[dotnet-linker] Use [DynamicDependency] attributes instead of manual marking when applying the [Preserve] attribute.

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

This was simplified a bit because we already had code to do this, but it was only
in effect when using NativeAOT. Generalize it, move it out of the marking phase of
the linker, and run it before marking.

Also move the removal of any [Preserve] attributes to the RemoveAttributes step.

Contributes towards #17693.

Author: rolfbjarne

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -551,6 +551,8 @@
 			<_UseDynamicDependenciesForSmartEnumPreservation Condition="'$(_UseDynamicDependenciesForSmartEnumPreservation)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForSmartEnumPreservation>
 			<_UseDynamicDependenciesForBlockCodePreservation Condition="'$(_UseDynamicDependenciesForBlockCodePreservation)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForBlockCodePreservation>
 			<_UseDynamicDependenciesForGeneratedCodeOptimizations Condition="'$(_UseDynamicDependenciesForGeneratedCodeOptimizations)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForGeneratedCodeOptimizations>
+			<_UseDynamicDependenciesForApplyPreserveAttribute Condition="'$(_UseDynamicDependenciesForApplyPreserveAttribute)' == '' And '$(_XamarinRuntime)' == 'NativeAOT'">true</_UseDynamicDependenciesForApplyPreserveAttribute>
+			<_UseDynamicDependenciesForApplyPreserveAttribute Condition="'$(_UseDynamicDependenciesForApplyPreserveAttribute)' == ''">$(_UseDynamicDependenciesInsteadOfMarking)</_UseDynamicDependenciesForApplyPreserveAttribute>
 		</PropertyGroup>
 
 		<PropertyGroup>
@@ -756,6 +758,7 @@
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="MarkStep" Type="Xamarin.Linker.Steps.PreserveSmartEnumConversionsStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseDynamicDependenciesForSmartEnumPreservation)' == 'true'" />
 			<_TrimmerCustomSteps Include="$(_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="'$(_AreAnyAssembliesTrimmed)' == 'true' And '$(_UseDynamicDependenciesForApplyPreserveAttribute)' == '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

@@ -48,6 +48,8 @@ public class DerivedLinkContext : LinkContext {
 		// so we need a second dictionary
 		Dictionary<TypeDefinition, LinkedAwayTypeReference> LinkedAwayTypeMap = new Dictionary<TypeDefinition, LinkedAwayTypeReference> ();
 
+		public bool DidRunApplyPreserveAttributeStep { get; set; }
+
 		public DerivedLinkContext (LinkerConfiguration configuration, Application app)
 #if !LEGACY_TOOLS
 			: base (configuration)

tools/dotnet-linker/AppBundleRewriter.cs

@@ -1384,7 +1384,7 @@ public bool AddAttributeToStaticConstructor (TypeDefinition onType, CustomAttrib
 			return modified;
 		}
 
-		MethodDefinition GetOrCreateStaticConstructor (TypeDefinition type, out bool modified)
+		public MethodDefinition GetOrCreateStaticConstructor (TypeDefinition type, out bool modified)
 		{
 			modified = false;
 
@@ -1408,7 +1408,7 @@ MethodDefinition GetOrCreateStaticConstructor (TypeDefinition type, out bool mod
 		/// <param name="provider">The provider to which the attribute should be added.</param>
 		/// <param name="attribute">The attribute to add.</param>
 		/// <returns>Whether the attribute was added or not.</returns>
-		bool AddAttributeOnlyOnce (ICustomAttributeProvider provider, CustomAttribute attribute)
+		public bool AddAttributeOnlyOnce (ICustomAttributeProvider provider, CustomAttribute attribute)
 		{
 			if (provider.HasCustomAttributes) {
 				foreach (var ca in provider.CustomAttributes) {