Make sure we emit diagnostics for "unsupported" marshalling for downlevel platforms

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

@@ -341,6 +341,8 @@ private static IncrementalStubGenerationContext CalculateStubInformation(
                     generatorFactory = new UnsupportedMarshallingFactory();
                 }
 
+                generatorFactory = new NoMarshallingInfoErrorMarshallingFactory(generatorFactory);
+
                 // The presence of System.Runtime.CompilerServices.DisableRuntimeMarshallingAttribute is tied to TFM,
                 // so we use TFM in the generator factory key instead of the Compilation as the compilation changes on every keystroke.
                 IAssemblySymbol coreLibraryAssembly = env.Compilation.GetSpecialType(SpecialType.System_Object).ContainingAssembly;

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/NoMarshallingInfoErrorMarshallingFactory.cs

@@ -0,0 +1,50 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Runtime.InteropServices;
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.Interop
+{
+    public sealed class NoMarshallingInfoErrorMarshallingFactory : IMarshallingGeneratorFactory
+    {
+        private readonly IMarshallingGeneratorFactory _inner;
+
+        public IMarshallingGenerator Create(
+            TypePositionInfo info,
+            StubCodeContext context)
+        {
+            if (info.MarshallingAttributeInfo is NoMarshallingInfo && CustomTypeToErrorMessageMap.TryGetValue(info.ManagedType, out string errorMessage))
+            {
+                throw new MarshallingNotSupportedException(info, context)
+                {
+                    NotSupportedDetails = errorMessage
+                };
+            }
+            return _inner.Create(info, context);
+        }
+
+        public NoMarshallingInfoErrorMarshallingFactory(IMarshallingGeneratorFactory inner)
+            : this(inner, DefaultTypeToErrorMessageMap)
+        {
+        }
+
+        private NoMarshallingInfoErrorMarshallingFactory(IMarshallingGeneratorFactory inner, ImmutableDictionary<ManagedTypeInfo, string> customTypeToErrorMessageMap)
+        {
+            _inner = inner;
+            CustomTypeToErrorMessageMap = customTypeToErrorMessageMap;
+        }
+
+        public ImmutableDictionary<ManagedTypeInfo, string> CustomTypeToErrorMessageMap { get; }
+
+        private static ImmutableDictionary<ManagedTypeInfo, string> DefaultTypeToErrorMessageMap { get; } =
+            ImmutableDictionary.CreateRange(new Dictionary<ManagedTypeInfo, string>
+            {
+                { SpecialTypeInfo.String, SR.MarshallingStringOrCharAsUndefinedNotSupported },
+                { SpecialTypeInfo.Boolean, SR.MarshallingBoolAsUndefinedNotSupported },
+            });
+    }
+}

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/UnsupportedMarshallingFactory.cs

@@ -11,38 +11,7 @@ namespace Microsoft.Interop
 {
     public sealed class UnsupportedMarshallingFactory : IMarshallingGeneratorFactory
     {
-        public IMarshallingGenerator Create(
-            TypePositionInfo info,
-            StubCodeContext context)
-        {
-            if (info.MarshallingAttributeInfo is NoMarshallingInfo && CustomTypeToErrorMessageMap.TryGetValue(info.ManagedType, out string errorMessage))
-            {
-                throw new MarshallingNotSupportedException(info, context)
-                {
-                    NotSupportedDetails = errorMessage
-                };
-            }
+        public IMarshallingGenerator Create(TypePositionInfo info, StubCodeContext context) =>
             throw new MarshallingNotSupportedException(info, context);
-        }
-
-        public UnsupportedMarshallingFactory()
-            : this(DefaultTypeToErrorMessageMap)
-        {
-
-        }
-
-        private UnsupportedMarshallingFactory(ImmutableDictionary<ManagedTypeInfo, string> customTypeToErrorMessageMap)
-        {
-            CustomTypeToErrorMessageMap = customTypeToErrorMessageMap;
-        }
-
-        public ImmutableDictionary<ManagedTypeInfo, string> CustomTypeToErrorMessageMap { get; }
-
-        private static ImmutableDictionary<ManagedTypeInfo, string> DefaultTypeToErrorMessageMap { get; } =
-            ImmutableDictionary.CreateRange(new Dictionary<ManagedTypeInfo, string>
-            {
-                { SpecialTypeInfo.String, SR.MarshallingStringOrCharAsUndefinedNotSupported },
-                { SpecialTypeInfo.Boolean, SR.MarshallingBoolAsUndefinedNotSupported },
-            });
     }
 }

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Diagnostics.cs

@@ -295,7 +295,7 @@ partial class Test
         }
 
         [Fact]
-        [OuterLoop("Uses the network for downlevel ref packs")]
+        //[OuterLoop("Uses the network for downlevel ref packs")]
         public async Task StringMarshallingForwardingNotSupported_ReportsDiagnostic()
         {
             string source = @"