Complete debugger story for ReadOnlySequence

[pakrym]

We need to add implementation for ReadOnlySequence<char> same as in Span<char> and Memory<char>.

We also need a better way to display positions - with masks removed, possibly segments visualized.

/cc @ahsonkhan @davidfowl @KrzysztofCwalina

[benaadams]

You've got two separate issues here; want to move

We need to add implementation for ReadOnlySequence same as in Span and Memory.

To a different issue as it will currently break for char; however have fixes; and handling for string additionally in dotnet/corefx#27455

[benaadams]

Code for

// IMemoryList<Byte>[90] 10 .. 100
// OwnedMemory<Byte>[100] 0 .. 100
// Char[8] 10 .. 18
// "Hello"

[DebuggerDisplay("{DebuggerDisplay,nq}")]
public readonly partial struct ReadOnlySequence<T>
{
    private string DebuggerDisplay
    {
        // Output:
        // IMemoryList<Byte>[90] 10 .. 100
        // OwnedMemory<Byte>[100] 0 .. 100
        // Char[8] 10 .. 18
        get
        {
            if (_sequenceStart.Segment == null)
            {
                return "(default)";
            }

            GetTypeAndIndices(_sequenceStart.Index, _sequenceEnd.Index, out SequenceType type, out var startIndex, out var endIndex);
            if (type == SequenceType.String)
            {
                return "\"" + ((string)_sequenceStart.Segment).Substring(startIndex, endIndex - startIndex) + "\"";
            }
            string typeName;
            if (type == SequenceType.Array)
            {
                typeName = $"{typeof(T).Name}[{Length}]";
            }
            else
            {
                typeName = $"{type}<{typeof(T).Name}>[{Length}]";
            }
            string range;
            if (type == SequenceType.IMemoryList)
            {

                var start = (IMemoryList<T>)_sequenceStart.Segment;
                var end = (IMemoryList<T>)_sequenceEnd.Segment;
                range = $"{start.RunningIndex + startIndex} .. {end.RunningIndex + endIndex}";
            }
            else
            {
                range = $"{startIndex} .. {endIndex}";
            }

            return $"{typeName} {range}";
        }
    }
}

[muratg]

@joshfree removing @pakrym until we are done with preview2.

[joshfree]

related dotnet/corefx#27338

[joshfree]

@pakrym can this issue be closed now?

[karelz]

@pakrym was it fully addressed in dotnet/corefx#28857 or were there additional changes for this? (double checking that all necessary parts made it into 2.1)