Improve codegen for merged activation factories

src/Authoring/WinRT.SourceGenerator/Generator.cs

@@ -13,6 +13,7 @@
 using System.Reflection.Metadata.Ecma335;
 using System.Reflection.PortableExecutable;
 using System.Text;
+using System.Threading;
 
 namespace Generator
 {
@@ -196,13 +197,43 @@ public void Generate()
             Logger.Close();
         }
 
+        private static bool ShouldEmitCallToTryGetDependentActivationFactory(GeneratorExecutionContext context)
+        {
+            if (!context.AnalyzerConfigOptions.GetCsWinRTMergeReferencedActivationFactories())
+            {
+                return false;
+            }
+
+            foreach (MetadataReference metadataReference in context.Compilation.References)
+            {
+                if (context.Compilation.GetAssemblyOrModuleSymbol(metadataReference) is not IAssemblySymbol assemblySymbol)
+                {
+                    continue;
+                }
+
+                // Check if the current assembly is a WinRT component (we just need one)
+                if (MergeReferencedActivationFactoriesGenerator.TryGetDependentAssemblyExportsTypeName(
+                    assemblySymbol,
+                    context.Compilation,
+                    CancellationToken.None,
+                    out _))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         /// <summary>
         /// Generates the native exports for a WinRT component.
         /// </summary>
         /// <param name="context">The <see cref="GeneratorExecutionContext"/> value to use to produce source files.</param>
         public static void GenerateWinRTNativeExports(GeneratorExecutionContext context)
         {
-            context.AddSource("NativeExports.g.cs", """
+            StringBuilder builder = new();
+
+            builder.AppendLine("""
                 // <auto-generated/>
                 #pragma warning disable
 
@@ -240,11 +271,22 @@ public static int DllGetActivationFactory(void* activatableClassId, void** facto
 
                                 IntPtr obj = GetActivationFactory(fullyQualifiedTypeName);
 
-                                if ((void*)obj is null)
-                                {
-                                    obj = TryGetDependentActivationFactory(fullyQualifiedTypeName);
-                                }
+                """);
+
+            // Only emit this call if we have actually generated that. We want to avoid generating
+            // that default implementation in every single assembly the generator runs on.
+            if (ShouldEmitCallToTryGetDependentActivationFactory(context))
+            {
+                builder.AppendLine("""
+                                     if ((void*)obj is null)
+                                     {
+                                         obj = TryGetDependentActivationFactory(fullyQualifiedTypeName);
+                                     }
+
+                    """);
+            }
 
+            builder.Append("""
                                 if ((void*)obj is null)
                                 {
                                     *factory = null;

src/Authoring/WinRT.SourceGenerator/MergeReferencedActivationFactoriesGenerator.cs

@@ -69,6 +69,11 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         // Generate the chaining helper
         context.RegisterImplementationSourceOutput(assemblyExportsTypeNames, static (context, assemblyExportsTypeNames) =>
         {
+            if (assemblyExportsTypeNames.IsEmpty)
+            {
+                return;
+            }
+
             StringBuilder builder = new();
 
             builder.AppendLine("""
@@ -88,14 +93,10 @@ partial class Module
                         /// <param name="fullyQualifiedTypeName">The marshalled fully qualified type name of the activation factory to retrieve.</param>
                         /// <returns>The pointer to the activation factory that corresponds with the class specified by <paramref name="fullyQualifiedTypeName"/>.</returns>
                         internal static unsafe IntPtr TryGetDependentActivationFactory(ReadOnlySpan<char> fullyQualifiedTypeName)
-                        {                  
-                """);
+                        {
+                            IntPtr obj;
 
-            if (!assemblyExportsTypeNames.IsEmpty)
-            {
-                builder.AppendLine("            IntPtr obj;");
-                builder.AppendLine();
-            }
+                """);
 
             foreach (string assemblyExportsTypeName in assemblyExportsTypeNames)
             {
@@ -129,7 +130,7 @@ internal static unsafe IntPtr TryGetDependentActivationFactory(ReadOnlySpan<char
     /// <param name="token">The <see cref="CancellationToken"/> instance to use.</param>
     /// <param name="name">The resulting type name, if found.</param>
     /// <returns>Whether a type name was found.</returns>
-    private static bool TryGetDependentAssemblyExportsTypeName(
+    internal static bool TryGetDependentAssemblyExportsTypeName(
         IAssemblySymbol assemblySymbol,
         Compilation compilation,
         CancellationToken token,