FormatarCpfSpan.cs

public ReadOnlySpan<char> FormatarCpf(string cpf)
{
    Span<char> formattedValue = stackalloc char[14];

    Copy(cpf, startIndex: 0, length: 3, ref formattedValue, insertIndex: 0);
    formattedValue[3] = '.';

    Copy(cpf, startIndex: 3, length: 3, ref formattedValue, insertIndex: 4);
    formattedValue[7] = '.';

    Copy(cpf, startIndex: 6, length: 3, ref formattedValue, insertIndex: 8);
    formattedValue[11] = '-';

    Copy(cpf, startIndex: 9, length: 2, ref formattedValue, insertIndex: 12);

    return formattedValue.ToArray();

    static void Copy(string origin, int startIndex, int length, ref Span<char> destination, int insertIndex)
    {
        for (int i = startIndex, j = insertIndex; i < (startIndex + length); i++, j++)
            destination[j] = origin[i];
    }
}