Change the semantics of Span ToString to return the contents for T=char

[ahsonkhan]

Fixes https://github.com/dotnet/corefx/issues/26584

Related PR: dotnet/coreclr#16143

This PR supersedes #26663

It is blocked:
After dotnet/coreclr#16087 is merged and CoreFX is updated to use the new coreclr version, this PR can be merged.

cc @pakrym, @jkotas, @stephentoub, @dotnet/corefxlab-contrib

[ahsonkhan]

I don't understand why I am seeing these compiler errors for netfx when I do:

Assert.Equal("System.Span<int>[3]", span.ToString());

Span\ToString.cs(17,49): error CS0029: Cannot implicitly convert type 'System.Span<int>' to 'System.ValueType'

Where is the implicit conversion happening?

And this error only occurs for Span (not ReadOnlySpan).

[jkotas]

Where is the implicit conversion happening?

There is implicit conversion because of System.Memory reference assembly is missing the ToString overload.

[ahsonkhan]

There is implicit conversion because of System.Memory reference assembly is missing the ToString overload.

Yes! For both Span and Memory.

Also, ReadOnlyMemory in S.Runtime ref is also missing it: https://github.com/dotnet/corefx/blob/master/src/System.Runtime/ref/System.Runtime.cs#L1933

[pakrym]

Visual Studio treats strings and ToString differently when returning non printable characters - it escapes them in strings but not ToString results, for example:

Does the same thing happen to spans in your change?

[khellang]

You can force the debugger to show the raw output, using [DebuggerDisplay]:

[DebuggerDisplay("{ToString(),raw}")]
public class c
{
    public override string ToString()
    {
        return "\r\b\0";
    }
}

Ouput:

[ahsonkhan]

You can force the debugger to show the raw output

I will make this change outside this PR to avoid mirroring issues (along with removing the redundant DebuggerDisplay property -

corefx/src/System.Memory/src/System/Span.Portable.cs

Line 110 in 4f36d83

private string DebuggerDisplay => string.Format("System.Span<{0}>[{1}]", typeof(T).Name, _length);

).

[ahsonkhan]

@dotnet-bot test this please

[ahsonkhan]

@dotnet-bot test UWP CoreCLR x64 Debug Build

[jkotas]

UWP failure is https://github.com/dotnet/corefx/issues/26802 (CI have not picked up the new pipeline definition for existing PRs)