Variables and Mutability
As mentioned in the “Storing Values with Variables” section, by default variables are immutable. This is one of many nudges Rust gives 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 sometimes you might want to opt out.
When a variable is immutable, once a value is bound to a name, you can’t change
that value. To illustrate this, let’s generate a new project called variables
in your projects directory by using cargo new variables.
Then, in your new variables directory, open src/main.rs and replace its code with the following code. This code won’t compile just yet, we’ll first examine the immutability error.
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:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
--> src/main.rs:4:5
|
2 | let x = 5;
| -
| |
| first assignment to `x`
| help: consider making this binding mutable: `mut x`
3 | println!("The value of x is: {}", x);
4 | x = 6;
| ^^^^^ cannot assign twice to immutable variable
For more information about this error, try `rustc --explain E0384`.
error: could not compile `variables` due to previous error
This example shows how the compiler helps you find errors in your programs. Compiler errors can be frustrating, but really 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 message indicates that the cause of the error is that you cannot assign twice to immutable variable `x`, because you 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’s 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. The cause of this kind of bug can be difficult to track down after the fact, especially when the second piece of code changes the value only sometimes. The Rust compiler guarantees that when you state a value won’t change, it really won’t change, so you don’t have to keep track of it yourself. Your code is thus easier to reason through.
But mutability can be very useful, and can make code more convenient to write.
Variables are immutable only by default; as you did in Chapter 2, you can make
them mutable by adding mut in front of the variable name. Adding mut also
conveys intent to future readers of the code by indicating that other parts of
the code will be changing this variable’s value.
For example, let’s 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 the program now, we get this:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
Finished dev [unoptimized + debuginfo] target(s) in 0.30s
Running `target/debug/variables`
The value of x is: 5
The value of x is: 6
We’re allowed to change the value bound to x from 5 to 6 when mut
is used. 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
think through, so lower performance might be a worthwhile penalty for gaining
that clarity.
Constants
Like immutable variables, constants are values that are bound to a name and are not allowed to change, but there are a few differences between constants and variables.
First, you aren’t allowed to use mut with constants. Constants aren’t just
immutable by default—they’re always immutable. You 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 you 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 be set only to a constant expression, not the result of a value that could only be computed at runtime.
Here’s an example of a constant declaration:
#![allow(unused)] fn main() { const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3; }
The constant’s name is THREE_HOURS_IN_SECONDS and its value is set to the
result of multiplying 60 (the number of seconds in a minute) by 60 (the number
of minutes in an hour) by 3 (the number of hours we want to count in this
program). Rust’s naming convention for constants is to use all uppercase with
underscores between words. The compiler is able to evaluate a limited set of
operations at compile time, which lets us choose to write out this value in a
way that’s easier to understand and verify, rather than setting this constant
to the value 10,800. See the Rust Reference’s section on constant
evaluation for more information on what operations can be used
when declaring constants.
Constants are valid for the entire time a program runs, within the scope they were declared in. This property makes constants useful for values in your application domain that multiple parts 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 you saw in the guessing game tutorial in Chapter
2, you can declare a
new variable with the same name as a previous variable. Rustaceans say that the
first variable is shadowed by the second, which means that the second
variable’s value is what the program sees when the variable is used. 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 in the inner scope is: {}", x); } 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. Then, within an inner scope, the third let statement also
shadows x, multiplying the previous value by 2 to give x a value of 12.
When that scope is over, the inner shadowing ends and x returns to being 6.
When we run this program, it will output the following:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
Running `target/debug/variables`
The value of x in the inner scope is: 12
The value of x is: 6
Shadowing is different from marking a variable as mut, because we’ll get a
compile-time error if we accidentally try to reassign to this variable without
using the let keyword. By using let, 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, and then we want to store that input as a number:
fn main() { let spaces = " "; let spaces = spaces.len(); }
The first spaces variable is a string type and the second spaces variable
is a number type. Shadowing thus spares us from having to come up with
different names, such as 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, we’ll get a compile-time error:
fn main() {
let mut spaces = " ";
spaces = spaces.len();
}
The error says we’re not allowed to mutate a variable’s type:
$ cargo run
Compiling variables v0.1.0 (file:///projects/variables)
error[E0308]: mismatched types
--> src/main.rs:3:14
|
2 | let mut spaces = " ";
| ----- expected due to this value
3 | spaces = spaces.len();
| ^^^^^^^^^^^^ expected `&str`, found `usize`
For more information about this error, try `rustc --explain E0308`.
error: could not compile `variables` due to previous error
Now that we’ve explored how variables work, let’s look at more data types they can have.