> You can cast between byte arrays and strings in Go.

Yes, you can. But, in the specific case you mentioned, no competent programmer would cast the bytes of an invalidly-encoded file to a string, then iterate through the runes of the string. That wouldn't even begin to make sense!

I really don't understand what you're trying to argue here.

Although it only works for smallish files, that seems fairly useful for getting as much info as you can out of a corrupt but mostly UTF-8 file?

Any runes that aren't valid will come back as the replacement character. And you can count newlines and print the location of the error(s). You also have the index of the error.