Declare first - Rust By Example
Description: A description

# 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.