[dotnet] Merge the 'RemoveAttributesStep' into the 'StoreAttributesStep'

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -709,11 +709,6 @@
 			-->
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="SweepStep" Type="Xamarin.Linker.ManagedRegistrarLookupTablesStep" Condition="'$(Registrar)' == 'managed-static'" />
 
-			<!--
-				post-sweep custom steps
-			-->
-			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" AfterStep="SweepStep" Condition="'$(_AreAnyAssembliesTrimmed)' == 'true'" Type="Xamarin.Linker.Steps.PostSweepDispatcher" />
-
 			<!--
 				pre-output custom steps
 			-->

tools/dotnet-linker/Steps/StoreAttributesStep.cs

@@ -5,7 +5,23 @@
 #nullable enable
 
 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.
+	// The .NET linker comes with a way to remove attributes (by passing '--link-attributes
+	// some.xml' as a command-line argument), but this has a few problems:
+	//
+	// * We'd need to figure out which attributes to remove before running the linker,
+	//   but the code to figure out which optimizations have been enabled (and which attributes
+	//   should be removed) is in our custom linker code. We'd need to refactor a big chunk
+	//   of code to move this logic out of our custom linker code.
+	// * We need to keep the removed attributes around, because the static registrar needs
+	//   them. Our custom linker logic is not notified for removed attributes, which means
+	//   we'd need to store all attributes for the attribute types we're interested in (as
+	//   opposed to this solution, where we only store attributes that are actually removed).
+	// * The attributes we want removed may contain references to types we don't want
+	//   linked away. If we ask the linker to remove those attributes, then the types may
+	//   be linked away as well, and there's no good way around this.
+	//
+	// The end result is that a custom step is the best solution for now.
+
 	public class StoreAttributesStep : AttributeIteratorBaseStep {
 		protected override void ProcessAttribute (ICustomAttributeProvider provider, CustomAttribute attribute, out bool remove)
 		{
@@ -31,10 +47,24 @@ protected override void ProcessAttribute (ICustomAttributeProvider provider, Cus
 					break;
 				}
 				break;
-			case "Foundation":
+			case Namespaces.Foundation:
 				switch (attr_type.Name) {
 				case "ProtocolAttribute":
+				case "ProtocolMemberAttribute":
+					store = LinkContext.App.Optimizations.RegisterProtocols == true;
+					remove = store;
+					break;
+				}
+				break;
+			case Namespaces.ObjCRuntime:
+				switch (attr_type.Name) {
+				case "NativeNameAttribute":
+					store = true;
+					remove = store;
+					break;
+				case "AdoptsAttribute":
 					store = LinkContext.App.Optimizations.RegisterProtocols == true;
+					remove = store;
 					break;
 				}
 				break;