Optimise HtmlEncode and HtmlDecode to produce less garbage

src/Common/src/CoreLib/System/Text/ValueStringBuilder.cs

@@ -22,6 +22,13 @@ public ValueStringBuilder(Span<char> initialBuffer)
             _pos = 0;
         }
 
+        public ValueStringBuilder(int initialCapacity)
+        {
+            _arrayToReturnToPool = ArrayPool<char>.Shared.Rent(initialCapacity);
+            _chars = _arrayToReturnToPool;
+            _pos = 0;
+        }
+
         public int Length
         {
             get => _pos;

src/Common/tests/Tests/System/Text/ValueStringBuilderTests.cs

@@ -30,6 +30,17 @@ public void Ctor_Span_CanAppend()
             Assert.Equal("a", vsb.ToString());
         }
 
+        [Fact]
+        public void Ctor_InitialCapacity_CanAppend()
+        {
+            var vsb = new ValueStringBuilder(1);
+            Assert.Equal(0, vsb.Length);
+
+            vsb.Append('a');
+            Assert.Equal(1, vsb.Length);
+            Assert.Equal("a", vsb.ToString());
+        }
+
         [Fact]
         public void Append_Char_MatchesStringBuilder()
         {
@@ -152,6 +163,26 @@ public void Insert_IntCharInt_MatchesStringBuilder()
             Assert.Equal(sb.ToString(), vsb.ToString());
         }
 
+        [Fact]
+        public void AsSpan_ReturnsCorrectValue_DoesntClearBuilder()
+        {
+            var sb = new StringBuilder();
+            var vsb = new ValueStringBuilder();
+
+            for (int i = 1; i <= 100; i++)
+            {
+                string s = i.ToString();
+                sb.Append(s);
+                vsb.Append(s);
+            }
+
+            var resultString = new string(vsb.AsSpan());
+            Assert.Equal(sb.ToString(), resultString);
+
+            Assert.NotEqual(0, sb.Length);
+            Assert.Equal(sb.Length, vsb.Length);
+        }
+
         [Fact]
         public void ToString_ClearsBuilder_ThenReusable()
         {
@@ -213,6 +244,27 @@ public void TryCopyTo_ClearsBuilder_ThenReusable()
             Assert.Equal(Text2, vsb.ToString());
         }
 
+        [Fact]
+        public void Dispose_ClearsBuilder_ThenReusable()
+        {
+            const string Text1 = "test";
+            var vsb = new ValueStringBuilder();
+
+            vsb.Append(Text1);
+            Assert.Equal(Text1.Length, vsb.Length);
+
+            vsb.Dispose();
+
+            Assert.Equal(0, vsb.Length);
+            Assert.Equal(string.Empty, vsb.ToString());
+            Assert.True(vsb.TryCopyTo(Span<char>.Empty, out _));
+
+            const string Text2 = "another test";
+            vsb.Append(Text2);
+            Assert.Equal(Text2.Length, vsb.Length);
+            Assert.Equal(Text2, vsb.ToString());
+        }
+
         [Fact]
         public unsafe void Indexer()
         {

src/System.Runtime.Extensions/src/System.Runtime.Extensions.csproj

@@ -67,9 +67,6 @@
     <Compile Include="$(CommonPath)\CoreLib\System\Threading\Tasks\TaskToApm.cs">
       <Link>Common\CoreLib\System\Threading\Tasks\TaskToApm.cs</Link>
     </Compile>
-    <Compile Include="$(CommonPath)\CoreLib\System\Text\StringBuilderCache.cs">
-      <Link>Common\System\Text\StringBuilderCache.cs</Link>
-    </Compile>
     <Compile Include="$(CommonPath)\System\HResults.cs">
       <Link>Common\System\HResults.cs</Link>
     </Compile>