src/variable_bindings/declare.md

# 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.

```rust,editable,ignore,mdbook-runnable
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.