What your tests will miss is the invalid data that was constructed in one corner of the codebase // apply special discount newPaymentInfo = {value: old

What your tests will miss is the invalid data that was constructed in one corner of the codebase // apply special discount newPaymentInfo = {value: oldPaymentInfo.value / 2} newPaymentInfo[tax] = applyRegionalTax(newPaymentInfo.value) return newPaymentInfo which only gets parsed by another corner of the codebase at runtime. // apply tax to tips only in some regions if (taxableTips) { paymentInfo.tip += applyRegionalTax(paymentInfo.value) // ERROR: tip is undefined (instead of zero) } You can't validate everything all the time, and if you try, it's easy for that validation to fall out of sync with the actual demands of the underlying logic. Errors like this crop up easily while refactoring. That's why one of the touted benefits of Rust's type system is "fearless refactoring."