Fix Stream.ReadAtLeast perf regression in DataContractSerializer

dotnet#69272 changed DCS to no longer call Stream.Read inside a loop, but instead call the new ReadAtLeast method. ReadAtLeast only takes a Span<byte>, and not a byte[]. This caused a regression because the internal encoding stream wrapper classes don't override Read(Span<byte>), so the base implementation is used. The base implementation is slower because it needs to rent a byte[] from the pool, and do 2 copies.

Overriding Read(Span<byte>) on the internal encoding stream wrapper classes allows ReadAtLeast to be just as fast.

Fix dotnet#69730

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonEncodingStreamWrapper.cs

@@ -174,15 +174,18 @@ public override void Flush()
             _stream.Flush();
         }
 
-        public override int Read(byte[] buffer, int offset, int count)
+        public override int Read(byte[] buffer, int offset, int count) =>
+            Read(new Span<byte>(buffer, offset, count));
+
+        public override int Read(Span<byte> buffer)
         {
             try
             {
                 if (_byteCount == 0)
                 {
                     if (_encodingCode == SupportedEncoding.UTF8)
                     {
-                        return _stream.Read(buffer, offset, count);
+                        return _stream.Read(buffer);
                     }
 
                     Debug.Assert(_bytes != null);
@@ -206,11 +209,13 @@ public override int Read(byte[] buffer, int offset, int count)
                 }
 
                 // Give them bytes
+                int count = buffer.Length;
                 if (_byteCount < count)
                 {
                     count = _byteCount;
                 }
-                Buffer.BlockCopy(_bytes!, _byteOffset, buffer, offset, count);
+
+                _bytes.AsSpan(_byteOffset, count).CopyTo(buffer);
                 _byteOffset += count;
                 _byteCount -= count;
                 return count;

src/libraries/System.Private.DataContractSerialization/src/System/Xml/EncodingStreamWrapper.cs

@@ -593,14 +593,17 @@ public override int ReadByte()
             return _byteBuffer[0];
         }
 
-        public override int Read(byte[] buffer, int offset, int count)
+        public override int Read(byte[] buffer, int offset, int count) =>
+            Read(new Span<byte>(buffer, offset, count));
+
+        public override int Read(Span<byte> buffer)
         {
             try
             {
                 if (_byteCount == 0)
                 {
                     if (_encodingCode == SupportedEncoding.UTF8)
-                        return _stream.Read(buffer, offset, count);
+                        return _stream.Read(buffer);
 
                     Debug.Assert(_bytes != null);
                     Debug.Assert(_chars != null);
@@ -622,9 +625,11 @@ public override int Read(byte[] buffer, int offset, int count)
                 }
 
                 // Give them bytes
+                int count = buffer.Length;
                 if (_byteCount < count)
                     count = _byteCount;
-                Buffer.BlockCopy(_bytes!, _byteOffset, buffer, offset, count);
+
+                _bytes.AsSpan(_byteOffset, count).CopyTo(buffer);
                 _byteOffset += count;
                 _byteCount -= count;
                 return count;