Failed to get address in while loop

help
1

Why can't I get address in the while loop?

struct Foo {
    elem1: u32,
    elem2: String,
}
fn demo1(x: &mut Foo) {
    let ref_1 = &mut x.elem1;
    let ref_2 = &mut x.elem2;
    let ptr_x = x as *mut Foo; // Ok!
}
fn demo2(x: &mut Foo) {
    let ref_1 = &mut x.elem1;
    while *ref_1 < 20 {
        let ptr_x1 = x as *mut Foo; // Failed: cannot borrow *x as mutable more than once at a time
        // let ptr_x2 = std::ptr::addr_of_mut!(x); // Failed: cannot borrow x as mutable more than once at a time
        // let ptr_x3 = std::ptr::addr_of_mut!(*x); // Failed: cannot borrow *x as mutable more than once at a time
    }
}
2

the loop is simply

struct Foo {
    elem1: u32,
    elem2: String,
}
fn demo1(x: &mut Foo) {
    let ref_1 = &mut x.elem1;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
    // .. forever
}

if you do only this you will see that it does not work

struct Foo {
    elem1: u32,
    elem2: String,
}
fn demo1(x: &mut Foo) {
    let ref_1 = &mut x.elem1;
    if *ref_1 < 20 { return }
    let ptr_x = x as *mut Foo;
    if *ref_1 < 20 { return }
}

ref_1 mutably borrows x, so x is unusable until ref_1 expires.
ref_1 expires only when the loop exists, so you can't use x until then