如何反转字符串中每个字母的大小写？

I know about ToUpper and ToLower from strings package but obviously they won't help here. Is there a built-in function or do I have to write one myself?

You need to write one yourself, but the building blocks are already in the standard library:

func swapCase(s string) string {
    return strings.Map(func(r rune) rune {
        switch {
        case unicode.IsLower(r):
            return unicode.ToUpper(r)
        case unicode.IsUpper(r):
            return unicode.ToLower(r)
        }
        return r
    }, s)
}