Moved the typeName assignment after checking for supported languages per:

https://github.com/dotnet/roslyn/issues/30449

src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerFileReference.cs

@@ -159,15 +159,21 @@ private static AnalyzerLoadFailureEventArgs CreateAnalyzerFailedArgs(Exception e
         /// </summary>
         /// <exception cref="BadImageFormatException">The PE image format is invalid.</exception>
         /// <exception cref="IOException">IO error reading the metadata.</exception>
+        [PerformanceSensitive( "https://github.com/dotnet/roslyn/issues/30449" )]
         private static ImmutableDictionary<string, ImmutableHashSet<string>> GetAnalyzerTypeNameMap(string fullPath, AttributePredicate attributePredicate)
         {
             using (var assembly = AssemblyMetadata.CreateFromFile(fullPath))
             {
+                // This is longer than strictly necessary to avoid thrashing the GC with string allocations
+                // in the call to GetFullyQualifiedTypeNames. Specifically, this checks for the presence of
+                // supported languages prior to creating the type names.
                 var typeNameMap = from module in assembly.GetModules()
                                   from typeDefHandle in module.MetadataReader.TypeDefinitions
-                                  let typeDef = module.MetadataReader.GetTypeDefinition(typeDefHandle)
-                                  let typeName = GetFullyQualifiedTypeName(typeDef, module.Module)
-                                  from supportedLanguage in GetSupportedLanguages(typeDef, module.Module, attributePredicate)
+                                  let typeDef = module.MetadataReader.GetTypeDefinition( typeDefHandle )
+                                  let supportedLanguages = GetSupportedLanguages( typeDef, module.Module, attributePredicate )
+                                  where supportedLanguages != null && supportedLanguages.Any()
+                                  let typeName = GetFullyQualifiedTypeName( typeDef, module.Module )
+                                  from supportedLanguage in supportedLanguages
                                   group typeName by supportedLanguage;
 
                 return typeNameMap.ToImmutableDictionary(g => g.Key, g => g.ToImmutableHashSet());