Expose the custom type marshalling types for source-generated interop. (#67052)

Co-authored-by: Elinor Fung <elfung@microsoft.com>

11 files changed, 175 insertions, 88 deletions

diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index ccebd9c7bae..cbcaf580b96 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
@@ -823,6 +823,10 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CurrencyWrapper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomQueryInterfaceMode.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomQueryInterfaceResult.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomTypeMarshallerAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomTypeMarshallerDirection.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomTypeMarshallerFeatures.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CustomTypeMarshallerKind.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\DefaultCharSetAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\DefaultDllImportSearchPathsAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\DefaultParameterValueAttribute.cs" />
@@ -852,8 +856,10 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\Marshal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\MarshalAsAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\MarshalDirectiveException.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\MarshalUsingAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\MemoryMarshal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\NativeLibrary.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\NativeMarshallingAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\NativeMemory.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\NFloat.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\OptionalAttribute.cs" />
diff --git a/src/libraries/Common/src/System/Runtime/InteropServices/GeneratedMarshallingAttribute.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerAttribute.cs
index 4a0aaf99e23..9b9bb3f4509 100644
--- a/src/libraries/Common/src/System/Runtime/InteropServices/GeneratedMarshallingAttribute.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerAttribute.cs
@@ -1,59 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#nullable enable
-
-//
-// Types in this file are used for generated p/invokes (docs/design/features/source-generator-pinvokes.md).
-//
 namespace System.Runtime.InteropServices
 {
-    [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Delegate)]
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    sealed class NativeMarshallingAttribute : Attribute
-    {
-        public NativeMarshallingAttribute(Type nativeType)
-        {
-            NativeType = nativeType;
-        }
-
-        public Type NativeType { get; }
-    }
-
-    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = true)]
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    sealed class MarshalUsingAttribute : Attribute
-    {
-        public MarshalUsingAttribute()
-        {
-            CountElementName = string.Empty;
-        }
-
-        public MarshalUsingAttribute(Type nativeType)
-            : this()
-        {
-            NativeType = nativeType;
-        }
-
-        public Type? NativeType { get; }
-
-        public string CountElementName { get; set; }
-
-        public int ConstantElementCount { get; set; }
-
-        public int ElementIndirectionDepth { get; set; }
-
-        public const string ReturnsCountValue = "return-value";
-    }
-
     /// <summary>
     /// Attribute used to indicate that the type can be used to convert a value of the provided <see cref="ManagedType"/> to a native representation.
     /// </summary>
@@ -63,12 +12,7 @@ namespace System.Runtime.InteropServices
     /// <seealso cref="LibraryImportAttribute"/>
     /// </remarks>
     [AttributeUsage(AttributeTargets.Struct)]
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    sealed class CustomTypeMarshallerAttribute : Attribute
+    public sealed class CustomTypeMarshallerAttribute : Attribute
     {
         public CustomTypeMarshallerAttribute(Type managedType, CustomTypeMarshallerKind marshallerKind = CustomTypeMarshallerKind.Value)
         {
diff --git a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerDirection.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerDirection.cs
index 0ee347d72b9..0a876b37a5a 100644
--- a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerDirection.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerDirection.cs
@@ -9,12 +9,7 @@ namespace System.Runtime.InteropServices
     /// A direction of marshalling data into or out of the managed environment
     /// </summary>
     [Flags]
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    enum CustomTypeMarshallerDirection
+    public enum CustomTypeMarshallerDirection
     {
         /// <summary>
         /// No marshalling direction
diff --git a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerFeatures.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerFeatures.cs
index fe58f386f72..cc0c88f86ee 100644
--- a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerFeatures.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerFeatures.cs
@@ -7,12 +7,7 @@ namespace System.Runtime.InteropServices
     /// Optional features supported by custom type marshallers.
     /// </summary>
     [Flags]
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    enum CustomTypeMarshallerFeatures
+    public enum CustomTypeMarshallerFeatures
     {
         /// <summary>
         /// No optional features supported
diff --git a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerKind.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerKind.cs
index 06c73cdf7b6..11171ab0006 100644
--- a/src/libraries/Common/src/System/Runtime/InteropServices/CustomTypeMarshallerKind.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomTypeMarshallerKind.cs
@@ -9,12 +9,7 @@ namespace System.Runtime.InteropServices
     /// <remarks>
     /// <seealso cref="LibraryImportAttribute"/>
     /// </remarks>
-#if LIBRARYIMPORT_GENERATOR_TEST
-    public
-#else
-    internal
-#endif
-    enum CustomTypeMarshallerKind
+    public enum CustomTypeMarshallerKind
     {
         /// <summary>
         /// This custom type marshaller represents a single value.
