Apply optimizations inside AppendDisplayName

(Note: The changed method's logic for dealing with generic type names
is way too simplistic. For example, it'll fail badly with nested types
or with types where the generic argument is yet another generic type.
Changing this isn't trivial, but perhaps should be done some time.)

Source/ExpressionStringBuilder.cs

@@ -1,13 +1,14 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Globalization;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Text;
-using System.Collections.ObjectModel;
+
 using Moq.Properties;
 
 namespace Moq
@@ -629,45 +630,38 @@ internal static string ToStringOperator(ExpressionType nodeType)
 		}
 	}
 
-	internal static class StringExtensions
-	{
-		public static string AsCommaSeparatedValues(this IEnumerable<string> source)
-		{
-			if (source == null)
-			{
-				return string.Empty;
-			}
-			var result = new StringBuilder(100);
-			bool appendComma = false;
-			foreach (var value in source)
-			{
-				if (appendComma)
-				{
-					result.Append(", ");
-				}
-				result.Append(value);
-				appendComma = true;
-			}
-			return result.ToString();
-		}
-	}
-
 	internal static class StringBuilderExtensions
 	{
 		public static StringBuilder AppendDisplayName(this StringBuilder builder, Type source, Func<Type, string> getName)
 		{
-			if (source == null)
+			Debug.Assert(source != null);
+
+			var name = getName(source);
+			var backtickIndex = name.IndexOf('`');
+			if (backtickIndex >= 0)
 			{
-				throw new ArgumentNullException("source");
+				builder.Append(name, 0, backtickIndex);
+			}
+			else
+			{
+				builder.Append(name);
 			}
 
-			builder.Append(getName(source).Split('`').First());
 			if (source.GetTypeInfo().IsGenericType)
 			{
+				var genericArguments = source.GetGenericArguments();
 				builder.Append('<');
-				builder.Append(source.GetGenericArguments().Select(t => getName(t)).AsCommaSeparatedValues());
+				for (int i = 0, n = genericArguments.Length; i < n; ++i)
+				{
+					if (i > 0)
+					{
+						builder.Append(", ");
+					}
+					builder.Append(getName(genericArguments[i]));
+				}
 				builder.Append('>');
 			}
+
 			return builder;
 		}
 	}