Cache what symbols a serialized defintion rehydrated to. That way we don't...

Cache what symbols a serialized defintion rehydrated to.  That way we don't have to recompute it every time we hear about a new reference.

src/Workspaces/Core/Portable/FindSymbols/SymbolFinder.ServerCallback.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.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Remote;
@@ -19,6 +20,10 @@ private class ServerCallback
             private readonly IStreamingFindReferencesProgress _progress;
             private readonly CancellationToken _cancellationToken;
 
+            private readonly object _gate = new object();
+            private readonly Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId> _definitionMap =
+                new Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId>();
+
             public ServerCallback(
                 Solution solution,
                 IStreamingFindReferencesProgress progress,
@@ -49,14 +54,24 @@ public async Task OnDefinitionFoundAsync(SerializableSymbolAndProjectId definiti
             {
                 var symbolAndProjectId = await definition.RehydrateAsync(
                     _solution, _cancellationToken).ConfigureAwait(false);
+
+                lock (_gate)
+                {
+                    _definitionMap[definition] = symbolAndProjectId;
+                }
+
                 await _progress.OnDefinitionFoundAsync(symbolAndProjectId).ConfigureAwait(false);
             }
 
             public async Task OnReferenceFoundAsync(
                 SerializableSymbolAndProjectId definition, SerializableReferenceLocation reference)
             {
-                var symbolAndProjectId = await definition.RehydrateAsync(
-                    _solution, _cancellationToken).ConfigureAwait(false);
+                SymbolAndProjectId symbolAndProjectId;
+                lock (_gate)
+                {
+                    symbolAndProjectId = _definitionMap[definition];
+                }
+
                 var referenceLocation = await reference.RehydrateAsync(
                     _solution, _cancellationToken).ConfigureAwait(false);
 

src/Workspaces/Core/Portable/Remote/RemoteArguments.cs

@@ -8,6 +8,7 @@
 using Microsoft.CodeAnalysis.FindSymbols;
 using Microsoft.CodeAnalysis.SymbolSearch;
 using Microsoft.CodeAnalysis.Text;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Remote
 {
@@ -16,11 +17,19 @@ namespace Microsoft.CodeAnalysis.Remote
     /// <summary>
     /// Arguments to pass from client to server when performing operations
     /// </summary>
-    internal class SerializableProjectId
+    internal class SerializableProjectId : IEquatable<SerializableProjectId>
     {
         public Guid Id;
         public string DebugName;
 
+        public override int GetHashCode()
+            => Hash.Combine(Id.GetHashCode(), DebugName.GetHashCode());
+
+        public override bool Equals(object obj) => Equals(obj as SerializableProjectId);
+
+        public bool Equals(SerializableProjectId obj)
+            => obj != null && Id.Equals(obj.Id) && DebugName.Equals(obj.DebugName);
+
         public static SerializableProjectId Dehydrate(ProjectId id)
         {
             return new SerializableProjectId { Id = id.Id, DebugName = id.DebugName };
@@ -80,11 +89,19 @@ public TextSpan Rehydrate()
 
     #region FindReferences
 
-    internal class SerializableSymbolAndProjectId
+    internal class SerializableSymbolAndProjectId : IEquatable<SerializableSymbolAndProjectId>
     {
         public string SymbolKeyData;
         public SerializableProjectId ProjectId;
 
+        public override int GetHashCode()
+            => Hash.Combine(SymbolKeyData, ProjectId.GetHashCode());
+
+        public override bool Equals(object obj) => Equals(obj as SerializableSymbolAndProjectId);
+
+        public bool Equals(SerializableSymbolAndProjectId other)
+            => other != null && SymbolKeyData.Equals(other.SymbolKeyData) && ProjectId.Equals(other.ProjectId);
+
         public static SerializableSymbolAndProjectId Dehydrate(
             IAliasSymbol alias, Document document)
         {