Generic string or byte slice read in one allocation

Question

Is it possible for a generic Go function to read data as either a string or a byte slice, and have only one memory allocation either way?

The following function does 1 allocation for byte slices and 2 allocations for strings.

func ReadN[S string | []byte](r io.Reader, n int) (S, error) {
    buf := make([]byte, n)
    n, err := io.ReadFull(r, buf)
    return S(buf)[:n], err
}

No unsafe permitted in this case.