alt Hacker News> debug_assert!() (only run in debug builds)
debug_assert!() (and it's equivalent in other languages, like C's assert with NDEBUG) is cursed. It states that you believe something to be true, but will take no automatic action if it is false; so you must implement the fallback behavior if your assumption is false manually (even if that fallback is just fallthrough). But you can't /test/ that fallback behavior in debug builds, which means you now need to run your test suite(s) in both debug and release build versions. While this is arguably a good habit anyway (although not as good a habit as just not having separate debug and release builds), deliberately diverging behavior between the two, and having tests that only work on one or the other, is pretty awful.
I hear you, but sometimes this is what I want.
For example, I’m pretty sure some complex invariant holds. Checking it is expensive, and I don’t want to actually check the invariant every time this function runs in the final build. However, if that invariant were false, I’d certainly like to know that when I run my unit tests.
Using debug_assert is a way to do this. It also communicates to anyone reading the code what the invariants are.
If all I had was assert(), there’s a bunch of assertions I’d leave out of my code because they’re too expensive. debug_assert lets me put them in without paying the cost.
And yes, you should run unit tests in release mode too.