借用

大多数情况下，我们希望在不获取所有权的情况下访问数据。为了实现这一点，Rust 使用了一种借用机制。对象可以通过引用传递 (&T)，而不是按值传递 (T)。

编译器静态地保证（通过其借用检查器）引用始终指向有效的对象。也就是说，只要对象的引用存在，该对象就不能被销毁。

// This function takes ownership of a box and destroys it
fn eat_box_i32(boxed_i32: Box<i32>) {
    println!("Destroying box that contains {}", boxed_i32);
}

// This function borrows an i32
fn borrow_i32(borrowed_i32: &i32) {
    println!("This int is: {}", borrowed_i32);
}

fn main() {
    // Create a boxed i32 in the heap, and a i32 on the stack
    // Remember: numbers can have arbitrary underscores added for readability
    // 5_i32 is the same as 5i32
    let boxed_i32 = Box::new(5_i32);
    let stacked_i32 = 6_i32;

    // Borrow the contents of the box. Ownership is not taken,
    // so the contents can be borrowed again.
    borrow_i32(&boxed_i32);
    borrow_i32(&stacked_i32);

    {
        // Take a reference to the data contained inside the box
        let _ref_to_i32: &i32 = &boxed_i32;

        // Error!
        // Can't destroy `boxed_i32` while the inner value is borrowed later in scope.
        eat_box_i32(boxed_i32);
        // FIXME ^ Comment out this line

        // Attempt to borrow `_ref_to_i32` after inner value is destroyed
        borrow_i32(_ref_to_i32);
        // `_ref_to_i32` goes out of scope and is no longer borrowed.
    }

    // `boxed_i32` can now give up ownership to `eat_box_32` and be destroyed
    eat_box_i32(boxed_i32);
}