The default option is dangerous and vulnerable being incorrect unless you explicitly use it in a different way. This is a symptom of it being a library and not a language feature.
As another example, imagine if span was a language feature. A compiler could bounds check at compile time, and fully elide the checks at runtime in many scenarios (like in a loop over the span).
As another example, imagine if span was a language feature. A compiler could bounds check at compile time, and fully elide the checks at runtime in many scenarios (like in a loop over the span).