Understanding rust borrowing and dereferencing

Question

I was reading through the Rust documentation and can't quite seem to be able to wrap my head around what is going on. For example, over here I see the following example:

// 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, and a stacked i32
    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` and be destroyed
    eat_box_i32(boxed_i32);
}

Things I believe:

1. eat_box_i32 takes a pointer to a Box
2. this line let boxed_i32 = Box::new(5_i32); makes is so that boxed_i32 now contains a pointer because Box is not a primitive

Things I don't understand:

1. why do we need to call borrow_i32(&boxed_i32); with the ampersand? Isn't boxed_i32 already a pointer?
2. on this line: let _ref_to_i32: &i32 = &boxed_i32; why is the ampersand required on the right hand side? Isn't boxed_i32 already an address?
3. how come borrow_i32 can be called with pointer to Box and pointer to i32 ?

Answer 1

Just to supplement @math4tots, the auto dereferencing is call Deref Coercion. It is explained in the rustbook here: https://doc.rust-lang.org/book/ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods