But I want it to fail and I'm happy for it to fail then. If I write a utility that is about to read a multi gigabyte file in a json format I know has only ascii

But I want it to fail and I'm happy for it to fail then. If I write a utility that is about to read a multi gigabyte file in a json format I know has only ascii identifiers, into a dictionary, then it's silly that the dictionary has UTF-16 keys stored in an array on the heap, with each value stored on the heap as well, so second pointer away. Just because I'm using json or xml or something doesn't mean I'm parsing any json, or that I'm not happy for it to catch fire if I parse a file that has some non-ascii in it.