Rust Variables and Data Types

Second post in the basic Rust series. Rust puts unusual care into how you declare and use variables — the rules here power its safety guarantees.

Variables are immutable by default#

let name = "Ada";
name = "Grace";   // error: cannot assign twice to immutable variable

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This is deliberate: immutable data is easy to reason about and safe to share. Make a variable mutable with mut:

let mut score = 10;
score += 5;

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Use mut only when you actually need it.

Rust infers types, but you can annotate them:

let count: u32 = 42;
let ratio: f64 = 3.14;
let name: &str = "Rust";
let done: bool = true;

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Default integer type is i32; default float is f64:

let x = 5;       // i32
let y = 2.5;     // f64

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Tuples group values of any types:

let point = (3, 5);
let (x, y) = point;   // destructuring
println!("{}", point.0);   // 3

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Arrays hold several values of the same type with fixed length:

let primes = [2, 3, 5, 7, 11];
println!("{}", primes[0]);   // 2

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const values are always immutable and must have a known type:

const MAX_SPEED: u32 = 120;

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Unlike let, const can be declared at any scope and is inlined at compile time.

You can reuse a variable name to transform its value, keeping the old value until the new one takes effect:

let x = 5;
let x = x + 1;

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Shadowing lets you change type too:

let label = "12";      // &str
let label = label.len();   // usize

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Rust’s variable rules — immutability by default, explicit mut, shadowing — make data flow predictable. Next in this series: control flow.