"If it compiles, and the tests pass, it works" is more accurate; if your tests are pretty thorough, it's very unlikely that you've got any major logical errors, and Rust rules out a massive class of lifetime, double-use, mutability etc. errors just by being Rust.

I've translated pure-logic business code (with fairly thorough, albeit static, tests that all passed) from Python to Rust, and the compiler's complained about subtle errors that would have taken down a business using the code in days had they been put into production. Turns out there was a flaw in the spec.

Leveraging Rust's type system to logically separate the meanings of different integers, and only permitting arithmetic operations where meaningful (e.g. no distance+time, distance/time=speed), also uncovered a couple of even more subtle bugs – though luckily assertions would've caught these in the event of them actually making it through to production.

If it compiles, it probably works. But, more importantly, Rust forces you to write code that works in the first place. You have to think through the ramifications of what you're doing, sometimes to hold the entire function in your head at once, and that means you have to understand what you're doing. If you don't, it complains, and the chances are you won't be able to get it to compile until you do understand it.

And if it does compile when it's wrong, it's obviously wrong and your tests will fail. Most of the time.

Isn't programming fun?

As I said, if it compiles it means that there should be no bugs other than logic bugs. No null pointer exceptions because there is no `null` in Rust, for instance. No passing a wrong pointer type because doing something like `void *` in Rust only works with unsafe code. Same with mutable pointers, which only work with unsafe code. No concurrency bugs due to shared memory because passing data between threads is not possible without specialized data types.

Basically, if you've programmed in unsafe languages such as C, C++, Assembly or any language that has `null`, you know what Rust brings to the table. The Rust compiler does prevent very large classes of bugs all the while being non garbage collected. That's quite a feat!

And I'm far from being a Rust fanboy. I work with PHP, Javascript and devops tools in my day job. I'm not doing PR here. Just sharing my experience. Feel free to share yours.

In my experience, though, the statement is largely true for refactorings. Even in a language like Java that doesn't have a very expressive type system, if I need to change the structure of my classes, rename some stuff, etc., just knowing that it compiles tells you that you probably didn't break anything.

Of course, once you add / change logic, I agree with your assessment that the statement is hopelessly naïve in most practical use cases.