AssemblyScanner doesn't scan message assemblies that reference Message Interfaces

src/NServiceBus.Core.Tests/AssemblyScanner/When_using_type_forwarding.cs

@@ -0,0 +1,44 @@
+namespace NServiceBus.Core.Tests.AssemblyScanner;
+
+using System.IO;
+using System.Linq;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+using Hosting.Helpers;
+using NUnit.Framework;
+
+[TestFixture]
+public class When_using_type_forwarding
+{
+    // This test is not perfect since it relies on existing binaries to covered assembly scanning scenarios. Since we
+    // already use those though the idea of this test is to make sure that the assembly scanner is able to scan all
+    // assemblies that have a type forwarding rule within the core assembly. This might turn out to be a broad assumption
+    // in the future, and we might have to explicitly remove some but in the meantime this test would have covered us
+    // when we moved ICommand, IEvent and IMessages to the message interfaces assembly.
+    [Test]
+    public void Should_scan_assemblies_indicated_by_the_forwarding_metadata()
+    {
+        using var fs = File.OpenRead(typeof(AssemblyScanner).Assembly.Location);
+        using var peReader = new PEReader(fs);
+        var metadataReader = peReader.GetMetadataReader();
+
+        // Exported types only contains a small subset of types, so it's safe to enumerate all of them
+        var assemblyNamesOfForwardedTypes = metadataReader.ExportedTypes
+            .Select(exportedTypeHandle => metadataReader.GetExportedType(exportedTypeHandle)).Where(exportedType => exportedType.IsForwarder)
+            .Select(exportedType => (AssemblyReferenceHandle)exportedType.Implementation)
+            .Select(assemblyReferenceHandle => metadataReader.GetAssemblyReference(assemblyReferenceHandle))
+            .Select(assemblyReference => metadataReader.GetString(assemblyReference.Name))
+            .Distinct()
+            .ToList();
+
+        var scanner = new AssemblyScanner(Path.Combine(TestContext.CurrentContext.TestDirectory, "TestDlls"))
+        {
+            ScanAppDomainAssemblies = false
+        };
+
+        var result = scanner.GetScannableAssemblies();
+        var assemblyFullNames = result.Assemblies.Select(a => a.GetName().Name).ToList();
+
+        CollectionAssert.IsSubsetOf(assemblyNamesOfForwardedTypes, assemblyFullNames);
+    }
+}