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MarshalUsingAttribute.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MarshalUsingAttribute.cs
new file mode 100644
index 00000000000..11482e10eb7
--- /dev/null
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MarshalUsingAttribute.cs
@@ -0,0 +1,73 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.InteropServices
+{
+    /// <summary>
+    /// Attribute used to provide a custom marshaller type or size information for marshalling.
+    /// </summary>
+    /// <remarks>
+    /// This attribute is recognized by the runtime-provided source generators for source-generated interop scenarios.
+    /// It is not used by the interop marshalling system at runtime.
+    /// <seealso cref="LibraryImportAttribute"/>
+    /// <seealso cref="CustomTypeMarshallerAttribute" />
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = true)]
+    public sealed class MarshalUsingAttribute : Attribute
+    {
+        /// <summary>
+        /// Create a <see cref="MarshalUsingAttribute" /> that provides only size information.
+        /// </summary>
+        public MarshalUsingAttribute()
+        {
+            CountElementName = string.Empty;
+        }
+
+        /// <summary>
+        /// Create a <see cref="MarshalUsingAttribute" /> that provides a native marshalling type and optionally size information.
+        /// </summary>
+        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" /></param>
+        public MarshalUsingAttribute(Type nativeType)
+            : this()
+        {
+            NativeType = nativeType;
+        }
+
+        /// <summary>
+        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" />
+        /// </summary>
+        public Type? NativeType { get; }
+
+        /// <summary>
+        /// The name of the parameter that will provide the size of the collection when marshalling from unmanaged to managed, or <see cref="ReturnsCountValue" /> if the return value provides the size.
+        /// </summary>
+        /// <remarks>
+        /// Cannot be provided when <see cref="ConstantElementCount" /> is set.
+        /// </remarks>
+        public string CountElementName { get; set; }
+
+        /// <summary>
+        /// If a collection is constant size, the size of the collection when marshalling from unmanaged to managed.
+        /// </summary>
+        /// <remarks>
+        /// Cannot be provided when <see cref="CountElementName" /> is set.
+        /// </remarks>
+        public int ConstantElementCount { get; set; }
+
+        /// <summary>
+        /// What indirection depth this marshalling info is provided for.
+        /// </summary>
+        /// <remarks>
+        /// This value corresponds to how many pointer indirections would be required to get to the corresponding value from the native representation.
+        /// For example, this attribute is on a parameter of type <see cref="int" />[][], then an <see cref="ElementIndirectionDepth"/> of 0 means that the marshalling info applies to the managed type of <see cref="int" />[][],
+        /// an <see cref="ElementIndirectionDepth"/> of 1 applies to the managed type of <see cref="int" />[], and an <see cref="ElementIndirectionDepth"/> of 2 applies to the managed type of <see cref="int" />.
+        /// Only one <see cref="MarshalUsingAttribute" /> with a given <see cref="ElementIndirectionDepth" /> can be provided on a given parameter or return value.
+        /// </remarks>
+        public int ElementIndirectionDepth { get; set; }
+
+        /// <summary>
+        /// A constant string that represents the name of the return value for <see cref="CountElementName" />.
+        /// </summary>
+        public const string ReturnsCountValue = "return-value";
+    }
+}
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/NativeMarshallingAttribute.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/NativeMarshallingAttribute.cs
new file mode 100644
index 00000000000..23e941af111
--- /dev/null
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/NativeMarshallingAttribute.cs
@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.InteropServices
+{
+    /// <summary>
+    /// Attribute used to provide a default custom marshaller type for a given managed type.
+    /// </summary>
+    /// <remarks>
+    /// This attribute is recognized by the runtime-provided source generators for source-generated interop scenarios.
+    /// It is not used by the interop marshalling system at runtime.
+    /// <seealso cref="LibraryImportAttribute"/>
+    /// <seealso cref="CustomTypeMarshallerAttribute" />
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Delegate)]
+    public sealed class NativeMarshallingAttribute : Attribute
+    {
+        /// <summary>
+        /// Create a <see cref="MarshalUsingAttribute" /> that provides a native marshalling type.
