Improve performance of async deserialization

src/Nerdbank.MessagePack/DecodeResultExtensions.cs

@@ -0,0 +1,35 @@
+// Copyright (c) Andrew Arnott. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Diagnostics;
+using static Nerdbank.MessagePack.MessagePackPrimitives;
+
+namespace Nerdbank.MessagePack;
+
+/// <summary>
+/// Extension methods for the <see cref="DecodeResult"/> enum.
+/// </summary>
+public static class DecodeResultExtensions
+{
+	/// <summary>
+	/// Processes a given <see cref="DecodeResult"/> value and returns a boolean indicating whether more bytes are needed.
+	/// </summary>
+	/// <param name="result">The <see cref="DecodeResult"/> value.</param>
+	/// <returns>
+	/// <see langword="true"/> if the value is <see cref="DecodeResult.InsufficientBuffer"/>;
+	/// <see langword="false"/> if the value is <see cref="DecodeResult.Success"/>.
+	/// </returns>
+	/// <exception cref="EndOfStreamException">Thrown if the value is <see cref="DecodeResult.EmptyBuffer"/>.</exception>
+	/// <exception cref="MessagePackSerializationException">Thrown if the value is <see cref="DecodeResult.TokenMismatch"/>.</exception>
+	public static bool NeedsMoreBytes(this DecodeResult result)
+	{
+		return result switch
+		{
+			DecodeResult.Success => false,
+			DecodeResult.InsufficientBuffer => true,
+			DecodeResult.EmptyBuffer => throw new EndOfStreamException(),
+			DecodeResult.TokenMismatch => throw new MessagePackSerializationException("Unexpected token encountered."), // TODO: Include the unexpected token into the exception message by packing MessagePackType into the DecodeResult.
+			_ => throw new UnreachableException(),
+		};
+	}
+}

src/Nerdbank.MessagePack/MessagePackPrimitives.Writers.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System.Buffers.Binary;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
@@ -1100,7 +1101,7 @@ private static bool TryWriteNegativeFixIntUnsafe(Span<byte> destination, sbyte v
 	}
 
 	[DoesNotReturn]
-	private static Exception ThrowUnreachable() => throw new Exception("Presumed unreachable point in code reached.");
+	private static Exception ThrowUnreachable() => throw new UnreachableException();
 
 	/// <summary>
 	/// Writes an integer value in big-endian order to the specified buffer.