Fix vararg P/Invoke cross-assembly metadata lookup using wrong module

src/coreclr/vm/dllimport.cpp

@@ -4556,7 +4556,7 @@ static void CreatePInvokeStubAccessMetadata(
 
     (*pNumArgs) = msig.NumFixedArgs();
 
-    IMDInternalImport* pInternalImport = pSigDesc->m_pModule->GetMDImport();
+    IMDInternalImport* pInternalImport = pSigDesc->m_pMetadataModule->GetMDImport();
 
     _ASSERTE(!SF_IsHRESULTSwapping(*pdwStubFlags));
 

src/tests/Interop/Interop.csproj

@@ -12,6 +12,7 @@
     <SupportProject Include="$(TestLibraryProjectPath)" />
     <SupportProject Include="PInvoke/Miscellaneous/CopyCtor/CopyCtorUtil.ilproj" />
     <SupportProject Include="PInvoke/Miscellaneous/MultipleAssembliesWithSamePInvoke/ManagedDll*/*.csproj" />
+    <SupportProject Include="PInvoke/Varargs/CrossAssembly/VarArgsPInvokeLib/*.csproj" />
     <MergedWrapperProjectReference Include="*/**/*.??proj" Exclude="@(SupportProject)" />
     <ProjectReference Include="@(SupportProject)" />
   </ItemGroup>
@@ -42,7 +43,7 @@
     <Compile Include="PInvoke/SafeHandles/**/*.cs" />
     <Compile Include="PInvoke/SetLastError/**/*.cs" />
     <Compile Include="PInvoke/SizeParamIndex/**/*.cs" />
-    <Compile Include="PInvoke/Varargs/**/*.cs" />
+    <Compile Include="PInvoke/Varargs/**/*.cs" Exclude="PInvoke/Varargs/CrossAssembly/VarArgsPInvokeLib/*.cs" />
     <Compile Include="PInvoke/Vector2_3_4/**/*.cs" />
 
     <!-- Reference the CMake projects for the directly-compiled-in tests -->

src/tests/Interop/PInvoke/Varargs/CrossAssembly/CrossAssemblyVarargsTest.cs

@@ -0,0 +1,28 @@
+// 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.Text;
+using TestLibrary;
+using VarArgsPInvokeLib;
+using Xunit;
+
+namespace PInvokeTests
+{
+    public class CrossAssemblyVarargsTest
+    {
+        [ConditionalFact(typeof(TestLibrary.PlatformDetection), nameof(TestLibrary.PlatformDetection.IsVarArgSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/91388", typeof(TestLibrary.PlatformDetection), nameof(TestLibrary.PlatformDetection.PlatformDoesNotSupportNativeTestAssets))]
+        public static void TestCrossAssemblyVarArgs()
+        {
+            int arg1 = 10;
+            int arg2 = 20;
+            double arg3 = 12.5;
+            string expected = FormattableString.Invariant($"{arg1}, {arg2}, {arg3:F1}");
+
+            var builder = new StringBuilder(30);
+            VarArgsWrapper.TestVarArgs(builder, (IntPtr)30, "%i, %i, %.1f", __arglist(arg1, arg2, arg3));
+            Assert.Equal(expected, builder.ToString());
+        }
+    }
+}

src/tests/Interop/PInvoke/Varargs/CrossAssembly/VarArgsPInvokeLib/VarArgsPInvokeLib.cs

@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using System.Text;
+
+namespace VarArgsPInvokeLib
+{
+    public static class VarArgsWrapper
+    {
+        [DllImport("VarargsNative", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void TestVarArgs(StringBuilder builder, nint bufferSize, string formatString, __arglist);
+    }
+}

src/tests/Interop/PInvoke/Varargs/CrossAssembly/VarArgsPInvokeLib/VarArgsPInvokeLib.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>