Add more contract guarantees

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

@@ -155,7 +155,7 @@ private static Metadata GetMetadataNoThrow(PortableExecutableReference reference
         {
             var filePath = reference.FilePath;
 
-            return TryLoadOrCreateAsync(
+            var result = TryLoadOrCreateAsync(
                 solution,
                 checksum,
                 filePath,
@@ -165,6 +165,8 @@ private static Metadata GetMetadataNoThrow(PortableExecutableReference reference
                 getPersistedChecksum: info => info.Checksum,
                 readObject: reader => ReadSymbolTreeInfo(reader, (names, nodes) => GetSpellCheckerTask(solution, checksum, filePath, names, nodes)),
                 cancellationToken: cancellationToken);
+            Contract.ThrowIfFalse(result != null || loadOnly == true, "Result can only be null if 'loadOnly: true' was passed.");
+            return result;
         }
 
         private static Task<SymbolTreeInfo> CreateMetadataSymbolTreeInfoAsync(

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

@@ -19,25 +19,6 @@ internal partial class SymbolTreeInfo : IObjectWritable
         private const string PrefixMetadataSymbolTreeInfo = "<SymbolTreeInfo>";
         private const string SerializationFormat = "17";
 
-        /// <summary>
-        /// Loads the SymbolTreeInfo for a given assembly symbol (metadata or project).  If the
-        /// info can't be loaded, it will be created (and persisted if possible).
-        /// </summary>
-        private static Task<SymbolTreeInfo> LoadOrCreateSourceSymbolTreeInfoAsync(
-            Project project, Checksum checksum, bool loadOnly, CancellationToken cancellationToken)
-        {
-            return TryLoadOrCreateAsync(
-                project.Solution,
-                checksum,
-                project.FilePath,
-                loadOnly,
-                createAsync: () => CreateSourceSymbolTreeInfoAsync(project, checksum, cancellationToken),
-                keySuffix: "_Source",
-                getPersistedChecksum: info => info.Checksum,
-                readObject: reader => ReadSymbolTreeInfo(reader, (names, nodes) => GetSpellCheckerTask(project.Solution, checksum, project.FilePath, names, nodes)),
-                cancellationToken: cancellationToken);
-        }
-
         /// <summary>
         /// Loads the SpellChecker for a given assembly symbol (metadata or project).  If the
         /// info can't be loaded, it will be created (and persisted if possible).
@@ -48,7 +29,7 @@ internal partial class SymbolTreeInfo : IObjectWritable
             string filePath,
             Func<Task<SpellChecker>> createAsync)
         {
-            return TryLoadOrCreateAsync(
+            var result = TryLoadOrCreateAsync(
                 solution,
                 checksum,
                 filePath,
@@ -58,6 +39,8 @@ internal partial class SymbolTreeInfo : IObjectWritable
                 getPersistedChecksum: s => s.Checksum,
                 readObject: SpellChecker.ReadFrom,
                 cancellationToken: CancellationToken.None);
+            Contract.ThrowIfNull(result, "Result should never be null as we passed 'loadOnly: false'.");
+            return result;
         }
 
         /// <summary>

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

@@ -29,9 +29,17 @@ private static void FreeSymbolMap(MultiDictionary<string, ISymbol> symbolMap)
         public static Task<SymbolTreeInfo> GetInfoForSourceAssemblyAsync(
             Project project, Checksum checksum, CancellationToken cancellationToken)
         {
-            var result = LoadOrCreateSourceSymbolTreeInfoAsync(
-                project, checksum, loadOnly: false, cancellationToken: cancellationToken);
-            Contract.ThrowIfNull(result);
+            var result = TryLoadOrCreateAsync(
+                project.Solution,
+                checksum,
+                project.FilePath,
+                loadOnly: false,
+                createAsync: () => CreateSourceSymbolTreeInfoAsync(project, checksum, cancellationToken),
+                keySuffix: "_Source",
+                getPersistedChecksum: info => info.Checksum,
+                readObject: reader => ReadSymbolTreeInfo(reader, (names, nodes) => GetSpellCheckerTask(project.Solution, checksum, project.FilePath, names, nodes)),
+                cancellationToken: cancellationToken);
+            Contract.ThrowIfNull(result, "Result should never be null as we passed 'loadOnly: false'.");
             return result;
         }