+        /// </summary>
+        /// <param name="nativeType">The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" /></param>
+        public NativeMarshallingAttribute(Type nativeType)
+        {
+            NativeType = nativeType;
+        }
+
+        /// <summary>
+        /// The marshaller type used to convert the attributed type from managed to native code. This type must be attributed with <see cref="CustomTypeMarshallerAttribute" />
+        /// </summary>
+        public Type NativeType { get; }
+    }
+}
diff --git a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
index 7d7e5fe7b22..c07ec0b6fc9 100644
--- a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
+++ b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Microsoft.Interop.SourceGeneration.csproj
@@ -5,15 +5,14 @@
     <Nullable>enable</Nullable>
     <RootNamespace>Microsoft.Interop</RootNamespace>
     <RunAnalyzers>true</RunAnalyzers>
-    <DefineConstants>$(DefineConstants);LIBRARYIMPORT_GENERATOR_TEST</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>
-    <Compile Include="$(LibrariesProjectRoot)Common\src\System\Runtime\InteropServices\CustomTypeMarshallerKind.cs"
+    <Compile Include="$(LibrariesProjectRoot)System.Private.CoreLib\src\System\Runtime\InteropServices\CustomTypeMarshallerKind.cs"
              Link="Common\System\Runtime\InteropServices\CustomTypeMarshallerKind.cs" />
-    <Compile Include="$(LibrariesProjectRoot)Common\src\System\Runtime\InteropServices\CustomTypeMarshallerDirection.cs"
+    <Compile Include="$(LibrariesProjectRoot)System.Private.CoreLib\src\System\Runtime\InteropServices\CustomTypeMarshallerDirection.cs"
              Link="Common\System\Runtime\InteropServices\CustomTypeMarshallerDirection.cs" />
-    <Compile Include="$(LibrariesProjectRoot)Common\src\System\Runtime\InteropServices\CustomTypeMarshallerFeatures.cs"
+    <Compile Include="$(LibrariesProjectRoot)System.Private.CoreLib\src\System\Runtime\InteropServices\CustomTypeMarshallerFeatures.cs"
              Link="Common\System\Runtime\InteropServices\CustomTypeMarshallerFeatures.cs" />
   </ItemGroup>
 
diff --git a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
index 663c8ad27d9..72adc4842f9 100644
--- a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
+++ b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
@@ -327,6 +327,41 @@ namespace System.Runtime.InteropServices
         NotHandled = 1,
         Failed = 2,
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Struct)]
+    public sealed partial class CustomTypeMarshallerAttribute : System.Attribute
+    {
+        public CustomTypeMarshallerAttribute(System.Type managedType, System.Runtime.InteropServices.CustomTypeMarshallerKind marshallerKind = System.Runtime.InteropServices.CustomTypeMarshallerKind.Value) { }
+        public System.Type ManagedType { get { throw null; } }
+        public System.Runtime.InteropServices.CustomTypeMarshallerKind MarshallerKind { get { throw null; } }
+        public int BufferSize { get { throw null; } set { } }
+        public System.Runtime.InteropServices.CustomTypeMarshallerDirection Direction { get { throw null; } set { } }
+        public System.Runtime.InteropServices.CustomTypeMarshallerFeatures Features { get { throw null; } set { } }
+        public struct GenericPlaceholder
+        {
+        }
+    }
+    [System.FlagsAttribute]
+    public enum CustomTypeMarshallerDirection
+    {
+        [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+        None = 0,
+        In = 0x1,
+        Out = 0x2,
+        Ref = In | Out,
+    }
+    [System.FlagsAttribute]
+    public enum CustomTypeMarshallerFeatures
+    {
+        None = 0,
+        UnmanagedResources = 0x1,
+        CallerAllocatedBuffer = 0x2,
+        TwoStageMarshalling = 0x4
+    }
+    public enum CustomTypeMarshallerKind
+    {
+        Value,
+        LinearCollection
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Module, Inherited=false)]
     public sealed partial class DefaultCharSetAttribute : System.Attribute
     {
@@ -781,6 +816,17 @@ namespace System.Runtime.InteropServices
         public MarshalDirectiveException(string? message) { }
         public MarshalDirectiveException(string? message, System.Exception? inner) { }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Parameter | System.AttributeTargets.ReturnValue, AllowMultiple = true)]
+    public sealed partial class MarshalUsingAttribute : System.Attribute
+    {
+        public MarshalUsingAttribute() { }
