[WIP] Don't Merge MemoryMarshal

src/mscorlib/System.Private.CoreLib.csproj

@@ -654,6 +654,7 @@
     <Compile Include="$(BclSourcesRoot)\mscorlib.Friends.cs" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="src\System\Runtime\InteropServices\MemoryMarshal.cs" />
     <Compile Include="src\System\Runtime\RuntimeImports.cs" />
   </ItemGroup>
   <Import Project="shared\System.Private.CoreLib.Shared.projitems" />
@@ -690,4 +691,4 @@
     <Win32Resource Condition="'$(GenerateNativeVersionInfo)'=='true'">$(IntermediateOutputPath)\System.Private.CoreLib.res</Win32Resource>
   </PropertyGroup>
   <Import Project="GenerateCompilerResponseFile.targets" />
-</Project>
+</Project>

src/mscorlib/shared/System/Memory.cs

@@ -90,6 +90,15 @@ internal Memory(OwnedMemory<T> owner, int index, int length)
             _length = length;
         }
 
+        // Constructor for internal use only
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal Memory(int start, int length, T[] array)
+        {
+            _arrayOrOwnedMemory = array;
+            _index = start;
+            _length = length;
+        }
+
         /// <summary>
         /// Defines an implicit conversion of an array to a <see cref="Memory{T}"/>
         /// </summary>

src/mscorlib/src/System/Runtime/InteropServices/MemoryMarshal.cs

@@ -0,0 +1,31 @@
+// 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.
+
+namespace src.System.Runtime.InteropServices
+{
+    using System;
+    using System.Runtime.CompilerServices;
+
+    public static class MemoryMarshal
+    {
+        /// <summary>
+        /// Creates a span over the range specified. However this does no bounds checking therefore is dangerous
+        /// all validation should be done prior to calling this method
+        /// </summary>
+        /// <param name="array">The target array.</param>
+        /// <param name="start">The index at which to begin the span.</param>
+        /// <param name="length">The number of items in the span.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Span<T> DangerousCreateSpan<T>(T[] array, int start, int length) => new Span<T>(ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), start), length);
+
+        /// <summary>
+        /// Creates a memory instance over the range. However this does no bounds checking therefore is dangerous
+        /// </summary>
+        /// <param name="array">The target array.</param>
+        /// <param name="start">The index at which to begin the span.</param>
+        /// <param name="length">The number of items in the span.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Memory<T> DangerousCreateMemory<T>(T[] array, int start, int length) => new Memory<T>(start, length, array);
+    }
+}