[kelnos]

Java's type system is pretty limited when compared to Rust's, and Rust as a language is designed to eliminate certain classes of bugs, some of which can still happen in a successfully-compiled Java program. Certainly javac can help you prove correctness in some cases, but rustc can prove that in many more. (Also consider that, during the time period you're referring to, Java's type system was even more limited than it is now.)

Put another way, I feel more confident about my Rust code being correct after it finishes compiling than I do about my Java code when it finishes compiling. And I also feel more confident about my Java code being correct after it finishes compiling than I do about my Python code after... I save the file.

The compiler isn't going to be able to tell you that your fibonacci function actually outputs a sequence of fibonacci numbers. You still have to write tests to prove that just as much in Rust as you do in Java as you do in Python. The fact that Java/JUnit really pushed unit testing hard for the first time is likely just due to the sheer number of Java developers in the enterprise world at the time, and that Java was the big up-and-coming language ecosystem with a lot of money and support and mindshare behind it.