+        public MarshalUsingAttribute(System.Type nativeType) { }
+        public System.Type? NativeType { get { throw null; } }
+        public string CountElementName { get { throw null; } set { } }
+        public int ConstantElementCount { get { throw null; } set { } }
+        public int ElementIndirectionDepth { get { throw null; } set { } }
+        public const string ReturnsCountValue = "return-value";
+    }
     public static partial class NativeLibrary
     {
         public static void Free(System.IntPtr handle) { }
@@ -793,6 +839,12 @@ namespace System.Runtime.InteropServices
         public static bool TryLoad(string libraryPath, out System.IntPtr handle) { throw null; }
         public static bool TryLoad(string libraryName, System.Reflection.Assembly assembly, System.Runtime.InteropServices.DllImportSearchPath? searchPath, out System.IntPtr handle) { throw null; }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Struct | System.AttributeTargets.Class | System.AttributeTargets.Enum | System.AttributeTargets.Delegate)]
+    public sealed partial class NativeMarshallingAttribute : System.Attribute
+    {
+        public NativeMarshallingAttribute(System.Type nativeType) { }
+        public System.Type NativeType { get { throw null; } }
+    }
     public static unsafe partial class NativeMemory
     {
         [System.CLSCompliantAttribute(false)]
diff --git a/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/Ancillary.Interop.csproj b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/Ancillary.Interop.csproj
index a1c0dd27574..b8e9081def8 100644
--- a/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/Ancillary.Interop.csproj
+++ b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/Ancillary.Interop.csproj
@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <AssemblyName>Microsoft.Interop.Ancillary</AssemblyName>
-    <TargetFramework>$(NetCoreAppMinimum)</TargetFramework>
+    <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
     <RootNamespace>System.Runtime.InteropServices</RootNamespace>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
@@ -11,10 +11,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <Compile Include="$(LibrariesProjectRoot)Common/src/System/Runtime/InteropServices/GeneratedMarshallingAttribute.cs" Link="GeneratedMarshallingAttribute.cs" />
-    <Compile Include="$(LibrariesProjectRoot)Common/src/System/Runtime/InteropServices/CustomTypeMarshallerKind.cs" Link="CustomTypeMarshallerKind.cs" />
-    <Compile Include="$(LibrariesProjectRoot)Common/src/System/Runtime/InteropServices/CustomTypeMarshallerDirection.cs" Link="CustomTypeMarshallerDirection.cs" />
-    <Compile Include="$(LibrariesProjectRoot)Common/src/System/Runtime/InteropServices/CustomTypeMarshallerFeatures.cs" Link="CustomTypeMarshallerFeatures.cs" />
     <Compile Include="$(LibrariesProjectRoot)Common/src/System/Runtime/InteropServices/ArrayMarshaller.cs" Link="ArrayMarshaller.cs" />
   </ItemGroup>
 </Project>
diff --git a/src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/TestUtils.cs b/src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/TestUtils.cs
index b90ffc84f6a..83f943478d6 100644
--- a/src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/TestUtils.cs
+++ b/src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/TestUtils.cs
@@ -132,7 +132,7 @@ namespace LibraryImportGenerator.UnitTests
             var referenceAssemblies = await GetReferenceAssemblies(targetFramework);
 
             // [TODO] Can remove once ancillary logic is removed.
-            if (targetFramework is TestTargetFramework.Net6 or TestTargetFramework.Net)
+            if (targetFramework is TestTargetFramework.Net)
             {
                 referenceAssemblies = referenceAssemblies.Add(GetAncillaryReference());
             }
@@ -181,9 +181,9 @@ namespace LibraryImportGenerator.UnitTests
         /// <returns></returns>
         internal static MetadataReference GetAncillaryReference()
         {
-            // Include the assembly containing the new attribute and all of its references.
-            // [TODO] Remove once the attribute has been added to the BCL
-            var attrAssem = typeof(MarshalUsingAttribute).GetTypeInfo().Assembly;
+            // Include the assembly containing the new types we are considering exposing publicly
+            // but haven't put through API review.
+            var attrAssem = typeof(MarshalEx).GetTypeInfo().Assembly;
             return MetadataReference.CreateFromFile(attrAssem.Location);
         }