[dotnet-linker] Add a StoreAttributesStep to store attributes that are removed by the linker.

Add a StoreAttributesStep to store attributes that are removed by the linker, but
that the static registrar needs.

In particular, in .NET 6 the linker removes the System.Runtime.CompilerServices.ExtensionAttribute,
which the static registrar needs to handle category methods properly.

This involved copying and slightly modifying the RemoveAttributesBase code.

tools/dotnet-linker/SetupStep.cs

@@ -80,6 +80,7 @@ namespace Xamarin {
 				prelink_substeps.Add (new MarkNSObjects ());
 				prelink_substeps.Add (new PreserveSmartEnumConversionsSubStep ());
 				prelink_substeps.Add (new CollectUnmarkedMembersSubStep ());
+				prelink_substeps.Add (new StoreAttributesStep ());
 
 				post_sweep_substeps.Add (new RemoveAttributesStep ());
 			}

tools/dotnet-linker/Steps/AttributeIteratorBaseStep.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections;
+
+using Mono.Linker;
+using Mono.Linker.Steps;
+
+using Mono.Cecil;
+using Xamarin.Tuner;
+
+namespace Xamarin.Linker.Steps {
+
+	public abstract class AttributeIteratorBaseStep : BaseSubStep {
+
+		protected DerivedLinkContext LinkContext {
+			get {
+				return LinkerConfiguration.GetInstance (Context).DerivedLinkContext;
+			}
+		}
+
+		public override SubStepTargets Targets {
+			get {
+				return SubStepTargets.Assembly
+					| SubStepTargets.Type
+					| SubStepTargets.Field
+					| SubStepTargets.Method
+					| SubStepTargets.Property
+					| SubStepTargets.Event;
+			}
+		}
+
+		public override bool IsActiveFor (AssemblyDefinition assembly)
+		{
+			return Annotations.GetAction (assembly) == AssemblyAction.Link;
+		}
+
+		public override void ProcessAssembly (AssemblyDefinition assembly)
+		{
+			ProcessAttributeProvider (assembly);
+			ProcessAttributeProvider (assembly.MainModule);
+		}
+
+		public override void ProcessType (TypeDefinition type)
+		{
+			ProcessAttributeProvider (type);
+
+			if (type.HasGenericParameters)
+				ProcessAttributeProviderCollection (type.GenericParameters);
+		}
+
+		void ProcessAttributeProviderCollection (IList list)
+		{
+			for (int i = 0; i < list.Count; i++)
+				ProcessAttributeProvider ((ICustomAttributeProvider) list [i]);
+		}
+
+		public override void ProcessField (FieldDefinition field)
+		{
+			ProcessAttributeProvider (field);
+		}
+
+		public override void ProcessMethod (MethodDefinition method)
+		{
+			ProcessMethodAttributeProvider (method);
+		}
+
+		void ProcessMethodAttributeProvider (MethodDefinition method)
+		{
+			ProcessAttributeProvider (method);
+			ProcessAttributeProvider (method.MethodReturnType);
+
+			if (method.HasParameters)
+				ProcessAttributeProviderCollection (method.Parameters);
+
+			if (method.HasGenericParameters)
+				ProcessAttributeProviderCollection (method.GenericParameters);
+		}
+
+		public override void ProcessProperty (PropertyDefinition property)
+		{
+			ProcessAttributeProvider (property);
+		}
+
+		public override void ProcessEvent (EventDefinition @event)
+		{
+			ProcessAttributeProvider (@event);
+		}
+
+		void ProcessAttributeProvider (ICustomAttributeProvider provider)
+		{
+			if (!provider.HasCustomAttributes)
+				return;
+
+			for (int i = 0; i < provider.CustomAttributes.Count; i++) {
+				var attrib = provider.CustomAttributes [i];
+				ProcessAttribute (provider, attrib, out var remove);
+
+				if (remove)
+					provider.CustomAttributes.RemoveAt (i--);
+			}
+		}
+
+		protected abstract void ProcessAttribute (ICustomAttributeProvider provider, CustomAttribute attribute, out bool remove);
+	}
+}

