前方宣言

It is possible to declare variable bindings first and initialize them later, but all variable bindings must be initialized before they are used: the compiler forbids use of uninitialized variable bindings, as it would lead to undefined behavior.

It is not common to declare a variable binding and initialize it later in the function. It is more difficult for a reader to find the initialization when initialization is separated from declaration. It is common to declare and initialize a variable binding near where the variable will be used.

fn main() {
    // 変数を宣言。
    let a_binding;

    {
        let x = 2;

        // 変数を初期化。
        a_binding = x * x;
    }

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

    let another_binding;

    // エラー！初期化していない変数の使用
    println!("another binding: {}", another_binding);
    // FIXME ^ この行をコメントアウトしましょう

    another_binding = 1;

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