This is a great way to create APIs that are hard or nearly impossible to implement properly or efficiently. I can imagine how this could easily lead to horrible

This is a great way to create APIs that are hard or nearly impossible to implement properly or efficiently. I can imagine how this could easily lead to horrible API design like a well-known stdlib `gets` function. From the user perspective, gets is a perfectly fine function. Unfortunately there is no way you could write a safe implementation, because in C you have no way of checking arbitrary buffer size. Low level implementation details do affect how we design APIs.