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

@@ -29,8 +29,14 @@ private static void FreeSymbolMap(MultiDictionary<string, ISymbol> symbolMap)
             Project project, CancellationToken cancellationToken)
         {
             var compilation = await project.GetCompilationAsync(cancellationToken).ConfigureAwait(false);
-            var stateChecksums = await project.State.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
 
+            // 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
+            // child symbols.  So we want to be able to reuse the index as long as none of these have 
+            // 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 stateChecksums = await project.State.GetStateChecksumsAsync(cancellationToken).ConfigureAwait(false);
             var checksum = Checksum.Create(nameof(SymbolTreeInfo),
                 new Checksum[] { stateChecksums.Documents.Checksum, stateChecksums.CompilationOptions, stateChecksums.ParseOptions });