Deduplicate diagnostics emitted for managed->unmanaged and unmanaged->managed stubs (#88803)

Author: jkoritzinsky

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComInterfaceGenerator.cs

@@ -99,24 +99,25 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 .Combine(interfaceContexts.Collect())
                 .SelectMany((data, ct) => GroupComContextsForInterfaceGeneration(data.Left, data.Right, ct));
 
-            // Generate the code for the managed-to-unmanaged stubs and the diagnostics from code-generation.
-            context.RegisterDiagnostics(interfaceAndMethodsContexts
-                .SelectMany((data, ct) => data.DeclaredMethods.SelectMany(m => m.ManagedToUnmanagedStub.Diagnostics)));
+            // Generate the code for the managed-to-unmanaged stubs.
             var managedToNativeInterfaceImplementations = interfaceAndMethodsContexts
                 .Select(GenerateImplementationInterface)
                 .WithTrackingName(StepNames.GenerateManagedToNativeInterfaceImplementation)
                 .WithComparer(SyntaxEquivalentComparer.Instance)
                 .SelectNormalized();
 
-            // Generate the code for the unmanaged-to-managed stubs and the diagnostics from code-generation.
-            context.RegisterDiagnostics(interfaceAndMethodsContexts
-                .SelectMany((data, ct) => data.DeclaredMethods.SelectMany(m => m.UnmanagedToManagedStub.Diagnostics)));
+            // Generate the code for the unmanaged-to-managed stubs.
             var nativeToManagedVtableMethods = interfaceAndMethodsContexts
                 .Select(GenerateImplementationVTableMethods)
                 .WithTrackingName(StepNames.GenerateNativeToManagedVTableMethods)
                 .WithComparer(SyntaxEquivalentComparer.Instance)
                 .SelectNormalized();
 
+            // Report diagnostics for managed-to-unmanaged and unmanaged-to-managed stubs, deduplicating diagnostics that are reported for both.
+            context.RegisterDiagnostics(
+                interfaceAndMethodsContexts
+                    .SelectMany((data, ct) => data.DeclaredMethods.SelectMany(m => m.ManagedToUnmanagedStub.Diagnostics).Union(data.DeclaredMethods.SelectMany(m => m.UnmanagedToManagedStub.Diagnostics))));
+
             // Generate the native interface metadata for each [GeneratedComInterface]-attributed interface.
             var nativeInterfaceInformation = interfaceContexts
                 .Select(static (data, ct) => data.Info)