crater

If you're talking about Rocket that seems to be the largest thing that requires nightly. That said I can count on my hands the number of times I've used nightly over the last 2-ish years. It's pretty rare that you need to do it. I'll also add that I've actually been really impressed with how much attention Rust takes to not break stable packages. Between crater[1] and how aggressively they've cut point releases to fix any issues that (rarely) happen to slip by. [1] https://github.com/rust-lang-nursery/crater

Rust is ambivalent about it. You can put tests in external files and force yourself to test only the public interface if you want. But you can also throw a `#[test]` function alongside the implementation. It's sometimes super convenient to test a leaf helper function directly instead of mocking a whole program around it. And because the test lives next to the implementation, it's easy to change or delete it when the impl changes. There's another nice side-effect of having one standard test framework - `cargo test` can test any Rust program. The Rust team can automatically test new compiler releases against all known Rust code: https://github.com/rust-lang-nursery/crater