Rust by Example

Declare first

It's possible to declare variable bindings first, and initialize them later. However, this form is seldom used, as it may lead to the use of uninitialized variables.

fn main() {

// Declare a variable binding

let a_binding;

{

let x = 2;

// Initialize the binding

a_binding = x * x;

}

println!("a binding: {}", a_binding);

let another_binding;

// Error! Use of uninitialized binding

println!("another binding: {}", another_binding);

// FIXME ^ Comment out this line

another_binding = 1;

println!("another binding: {}", another_binding);

}

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The compiler forbids use of uninitialized variables, as this would lead to undefined behavior.