Technically, code that contains UB is just UB, and unsound code is code that allows you to trigger UB using safe code only. For example, this is UB: unsafe

Technically, code that contains UB is just UB, and unsound code is code that allows you to trigger UB using safe code only. For example, this is UB: unsafe { std::hint::unreachable_unchecked() }; And this is an unsound code, that may not contain any UB if I call `unsound()` with `false`: fn unsound(trigger: bool) { if trigger { unsafe { std::hint::unreachable_unchecked() }; } }