Fix bad assembly when a nested exported type is marked via link.xml

[mrvoorhe]

When a nested exported type is marked via link.xml and the declaring type is not marked, cecil will write out an invalid exported type table which will lead to exceptions such as

System.ArgumentOutOfRangeException : Specified argument was out of the range of valid values.
   at Mono.Collections.Generic.Collection`1.get_Item(Int32 index)
   at Mono.Cecil.MetadataReader.ReadExportedTypes()
   at Mono.Cecil.ModuleDefinition.<>c.<get_ExportedTypes>b__116_0(ModuleDefinition _, MetadataReader reader)
   at Mono.Cecil.ModuleDefinition.Read[TItem,TRet](TRet& variable, TItem item, Func`3 read)
   at Mono.Cecil.ModuleDefinition.get_ExportedTypes()
   at Mono.Cecil.MetadataResolver.GetType(ModuleDefinition module, TypeReference reference)
   at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
   at Mono.Linker.Tests.TestCasesRunner.TestResolver.TryResolve(TypeReference typeReference) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Trimming.Tests.Shared\TestResolver.cs:line 12
   at Mono.Linker.ModuleDefinitionExtensions.ResolveType(ModuleDefinition module, String typeFullName, ITryResolveMetadata resolver) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\ModuleDefinitionExtensions.cs:line 50
   at Mono.Linker.TypeNameResolver.<ResolveTypeName>g__GetSimpleTypeFromModule|6_0(TypeName typeName, ModuleDefinition module) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 135
   at Mono.Linker.TypeNameResolver.ResolveTypeName(AssemblyDefinition originalAssembly, TypeName typeName, List`1 typeResolutionRecords) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 101
   at Mono.Linker.TypeNameResolver.TryResolveTypeName(AssemblyDefinition assembly, String typeNameString, TypeReference& typeReference, List`1& typeResolutionRecords) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 44
   at Mono.Linker.Tests.TestCasesRunner.ResultChecker.VerifyLinkingOfOtherAssemblies(AssemblyDefinition original) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCasesRunner\ResultChecker.cs:line 320
   at Mono.Linker.Tests.TestCasesRunner.ResultChecker.Check(TrimmedTestCaseResult linkResult) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCasesRunner\ResultChecker.cs:line 114
   at Mono.Linker.Tests.TestCases.All.Run(TestCase testCase) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCases\TestSuites.cs:line 305
   at Mono.Linker.Tests.TestCases.All.TypeForwardingTests(TestCase testCase) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCases\TestSuites.cs:line 262
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)

Or if you were to try and open the linked Forwarder.dll without the fix in something like ILSpy, it would crash ILSpy.

An easy way to fix this seems to be marking the declaring type.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @sbomer, @vitek-karas
See info in area-owners.md if you want to be subscribed.