and Mutability

As mentioned in Chapter 2, by default variables are immutable. This is one of many nudges in Rust that encourages you to write your code in a way that takes advantage of the safety and easy concurrency that Rust offers. However, you still have the option to make your variables mutable. Let’s explore how and why Rust encourages you to favor immutability, and why you might want to opt out.

When a variable is immutable, that means once a value is bound to a name, you can’t change that value. To illustrate, let’s generate a new project called variables in your projects directory by using cargo new --bin variables.

Then, in your new variables directory, open src/main.rs and replace its code with the following:

Filename: src/main.rs

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}

Save and run the program using cargo run. You should receive an error message, as shown in this output:

error[E0384]: re-assignment of immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         - first assignment to `x`
3 |     println!("The value of x is: {}", x);
4 |     x = 6;
  |     ^^^^^ re-assignment of immutable variable

This example shows how the compiler helps you find errors in your programs. Even though compiler errors can be frustrating, they only mean your program isn’t safely doing what you want it to do yet; they do not mean that you’re not a good programmer! Experienced Rustaceans still get compiler errors. The error indicates that the cause of the error is re-assignment of immutable variable, because we tried to assign a second value to the immutable x variable.

It’s important that we get compile-time errors when we attempt to change a value that we previously designated as immutable because this very situation can lead to bugs. If one part of our code operates on the assumption that a value will never change and another part of our code changes that value, it’s possible that the first part of the code won’t do what it was designed to do. This cause of bugs can be difficult to track down after the fact, especially when the second piece of code changes the value only sometimes.

In Rust the compiler guarantees that when we state that a value won’t change, it really won’t change. That means that when you’re reading and writing code, you don’t have to keep track of how and where a value might change, which can make code easier to reason about.

But mutability can be very useful. Variables are immutable only by default; we can make them mutable by adding mut in front of the variable name. In addition to allowing this value to change, it conveys intent to future readers of the code by indicating that other parts of the code will be changing this variable value.

For example, change src/main.rs to the following:

Filename: src/main.rs

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}

When we run this program, we get the following:

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
     Running `target/debug/variables`
The value of x is: 5
The value of x is: 6

Using mut, we’re allowed to change the value that x binds to from 5 to 6. In some cases, you’ll want to make a variable mutable because it makes the code more convenient to write than an implementation that only uses immutable variables.

There are multiple trade-offs to consider, in addition to the prevention of bugs. For example, in cases where you’re using large data structures, mutating an instance in place may be faster than copying and returning newly allocated instances. With smaller data structures, creating new instances and writing in a more functional programming style may be easier to reason about, so the lower performance might be a worthwhile penalty for gaining that clarity.

Differences Between Variables and Constants

Being unable to change the value of a variable might have reminded you of another programming concept that most other languages have: constants. Like immutable variables, constants are also values that are bound to a name and are not allowed to change, but there are a few differences between constants and variables.

First, we aren’t allowed to use mut with constants: constants aren't only immutable by default, they're always immutable.

We declare constants using the const keyword instead of the let keyword, and the type of the value must be annotated. We're about to cover types and type annotations in the next section, “Data Types,” so don't worry about the details right now, just know that we must always annotate the type.

Constants can be declared in any scope, including the global scope, which makes them useful for values that many parts of code need to know about.

The last difference is that constants may only be set to a constant expression, not the result of a function call or any other value that could only be computed at runtime.

Here's an example of a constant declaration where the constant's name is MAX_POINTS and its value is set to 100,000. (Rust constant naming convention is to use all upper case with underscores between words):

const MAX_POINTS: u32 = 100_000;

Constants are valid for the entire time a program runs, within the scope they were declared in, making them a useful choice for values in your application domain that multiple part of the program might need to know about, such as the maximum number of points any player of a game is allowed to earn or the speed of light.

Naming hardcoded values used throughout your program as constants is useful in conveying the meaning of that value to future maintainers of the code. It also helps to have only one place in your code you would need to change if the hardcoded value needed to be updated in the future.

Shadowing

As we saw in the guessing game tutorial in Chapter 2, we can declare new variables with the same name as a previous variables, and the new variable shadows the previous variable. Rustaceans say that the first variable is shadowed by the second, which means that the second variable’s value is what we’ll see when we use the variable. We can shadow a variable by using the same variable’s name and repeating the use of the let keyword as follows:

Filename: src/main.rs

fn main() {
    let x = 5;

    let x = x + 1;

    let x = x * 2;

    println!("The value of x is: {}", x);
}

This program first binds x to a value of 5. Then it shadows x by repeating let x =, taking the original value and adding 1 so the value of x is then 6. The third let statement also shadows x, taking the previous value and multiplying it by 2 to give x a final value of 12. When you run this program, it will output the following:

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
     Running `target/debug/variables`
The value of x is: 12

This is different than marking a variable as mut, because unless we use the let keyword again, we’ll get a compile-time error if we accidentally try to reassign to this variable. We can perform a few transformations on a value but have the variable be immutable after those transformations have been completed.

The other difference between mut and shadowing is that because we’re effectively creating a new variable when we use the let keyword again, we can change the type of the value, but reuse the same name. For example, say our program asks a user to show how many spaces they want between some text by inputting space characters, but we really want to store that input as a number:

let spaces = "   ";
let spaces = spaces.len();

This construct is allowed because the first spaces variable is a string type, and the second spaces variable, which is a brand-new variable that happens to have the same name as the first one, is a number type. Shadowing thus spares us from having to come up with different names, like spaces_str and spaces_num; instead, we can reuse the simpler spaces name. However, if we try to use mut for this, as shown here:

let mut spaces = "   ";
spaces = spaces.len();

we’ll get a compile-time error because we’re not allowed to mutate a variable’s type:

error[E0308]: mismatched types
 --> src/main.rs:3:14
  |
3 |     spaces = spaces.len();
  |              ^^^^^^^^^^^^ expected &str, found usize
  |
  = note: expected type `&str`
             found type `usize`

Now that we’ve explored how variables work, let’s look at more data types they can have.