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.