[dotnet-linker] Use [DynamicDependency] attributes instead of manual marking when marking static registrar methods.

This makes it easier to move this code out of a custom linker step in the future.

Contributes towards #17693.

Author: rolfbjarne

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -553,6 +553,7 @@
 			<_UseDynamicDependenciesForGeneratedCodeOptimizations Condition="'$(_UseDynamicDependenciesForGeneratedCodeOptimizations)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForGeneratedCodeOptimizations>
 			<_UseLinkDescriptionForApplyPreserveAttribute Condition="'$(_UseLinkDescriptionForApplyPreserveAttribute)' == '' And '$(_XamarinRuntime)' == 'NativeAOT'">true</_UseLinkDescriptionForApplyPreserveAttribute>
 			<_UseLinkDescriptionForApplyPreserveAttribute Condition="'$(_UseLinkDescriptionForApplyPreserveAttribute)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseLinkDescriptionForApplyPreserveAttribute>
+			<_UseDynamicDependenciesForMarkStaticRegistrar Condition="'$(_UseDynamicDependenciesForMarkStaticRegistrar)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForMarkStaticRegistrar>
 		</PropertyGroup>
 
 		<PropertyGroup>
@@ -759,6 +760,7 @@
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="Xamarin.Linker.Steps.PreserveBlockCodeStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseDynamicDependenciesForBlockCodePreservation)' == 'true'" />
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="Xamarin.Linker.Steps.OptimizeGeneratedCodeStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseDynamicDependenciesForGeneratedCodeOptimizations)' == 'true'" />
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="Xamarin.Linker.Steps.ApplyPreserveAttributeStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseLinkDescriptionForApplyPreserveAttribute)' == 'true'" />
+			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="Xamarin.Linker.Steps.MarkForStaticRegistrarStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseDynamicDependenciesForMarkStaticRegistrar)' == 'true'" />
 			<!-- The final decision to remove/keep the dynamic registrar must be done before the linking step -->
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="MonoTouch.Tuner.RegistrarRemovalTrackingStep" />
 			<!-- TODO: these steps should probably run after mark. -->

tools/common/DerivedLinkContext.cs

@@ -49,6 +49,7 @@ public class DerivedLinkContext : LinkContext {
 		Dictionary<TypeDefinition, LinkedAwayTypeReference> LinkedAwayTypeMap = new Dictionary<TypeDefinition, LinkedAwayTypeReference> ();
 
 		public bool DidRunApplyPreserveAttributeStep { get; set; }
+		public bool DidRunMarkForStaticRegistrarStep { get; set; }
 
 		public DerivedLinkContext (LinkerConfiguration configuration, Application app)
 #if !LEGACY_TOOLS

tools/dotnet-linker/MarkForStaticRegistrar.cs

@@ -23,6 +23,10 @@ public class MarkForStaticRegistrar : ConfigurationAwareSubStep {
 
 		public override bool IsActiveFor (AssemblyDefinition assembly)
 		{
+			// It's either this step, or MarkForStaticRegistrarStep. If MarkForStaticRegistrarStep already ran, then we shouldn't run this step.
+			if (Configuration.DerivedLinkContext.DidRunMarkForStaticRegistrarStep)
+				return false;
+
 			if (Configuration.Application.Optimizations.OptimizeBlockLiteralSetupBlock != true)
 				return false;
 

tools/dotnet-linker/MarkForStaticRegistrarStep.cs

@@ -0,0 +1,66 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Linq;
+
+using Mono.Cecil;
+using Mono.Linker;
+using Registrar;
+using Xamarin.Bundler;
+
+#nullable enable
+
+namespace Xamarin.Linker.Steps {
+
+	// This class marks APIs referenced by the native code generated by the static registrar.
+	public class MarkForStaticRegistrarStep : AssemblyModifierStep {
+		protected override string Name { get => "Mark For Static Registrar"; }
+
+		protected override int ErrorCode { get => 2460; }
+
+		protected override bool IsActiveFor (AssemblyDefinition assembly)
+		{
+			if (Configuration.Application.Optimizations.OptimizeBlockLiteralSetupBlock != true)
+				return false;
+
+			if (Configuration.Application.Registrar != RegistrarMode.Static)
+				return false;
+
+			return Annotations.GetAction (assembly) == AssemblyAction.Link;
+		}
+
+		protected override void TryProcess ()
+		{
+			DerivedLinkContext.DidRunMarkForStaticRegistrarStep = true;
+			base.TryProcess ();
+		}
+
+		protected override bool ProcessType (TypeDefinition type)
+		{
+			return base.ProcessMethods (type);
+		}
+
+		protected override bool ProcessMethod (MethodDefinition method)
+		{
+			return ProcessDelegateProxyAttribute (method);
+		}
+
+		// Mark the Invoke method in the type pointed to by the DelegateProxy attribute,
+		// because it may only be referenced from native code when using the static registrar.
+		bool ProcessDelegateProxyAttribute (MethodDefinition method)
+		{
+			if (!StaticRegistrar.IsDelegate (method.ReturnType.Resolve ()))
+				return false;
+
+			var getDelegateProxyType = DerivedLinkContext.StaticRegistrar.GetDelegateProxyType (method);
+			if (getDelegateProxyType is null)
+				return false;
+
+			var invokeMethod = getDelegateProxyType.Methods.SingleOrDefault (m => m.Name == "Invoke");
+			if (invokeMethod is null)
+				return false;
+
+			return abr.AddDynamicDependencyAttribute (method, invokeMethod);
+		}
+	}
+}