[illink] refactor code sharing between ILLink and MSBuild tasks (#9688)

We already have a few trimmer steps that run under both ILLink and the
`<LinkAssembliesNoShrink/>` MSBuild task:

* `AddKeepAlivesStep`
* `FixAbstractMethodsStep`
* `FixLegacyResourceDesignerStep`

We want to move various parts of the build to become new trimmer steps
like above.

Previously, we also had to share these steps with the Xamarin.Android
linker pipeline, but that's no longer the case. We can now cleanup how
these trimmer steps are shared between ILLink and MSBuild.

The benefits are:

* The `<LinkAssembliesNoShrink/>` task no longer has a list of subclassed
  trimmer steps.

* Much less `#if ILLINK` calls in shared trimmer steps.

A new `MSBuildLinkContext` class will be used for running trimmer steps in
an MSBuild task context. It handles MSBuild logging, etc.

I also removed various Mono linker source code that is unused.

Author: jonathanpeppers

src/Microsoft.Android.Sdk.ILLink/Microsoft.Android.Sdk.ILLink.csproj

@@ -12,6 +12,7 @@
     <ProjectReference Include="..\Xamarin.Android.Build.Tasks\Xamarin.Android.Build.Tasks.csproj" ReferenceOutputAssembly="False" />
 
     <!--Include shared linker sources-->
+    <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\External\Linker\BaseMarkHandler.cs" Link="External\BaseMarkHandler.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\AddKeepAlivesStep.cs" Link="MonoDroid.Tuner\AddKeepAlivesStep.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\AndroidLinkConfiguration.cs" Link="MonoDroid.Tuner\AndroidLinkConfiguration.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\Extensions.cs" Link="MonoDroid.Tuner\Extensions.cs" />

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/BaseStep.cs

@@ -46,7 +46,7 @@ public Tracer Tracer {
 			get { return _context.Tracer; }
 		}
 
-		public MarkingHelpers MarkingHelpers => _context.MarkingHelpers;
+		public void Initialize (LinkContext context) => _context = context;
 
 		public void Process (LinkContext context)
 		{
@@ -79,5 +79,19 @@ protected virtual void EndProcess ()
 		protected virtual void ProcessAssembly (AssemblyDefinition assembly)
 		{
 		}
+
+		public virtual void LogMessage (string message)
+		{
+			Context.LogMessage (message);
+		}
+
+		public virtual void LogError (int code, string message)
+		{
+#if ILLINK
+			Context.LogMessage (MessageContainer.CreateCustomErrorMessage (message, code, origin: new MessageOrigin ()));
+#else   // !ILLINK
+			Context.LogError ($"XA{code}", message);
+#endif  // !ILLINK
+		}
 	}
 }

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/IMarkHandler.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Mono.Linker.Steps
+{
+
+	/// <summary>
+	/// This API supports the product infrastructure and is not intended to be used directly from your code.
+	/// Extensibility point for custom logic that run during MarkStep, for marked members.
+	/// </summary>
+	public interface IMarkHandler
+	{
+		/// <summary>
+		/// Initialize is called at the beginning of MarkStep. This should be
+		/// used to perform global setup, and register callbacks through the
+		/// MarkContext.Register* methods) to be called when pieces of IL are marked.
+		/// </summary>
+		void Initialize (LinkContext context, MarkContext markContext);
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/MarkContext.cs

@@ -0,0 +1,40 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using Mono.Cecil;
+
+namespace Mono.Linker.Steps
+{
+	/// <summary>
+	/// Context which can be used to register actions to call during MarkStep
+	/// when various members are marked.
+	/// </summary>
+	public abstract class MarkContext
+	{
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked assembly.
+		/// </summary>
+		public abstract void RegisterMarkAssemblyAction (Action<AssemblyDefinition> action);
+
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked type.
+		/// </summary>
+		public abstract void RegisterMarkTypeAction (Action<TypeDefinition> action);
+
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked method.
+		/// </summary>
+		public abstract void RegisterMarkMethodAction (Action<MethodDefinition> action);
+	}
+
+	// Used within MSBuild and tests
+	class EmptyMarkContext : MarkContext
+	{
+		public override void RegisterMarkAssemblyAction (Action<AssemblyDefinition> action) { }
+
+		public override void RegisterMarkTypeAction (Action<TypeDefinition> action) { }
+
+		public override void RegisterMarkMethodAction (Action<MethodDefinition> action) { }
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/Annotations.cs

@@ -59,7 +59,6 @@ public partial class AnnotationStore {
 		protected readonly HashSet<CustomAttribute> marked_attributes = new HashSet<CustomAttribute> ();
 		readonly HashSet<TypeDefinition> marked_types_with_cctor = new HashSet<TypeDefinition> ();
 		protected readonly HashSet<TypeDefinition> marked_instantiated = new HashSet<TypeDefinition> ();
-		protected readonly HashSet<MethodDefinition> indirectly_called = new HashSet<MethodDefinition>();
 
 
 		public AnnotationStore (LinkContext context) => this.context = context;
@@ -167,24 +166,6 @@ public bool IsMarked (CustomAttribute attribute)
 			return marked_attributes.Contains (attribute);
 		}
 
-		public void MarkIndirectlyCalledMethod (MethodDefinition method)
-		{
-			if (!context.AddReflectionAnnotations)
-				return;
-
-			indirectly_called.Add (method);
-		}
-
-		public bool HasMarkedAnyIndirectlyCalledMethods ()
-		{
-			return indirectly_called.Count != 0;
-		}
-
-		public bool IsIndirectlyCalled (MethodDefinition method)
-		{
-			return indirectly_called.Contains (method);
-		}
-
 		public void MarkInstantiated (TypeDefinition type)
 		{
 			marked_instantiated.Add (type);

src/Microsoft.Android.Sdk.ILLink/BaseMarkHandler.cs renamed to src/Xamarin.Android.Build.Tasks/Linker/External/Linker/BaseMarkHandler.cs

@@ -1,4 +1,3 @@
-using Microsoft.Android.Sdk.ILLink;
 using Mono.Cecil;
 using Mono.Linker.Steps;