可变性

当所有权转移时，数据可变性可能会改变。

fn main() {
    let immutable_box = Box::new(5u32);

    println!("immutable_box contains {}", immutable_box);

    // Mutability error
    //*immutable_box = 4;

    // *Move* the box, changing the ownership (and mutability)
    let mut mutable_box = immutable_box;

    println!("mutable_box contains {}", mutable_box);

    // Modify the contents of the box
    *mutable_box = 4;

    println!("mutable_box now contains {}", mutable_box);
}