Invalid UTF-8 input is considered undefined behavior. That said, we’ve worked to ensure that such inputs will not cause pathological outcomes, such as a panic or infinite loop. Callers should expect “garbage-in, garbage-out”.

There are two basic tests in each package, called TestInvalidUTF8 and TestRandomBytes. Those tests pass, returning the invalid bytes verbatim, without a guarantee as to how they will be segmented.