Variables

Mutability

Variables in Rust are immutable by default. To modify them, the mut keyword must be used.

let x = 5;       // Immutable
let mut y = 10;  // Mutable
y += 5;          // OK

Constants

They are immutable and must be initialized at compile time. They are declared with const.

const PI: f64 = 3.141592653589793;
const MAX: i32 = 100;

Note

Unlike let, constants cannot omit the type and their value must be a compile-time constant (it cannot depend on a runtime value).

Shadowing

Allows declaring a new variable with the same name as an existing one, hiding the original variable. Unlike mut, shadowing creates a completely new variable.

let x = 5;        // First variable x
let x = x + 1;    // New variable x (hides the previous one)
let x = x * 2;    // Another new variable x

Tip

Shadowing is useful when you need to transform a value and change its type, something mut does not allow:

let spaces = "   ";          // &str
let spaces = spaces.len();   // usize (same name, different type!)

Next: Data Types