I have a struct with a function that returns a String from a json using serde_json::to_string_pretty(). I am trying to write a unit test to verify the output of the string.

 #[test]
    fn test() {
        ...
        let j =
            json!({
            "mock_key": "val",
            "another_key": "another_val",
        });
        assert_eq!(serde_json::to_string_pretty(&j).unwrap(), json_string);

When I run the test, it passed for the first time. And if I try to run the test for the second time , the test fails because:

Diff < left / right > :
 {
<  "mock_key": "val",
<  "another_key": "another_val"
>  "another_key": "another_val",
>  "mock_key": "val"
 }

If I try to run it again, the test passes, and the cycle continues.
Why is that? And is there a better way to do it?

I tried changing the order in let j = json!({...}).
The test should pass (or fail) each time I run it with no changes made to the code.