Key off of ProjectState so that we don't recompute when a project hasn't actually changed.

src/Workspaces/Core/Portable/FindSymbols/SymbolTree/SymbolTreeInfo_Source.cs

@@ -47,18 +47,20 @@ private static void FreeSymbolMap(MultiDictionary<string, ISymbol> symbolMap)
         /// Cache of project to the checksum for it so that we don't have to expensively recompute
         /// this each time we get a project.
         /// </summary>
-        private static ConditionalWeakTable<Project, AsyncLazy<Checksum>> s_projectToSourceChecksum =
-            new ConditionalWeakTable<Project, AsyncLazy<Checksum>>();
+        private static ConditionalWeakTable<ProjectState, AsyncLazy<Checksum>> s_projectToSourceChecksum =
+            new ConditionalWeakTable<ProjectState, AsyncLazy<Checksum>>();
 
         public static Task<Checksum> GetSourceSymbolsChecksumAsync(Project project, CancellationToken cancellationToken)
         {
+            var workspace = project.Solution.Workspace;
             var lazy = s_projectToSourceChecksum.GetValue(
-                project, p => new AsyncLazy<Checksum>(c => ComputeSourceSymbolsChecksumAsync(p, c), cacheResult: true));
+                project.State, p => new AsyncLazy<Checksum>(c => ComputeSourceSymbolsChecksumAsync(workspace, p, c), cacheResult: true));
 
             return lazy.GetValueAsync(cancellationToken);
         }
 
-        private static async Task<Checksum> ComputeSourceSymbolsChecksumAsync(Project project, CancellationToken cancellationToken)
+        private static async Task<Checksum> ComputeSourceSymbolsChecksumAsync(
+            Workspace workspace, ProjectState projectState, CancellationToken cancellationToken)
         {
             // The SymbolTree for source is built from the source-symbols from the project's compilation's
             // assembly.  Specifically, we only get the name, kind and parent/child relationship of all the
@@ -66,14 +68,14 @@ private static async Task<Checksum> ComputeSourceSymbolsChecksumAsync(Project pr
             // changed.  The only thing that can make those source-symbols change in that manner are if
             // the text of any document changes, or if options for the project change.  So we build our
             // checksum out of that data.
-            var serializer = new Serializer(project.Solution.Workspace);
-            var projectStateChecksums = await project.State.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
+            var serializer = new Serializer(workspace);
+            var projectStateChecksums = await projectState.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
 
             // Order the documents by FilePath.  Default ordering in the RemoteWorkspace is
             // to be ordered by Guid (which is not consistent across VS sessions).
-            var textChecksumsTasks = project.Documents.OrderBy(d => d.FilePath, StringComparer.Ordinal).Select(async d =>
+            var textChecksumsTasks = projectState.DocumentStates.OrderBy(d => d.Value.FilePath, StringComparer.Ordinal).Select(async d =>
             {
-                var documentStateChecksum = await d.State.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
+                var documentStateChecksum = await d.Value.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
                 return documentStateChecksum.Text;
             });