If you use the type system well you rarely get wiring issues you would normally get at runtime (wasting an entire "programming cycle" of writing code and trying

If you use the type system well you rarely get wiring issues you would normally get at runtime (wasting an entire "programming cycle" of writing code and trying it out). But overall I agree: you need tests for business logic. Another thing that Rust gets right is that you're incentivised to write some tests right in the same file where you're writing the implementation (it just works out of the box with cargo test), so even complex logic is easy to test out. You don't have to go through the "oh, let me setup mocha to add a test... never mind I won't bother right now"