[dotnet] Prevent linking out code referenced by P/Invoke

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -314,6 +314,10 @@
 		<ReadItemsFromFile File="$(_LinkerItemsDirectory)/_RegistrarFile.items" Condition="Exists('$(_LinkerItemsDirectory)/_RegistrarFile.items')">
 			<Output TaskParameter="Items" ItemName="_RegistrarFile" />
 		</ReadItemsFromFile>
+		<!-- Load _RegistrarFile -->
+		<ReadItemsFromFile File="$(_LinkerItemsDirectory)/_ReferencesFile.items" Condition="Exists('$(_LinkerItemsDirectory)/_ReferencesFile.items')">
+			<Output TaskParameter="Items" ItemName="_ReferencesFile" />
+		</ReadItemsFromFile>
 
 		<ItemGroup>
 			<_AssembliesToAOT Include="@(ResolvedFileToPublish)" Condition="'%(Extension)' == '.dll' Or '%(Extension)' == '.exe' " />
@@ -467,6 +471,9 @@
 			<_CompileNativeExecutableFile Include="@(_RegistrarFile)">
 				<OutputFile>$(_IntermediateNativeLibraryDir)%(Filename).o</OutputFile>
 			</_CompileNativeExecutableFile>
+			<_CompileNativeExecutableFile Include="@(_ReferencesFile)">
+				<OutputFile>$(_IntermediateNativeLibraryDir)%(Filename).o</OutputFile>
+			</_CompileNativeExecutableFile>
 			<_XamarinMainIncludeDirectory Include="$(_XamarinIncludeDirectory)" />
 		</ItemGroup>
 	</Target>

tools/dotnet-linker/SetupStep.cs

@@ -89,6 +89,7 @@ protected override void TryProcess ()
 			Steps.Add (new ExtractBindingLibrariesStep ());
 			Steps.Add (new RegistrarStep ());
 			Steps.Add (new GenerateMainStep ());
+			Steps.Add (new GenerateReferencesStep ());
 			Steps.Add (new GatherFrameworksStep ());
 
 			Configuration.Write ();

tools/dotnet-linker/Steps/GenerateReferencesStep.cs

@@ -0,0 +1,74 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+using Mono.Cecil;
+
+using Xamarin.Bundler;
+using Xamarin.Linker;
+
+namespace Xamarin {
+
+	public class GenerateReferencesStep : ConfigurationAwareStep {
+		private Symbols required_symbols = new Symbols ();
+
+		protected override string Name { get; } = "Generate References";
+		protected override int ErrorCode { get; } = 2320;
+
+		protected override void TryEndProcess ()
+		{
+			base.TryEndProcess ();
+
+			var items = new List<MSBuildItem> ();
+			var file = Path.Combine (Configuration.CacheDirectory, $"references.mm");
+			if (Configuration.Target.GenerateReferencingSource (file, required_symbols) != null) {
+				var item = new MSBuildItem { Include = file };
+				items.Add (item);
+			}
+			Configuration.WriteOutputForMSBuild ("_ReferencesFile", items);
+		}
+
+		protected override void TryProcessAssembly (AssemblyDefinition assembly)
+		{
+			base.TryProcessAssembly (assembly);
+
+			if (!assembly.MainModule.HasTypes)
+				return;
+
+			var hasSymbols = false;
+			if (assembly.MainModule.HasModuleReferences) {
+				hasSymbols = true;
+			} else if (assembly.MainModule.HasTypeReference (Namespaces.Foundation + ".FieldAttribute")) {
+				hasSymbols = true;
+			}
+			if (!hasSymbols)
+				return;
+
+			foreach (var type in assembly.MainModule.Types)
+				ProcessType (type);
+		}
+
+		void ProcessType (TypeDefinition type)
+		{
+			if (type.HasNestedTypes) {
+				foreach (var nested in type.NestedTypes)
+					ProcessType (nested);
+			}
+
+			if (type.HasMethods) {
+				foreach (var method in type.Methods)
+					ProcessMethod (method);
+			}
+		}
+
+		void ProcessMethod (MethodDefinition method)
+		{
+			if (method.IsPInvokeImpl && method.HasPInvokeInfo && method.PInvokeInfo != null) {
+				var pinfo = method.PInvokeInfo;
+				if (pinfo.Module.Name == "__Internal") {
+					required_symbols.AddFunction (pinfo.EntryPoint).AddMember (method);
+				}
+			}
+		}
+	}
+}