> While replacing the array of zeros by an array of uninitialised values may work in specific circumstances, the code is unsound. Change to the compiler, its options, modification of unrelated parts of the code or using the function for a different Read trait implementation may break the program in unpredictable ways.

Why? It seems the only thing on that list that will cause UB is using the function with a different reader (one that inspects the uninitialized bytes). Why would any of the other listed possible changes break it?