Rust Variables

Rust Variables

In Rust, a variable is a value that is paired with a name. Variables can be either mutable or immutable, and are immutable by default.

Rust uses the let keyword to declare a variable:

let my_variable = 42;

Let’s break this down:

let is used to declare the variable
my_variable is the name of the variable
42 is the value being assigned to my_variable

Variable Types in Rust

Rust will automatically assign a data type to a variable if one is not specified. Take the previous example:

let my_variable = 42;

Since we set my_variable equal to 42, Rust will use the default integer type, which is i32 – a signed 32 bit integer.

Rust allows us to specify the variable type:

let my_variable: u8 = 42;

In this case, we have assigned a data type of u8 – an unsigned 8 bit integer.

Rust Variable Mutability

Variables are immutable by default but can be made mutable by including the keyword mut in the variable declaration:

let mut my_variable = 42;

This will allow us to change my_variable after an initial value has been assigned:

fn main() {
    let mut my_variable = 21;
    println!("The value of my_variable is: {}", my_variable);
    my_variable = 42;
    println!("The value of my_variable is: {}", my_variable);


Mutable variables in Rust

Declaring Multiple Variables

We can create multiple variables with a single line of code:

let (name,experience) = ("Bill","beginner"); 
println!("My name is {} and I am a {} in Rust.", name, experience); 

Variables vs. Constants in Rust

There are two kinds of name-binding declarations in Rust: variable declarations and constant declarations. Constants are also commonly referred to as constant variables.

There are several differences between variables and constants:

  1. Variables can be either mutable or immutable (constants are always immutable).
  2. Variables are declared using let, while constants are declared using const.
  3. Variables do not require the data type to be specified (constants require data type annotation).
  4. Variables are constrained to live in a block (between curly braces {}); constants can be global in scope.
  5. Variables should use snake_case while constants should use SCREAMING_SNAKE_CASE.
Declaration KeywordSyntaxType
Variablesletsnake_caseNot required

Naming Convention for Variables in Rust

Carbon uses snake_case for variables. This means that all letters are lowercase, and words are separated using an underscore. The snake_case style should be used for all variables, whether mutable or immutable.

In contrast, constants should always use SCREAMING_SNAKE_CASE. This helps to differentiate variables from constants.

Variable Errors in Rust

Rust has a lot of features to help us improve our code, and many of these features are built-in using errors.

When it comes to variables, Rust will not compile code that contains unnecessary variables or variables that are unnecessarily made mutable. As a result, Rust will throw errors for many variable-related issues including:

  • Variables declared but not used
  • Mutable variables that are never changed (variables that don’t need to be mutable)
  • Immutable variables that are changed (variables that should be mutable)
  • Mutable variables that are assigned an unused value (if a mutable value is assigned a value that is unused but later changed)

These errors can help us to streamline our code base and reduce unneeded bloat that can accumulate in larger projects.