Use stackalloc in string.Split (#15435)

* Use stackalloc in string.Split

* Added initial usage of ValueListBuilder

* Added usage of list builder to string separator Split overloads

Author: Alex

src/mscorlib/System.Private.CoreLib.csproj

@@ -670,4 +670,4 @@
   </ItemGroup>
 
   <Import Project="GenerateCompilerResponseFile.targets" />
-</Project>
+</Project>

src/mscorlib/shared/System.Private.CoreLib.Shared.projitems

@@ -73,6 +73,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\KeyNotFoundException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\KeyValuePair.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\NonRandomizedStringEqualityComparer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\ValueListBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\List.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\HashHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ICollection.cs" />

src/mscorlib/shared/System/Collections/Generic/ValueListBuilder.cs

@@ -0,0 +1,67 @@
+// 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.Buffers;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace System.Collections.Generic
+{
+    internal ref struct ValueListBuilder<T>
+    {
+        private Span<T> _span;
+        private T[] _arrayFromPool;
+        private int _pos;
+
+        public ValueListBuilder(Span<T> initialSpan)
+        {
+            _span = initialSpan;
+            _arrayFromPool = null;
+            _pos = 0;
+        }
+
+        public int Length => _pos;
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Append(T item)
+        {
+            int pos = _pos;
+            if (pos >= _span.Length)
+                Grow();
+
+            _span[pos] = item;
+            _pos = pos + 1;
+        }
+
+        public ReadOnlySpan<T> AsReadOnlySpan()
+        {
+            return _span.Slice(0, _pos);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Dispose()
+        {
+            if (_arrayFromPool != null)
+            {
+                ArrayPool<T>.Shared.Return(_arrayFromPool);
+                _arrayFromPool = null;
+            }
+        }
+
+        private void Grow()
+        {
+            T[] array = ArrayPool<T>.Shared.Rent(_span.Length * 2);
+
+            bool success = _span.TryCopyTo(array);
+            Debug.Assert(success);
+
+            T[] toReturn = _arrayFromPool;
+            _span = _arrayFromPool = array;
+            if (toReturn != null)
+            {
+                ArrayPool<T>.Shared.Return(toReturn);
+            }
+        }
+    }
+}