tools/dotnet-linker/Steps/RemoveAttributesStep.cs

@@ -1,8 +1,6 @@
 using System;
 
 using Mono.Cecil;
-using Mono.Tuner;
-using Xamarin.Tuner;
 
 namespace Xamarin.Linker.Steps {
 	// The .NET linker comes with a way to remove attributes (by passing '--link-attributes
@@ -22,14 +20,16 @@ namespace Xamarin.Linker.Steps {
 	// 
 	// The end result is that a custom step is the best solution for now.
 
-	public class RemoveAttributesStep : RemoveAttributesBase {
+	public class RemoveAttributesStep : AttributeIteratorBaseStep {
-		protected DerivedLinkContext LinkContext {
+		protected override void ProcessAttribute (ICustomAttributeProvider provider, CustomAttribute attribute, out bool remove)
-			get {
+		{
-				return LinkerConfiguration.GetInstance (Context).DerivedLinkContext;
+			remove = IsRemovedAttribute (attribute);
-			}
+
+			if (remove)
+				LinkContext.StoreCustomAttribute (provider, attribute);
 		}
 
-		protected override bool IsRemovedAttribute (CustomAttribute attribute)
+		bool IsRemovedAttribute (CustomAttribute attribute)
 		{
 			// this avoids calling FullName (which allocates a string)
 			var attr_type = attribute.Constructor.DeclaringType;
@@ -51,11 +51,5 @@ namespace Xamarin.Linker.Steps {
 				return false;
 			}
 		}
-
-		protected override void WillRemoveAttribute (ICustomAttributeProvider provider, CustomAttribute attribute)
-		{
-			LinkContext.StoreCustomAttribute (provider, attribute);
-			base.WillRemoveAttribute (provider, attribute);
-		}
 	}
 }

tools/dotnet-linker/Steps/StoreAttributesStep.cs

@@ -0,0 +1,30 @@
+using System;
+
+using Mono.Cecil;
+
+namespace Xamarin.Linker.Steps {
+	// The registrar needs some of the system attributes that the linker might remove, so store those elsewhere for the static registrar's use.
+	public class StoreAttributesStep : AttributeIteratorBaseStep {
+		protected override void ProcessAttribute (ICustomAttributeProvider provider, CustomAttribute attribute, out bool remove)
+		{
+			// don't remove any of these
+			remove = false;
+
+			// this avoids calling FullName (which allocates a string)
+			var attr_type = attribute.Constructor.DeclaringType;
+			var store = false;
+			switch (attr_type.Namespace) {
+			case "System.Runtime.CompilerServices":
+				switch (attr_type.Name) {
+				case "ExtensionAttribute":
+					store = true;
+					break;
+				}
+				break;
+			}
+
+			if (store)
+				LinkContext.StoreCustomAttribute (provider, attribute);
+		}
+	}
+}

tools/dotnet-linker/dotnet-linker.csproj

@@ -176,9 +176,6 @@
     <Compile Include="..\..\builds\mono-ios-sdk-destdir\ios-sources\external\linker\src\tuner\Mono.Tuner\CecilRocks.cs">
       <Link>external\mono-archive\Mono.Tuner\CecilRocks.cs</Link>
     </Compile>
-    <Compile Include="..\..\builds\mono-ios-sdk-destdir\ios-sources\external\linker\src\tuner\Mono.Tuner\RemoveAttributesBase.cs">
-      <Link>external\mono-archive\Mono.Tuner\RemoveAttributesBase.cs</Link>
-    </Compile>
     <Compile Include="..\..\external\Xamarin.MacDev\Xamarin.MacDev\PListObject.cs">
       <Link>external\Xamarin.MacDev\Xamarin.MacDev\PListObject.cs</Link>
     </Compile>