Rust by Example

4.3 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);
}

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