Adding placeholder Span debugger proxy (dotnet/coreclr#14749)

* Adding placeholder Span debugger proxy.

* Remove unnecessary unsafe keyword.

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

src/Common/src/CoreLib/System.Private.CoreLib.Shared.projitems

@@ -441,6 +441,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\SerializableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Single.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Span.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\SpanDebugView.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Span.NonGeneric.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\String.Searching.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\StringSpanHelpers.cs" />

src/Common/src/CoreLib/System/ReadOnlySpan.cs

@@ -15,6 +15,7 @@ namespace System
     /// ReadOnlySpan represents a contiguous region of arbitrary memory. Unlike arrays, it can point to either managed
     /// or native memory, or to memory allocated on the stack. It is type- and memory-safe.
     /// </summary>
+    [DebuggerTypeProxy(typeof(SpanDebugView<>))]
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     [NonVersionable]
     public readonly ref struct ReadOnlySpan<T>

src/Common/src/CoreLib/System/Span.cs

@@ -21,6 +21,7 @@ namespace System
     /// Span represents a contiguous region of arbitrary memory. Unlike arrays, it can point to either managed
     /// or native memory, or to memory allocated on the stack. It is type- and memory-safe.
     /// </summary>
+    [DebuggerTypeProxy(typeof(SpanDebugView<>))]
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     [NonVersionable]
     public readonly ref struct Span<T>

src/Common/src/CoreLib/System/SpanDebugView.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+
+namespace System
+{
+    internal sealed class SpanDebugView<T>
+    {
+        private readonly T[] _array;
+
+        public SpanDebugView(Span<T> span)
+        {
+           _array = span.ToArray();
+        }
+
+        public SpanDebugView(ReadOnlySpan<T> span)
+        {
+            _array = span.ToArray();
+        }
+
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public T[] Items => _array;
+    }
+}