A new test about *Arc::make_mut() #974
Description
When I was implementing Arc, I found that the testing for *Arc:: make_cut() was not very comprehensive. The documentation only explained that it need to clone an inner value, but did not mention how to update the internal count. I made a mistake during my first implementation. Later, I compared with the standard library and updated the following tests. I believe this will make the test samples in this section more comprehensive.
#[test]
fn test_try_make_mut_count() {
let mut data1 = Arc::new(5);
let mut data2 = Arc::clone(&data1); // Won't clone inner data
let mut data3 = Arc::clone(&data1);
assert_eq!(Arc::count(&data1), 3);
assert_eq!(Arc::count(&data2), 3);
assert_eq!(Arc::count(&data3), 3);
*Arc::make_mut(&mut data1) += 1;
assert_eq!(Arc::count(&data1), 1);
assert_eq!(Arc::count(&data2), 2);
assert_eq!(Arc::count(&data3), 2);
*Arc::make_mut(&mut data2) *= 2; // clone
assert_eq!(Arc::count(&data1), 1);
assert_eq!(Arc::count(&data2), 1);
assert_eq!(Arc::count(&data3), 1);
*Arc::make_mut(&mut data3) += 1; // clone
assert_eq!(Arc::count(&data1), 1);
assert_eq!(Arc::count(&data2), 1);
assert_eq!(Arc::count(&data3), 1);
}