Declare a variable in Rust and mutability

This is a common programming concepts. That might be not unique to rust. Let’s see how to declare a variable in Rust.

Immutable variable

By default, a variable in Rust is immutable. Meaning this variable is a constant or will not change.

It’s intentionally made immutable by default for safety and easy concurrency.

let x = 5;

or

let y = "hallo!";

If you try to change one of the variable, it will return

cannot assign twice to immutable variable `x`

Mutable variable

To make a mutable variable, use mut keyword

let mut x = 5;

Now you can assign a new variable

let mut x = 5;
x = 10;

Const variable

What about const?
It also available in Rust, using const keyword for a constant value.

const MAX_POINTS: u32 = 100_000;

It might be similar to immutable let. What’s the difference then?

• const can be declared in the global scope.
• It represents a constant expression, not a result of something
• The naming convention is UPPER_CASE