Extension methods needed from C# 8 range and index features

Extensions.cs

static class Extensions
{
  public static int get_IndexerExtension(this int[] array, Index index) =>
    index.FromEnd ? array[array.Length - index.Value] : array[index.Value];

  public static int get_IndexerExtension(this Span<int> span, Index index) =>
    index.FromEnd ? span[span.Length - index.Value] : span[index.Value];

  public static char get_IndexerExtension(this string s, Index index) =>
    index.FromEnd ? s[s.Length - index.Value] : s[index.Value];

  public static Span<int> get_IndexerExtension(this int[] array, Range range) =>
    array.Slice(range);

  public static Span<int> get_IndexerExtension(this Span<int> span, Range range) =>
    span.Slice(range);

  public static string get_IndexerExtension(this string s, Range range) =>
    s.Substring(range);

  public static Span<T> Slice<T>(this T[] array, Range range) =>
    => array.AsSpan().Slice(range);

  public static Span<T> Slice<T>(this Span<T> span, Range range)
  {
    var (start, length) = GetStartAndLength(range, span.Length);
    return span.Slice(start, length);
   }

  public static string Substring(this string s, Range range)
  {
    var (start, length) = GetStartAndLength(range, s.Length);
    return s.Substring(start, length);
  }

  private static (int start, int length) GetStartAndLength(Range range, int entityLength)
  {
    var start = range.Start.FromEnd ? entityLength - range.Start.Value : range.Start.Value;
    var end = range.End.FromEnd ? entityLength - range.End.Value : range.End.Value;
    var length = end - start;

    return (start, length);
  }
}