-Improved SplitFullyQualifiedTypeName allocations

Src/Newtonsoft.Json/Utilities/ReflectionUtils.cs

@@ -866,8 +866,8 @@ namespace Newtonsoft.Json.Utilities
 
             if (assemblyDelimiterIndex != null)
             {
-                typeName = fullyQualifiedTypeName.Substring(0, assemblyDelimiterIndex.GetValueOrDefault()).Trim();
-                assemblyName = fullyQualifiedTypeName.Substring(assemblyDelimiterIndex.GetValueOrDefault() + 1, fullyQualifiedTypeName.Length - assemblyDelimiterIndex.GetValueOrDefault() - 1).Trim();
+                typeName = fullyQualifiedTypeName.Trim(0, assemblyDelimiterIndex.GetValueOrDefault());
+                assemblyName = fullyQualifiedTypeName.Trim(assemblyDelimiterIndex.GetValueOrDefault() + 1, fullyQualifiedTypeName.Length - assemblyDelimiterIndex.GetValueOrDefault() - 1);
             }
             else
             {

Src/Newtonsoft.Json/Utilities/StringUtils.cs

@@ -285,5 +285,43 @@ namespace Newtonsoft.Json.Utilities
         {
             return (source.Length > 0 && source[source.Length - 1] == value);
         }
+
+        public static string Trim(this string s, int start, int length)
+        {
+            // References: https://referencesource.microsoft.com/#mscorlib/system/string.cs,2691
+            // https://referencesource.microsoft.com/#mscorlib/system/string.cs,1226
+            if (s == null)
+            {
+                throw new ArgumentNullException();
+            }
+            if (start < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(start));
+            }
+            if (length < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(length));
+            }
+            int end = start + length - 1;
+            if (end >= s.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(length));
+            }
+            for (; start < end; start++)
+            {
+                if (!char.IsWhiteSpace(s[start]))
+                {
+                    break;
+                }
+            }
+            for (; end >= start; end--)
+            {
+                if (!char.IsWhiteSpace(s[end]))
+                {
+                    break;
+                }
+            }
+            return s.Substring(start, end - start + 1);
+        }
     }
 }