Allocation free int to string (incomplete)

FormatExtensions.cs

public static class FormatExtensions
{
    public static string Int32ToString(int a)
    {
        const int radix = 10;
        const int maxLength = 11;
        Span<char> str = stackalloc char[maxLength];
        int i = str.Length;

        bool isNegative = a < 0;
        if (a <= 0) // handles 0 and int.MinValue special cases
        {
            a = -Math.DivRem(a, radix, out int remainder);
            str[--i] = (char)(-remainder + '0');
        }

        while (a != 0)
        {
            a = Math.DivRem(a, radix, out int remainder);
            str[--i] = (char)(remainder + '0');
        }

        if (isNegative)
        {
            str[--i] = '-';
        }

        return str.Slice(i, str.Length - i).ToString();
    }
}