Strange behavior of borrow checker

I tried this code:

fn test() {
    let a_mut = &mut ();
    let mut a = &*a_mut;
    let temp = a;
    a = &();
    do_smth(temp);
    *a_mut = ();
    do_smth(a);
}
fn do_smth(_: &()) {}

And this:

fn test() {
    let a_mut = &mut ();
    let mut a = &*a_mut;
    let temp = a;
    do_smth(temp);
    a = &();
    *a_mut = ();
    do_smth(a);
}
fn do_smth(_: &()) {}

I expect both options to work. But only the second option works. It’s strange that if you swap the lines “do_smth(temp)” and “a = &()”, then everything starts to work. In both variants, the essence of the code is preserved: after "*a_mut = ()" the variable "temp" is no longer used, and the variable "a" is not associated in lifetime with the variable "a_mut", so borrow checker can ignore them. But the results of the borrow checker are different in these two cases. I wonder why?

Start by compiling the two test examples and comparing the borrow-checker diagnostics when do_smth(temp) and a = &() are reordered. Trace the compiler's borrow-checking treatment of a_mut, a, and temp; done means the differing behavior is explained and, if incorrect, covered by a regression test.