Add fast path for ToString for this enum.

src/Workspaces/Core/Portable/Execution/Serializer.cs

@@ -52,7 +52,7 @@ public Checksum CreateChecksum(object value, CancellationToken cancellationToken
         {
             var kind = value.GetWellKnownSynchronizationKind();
 
-            using (Logger.LogBlock(FunctionId.Serializer_CreateChecksum, kind.ToString(), cancellationToken))
+            using (Logger.LogBlock(FunctionId.Serializer_CreateChecksum, kind.ToStringFast(), cancellationToken))
             {
                 cancellationToken.ThrowIfCancellationRequested();
 

src/Workspaces/Core/Portable/Execution/WellKnownSynchronizationKind.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System.Linq;
+using System.Reflection;
+
 namespace Microsoft.CodeAnalysis.Serialization
 {
     // TODO: Kind might not actually needed. see whether we can get rid of this
@@ -47,4 +50,35 @@ internal enum WellKnownSynchronizationKind
         ProjectStateChecksums,
         DocumentStateChecksums,
     }
+
+    internal static class WellKnownSynchronizationKindExtensions
+    {
+        private static readonly string[] s_strings;
+
+        static WellKnownSynchronizationKindExtensions()
+        {
+            var fields = typeof(WellKnownSynchronizationKind).GetTypeInfo().DeclaredFields.Where(f => f.IsStatic);
+
+            var maxValue = 0;
+            foreach (var field in fields)
+            {
+                var value = (int)field.GetValue(null);
+                if (value > maxValue)
+                {
+                    maxValue = value;
+                }
+            }
+
+            s_strings = new string[maxValue + 1];
+
+            foreach (var field in fields)
+            {
+                var value = (int)field.GetValue(null);
+                s_strings[value] = field.Name;
+            }
+        }
+
+        public static string ToStringFast(this WellKnownSynchronizationKind kind)
+            => s_strings[(int)kind];
+    }
 }
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