[illink] Move FixLegacyResourceDesignerStep to post-trim pipeline (#11059)

Migrate `FixLegacyResourceDesignerStep` out of the ILLink trimmer process
and into PostTrimmingPipeline, continuing the work in #10842 to remove
custom ILLink steps. The step now runs after ILLink via a thin wrapper
(PostTrimmingFixLegacyResourceDesignerStep) that calls
ProcessAssemblyDesigner() directly, matching the former ILLink behavior.

- Remove all #if ILLINK conditionals from FixLegacyResourceDesignerStep,
  LinkDesignerBase, and BaseStep
- Remove FixLegacyResourceDesignerStep and LinkDesignerBase from the
  ILLink project (no longer compiled as a trimmer step)
- Add UseDesignerAssembly property to PostTrimmingPipeline task
- Wire AndroidUseDesignerAssembly through targets to PostTrimmingPipeline
- Remove _TrimmerCustomSteps entry for FixLegacyResourceDesignerStep

### [targets] Fix NativeAOT designer assembly trimming

In NativeAOT builds, _AddResourceDesignerToPublishFiles ran after
_ComputeManagedAssemblyToLink, so the designer assembly was not in
ManagedAssemblyToLink. ILLink skipped the designer assembly entirely
and did not rewrite its netstandard references.

Fix by adding _AddResourceDesignerToPublishFiles to _PrepareLinking's
DependsOnTargets, so the designer is passed to ILLink.

### Fix merge conflict: remove FixAbstractMethodsStep from ILLink csproj

FixAbstractMethodsStep was moved from ILLink to the post-trim pipeline
on main (7ae24aa). The merge conflict resolution incorrectly kept the
Compile include, but the file now uses types not available in the ILLink
project (IAssemblyModifierPipelineStep, StepContext, Properties.Resources).

### Root designer assembly to prevent ILLink from trimming resource properties

FixLegacyResourceDesignerStep now runs in PostTrimmingPipeline (after ILLink)
instead of as an ILLink custom step. Previously, the step ran before MarkStep
and rewrote library assemblies to reference designer properties, which caused
MarkStep to preserve them. Now that the rewriting happens after ILLink, we
must root the designer assembly so all resource properties survive trimming.

Add TrimmerRootAssembly for the designer in _AddResourceDesignerToPublishFiles.
This keeps IsTrimmable=true (action=link) so ILLink still rewrites the
netstandard assembly reference, but roots all types so nothing is trimmed.

An alternative approach would be to run FixLegacyResourceDesignerStep before
ILLink instead of after. That would allow ILLink to trim unused resource
properties from the designer (since the rewritten references would already be
in place for MarkStep), but it would also process resource references in
assemblies that ILLink may later remove entirely.

### Use TrimmerRootDescriptor instead of TrimmerRootAssembly for designer assembly

TrimmerRootAssembly (-a flag) makes the designer an entry point, which
causes ILLink to retain netstandard.dll as a dependency, leaking it into
the APK (+19KB). TrimmerRootDescriptor (-x flag) preserves all designer
types/members without making it an entry point, avoiding the regression.

### Update SkiaSharp test to expect XA8000 instead of IL8000 for release builds

FixLegacyResourceDesignerStep now runs in PostTrimmingPipeline (MSBuild
task) instead of inside ILLink, so the error code is always XA8000
regardless of build configuration.

### Fix incremental build: use WriteOnlyWhenDifferent for linker descriptor

WriteLinesToFile was regenerating the TrimmerRootDescriptor XML on every
build, changing its timestamp and causing ILLink to re-run on incremental
builds. Adding WriteOnlyWhenDifferent preserves the timestamp when the
content hasn't changed.

- Add TODO to simplify TrimmerRootDescriptor once dotnet/runtime#126518 is fixed

### Update APK size reference baselines

FixLegacyResourceDesignerStep now runs post-ILLink with preserve=all,
keeping all designer properties instead of only those referenced by
libraries. This increases the designer assembly from ~19KB to ~135KB
in the XForms MonoVM test app (1% of total APK size). Other baselines
updated for SDK changes from the main branch merge.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sbomer

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

@@ -16,8 +16,6 @@
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\External\Linker\BaseMarkHandler.cs" Link="External\BaseMarkHandler.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" />
-    <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\FixLegacyResourceDesignerStep.cs" Link="MonoDroid.Tuner\FixLegacyResourceDesignerStep.cs" />
-    <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\LinkDesignerBase.cs" Link="MonoDroid.Tuner\LinkDesignerBase.cs" />
 
     <!--Other .NET for Android / Java.Interop files-->
     <Compile Include="..\..\external\Java.Interop\src\Java.Interop.Tools.Cecil\Java.Interop.Tools.Cecil\CustomAttributeProviderRocks.cs" Link="Java.Interop\CustomAttributeProviderRocks.cs" />

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

@@ -89,11 +89,7 @@ public virtual void LogMessage (string 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/MonoDroid.Tuner/FixLegacyResourceDesignerStep.cs

@@ -14,18 +14,11 @@
 using Mono.Linker.Steps;
 
 using Mono.Tuner;
-#if ILLINK
-using Resources = Microsoft.Android.Sdk.ILLink.Properties.Resources;
-#else   // !ILLINK
 using Resources = Xamarin.Android.Tasks.Properties.Resources;
-#endif  // ILLINK
 
 namespace MonoDroid.Tuner
 {
-	public class FixLegacyResourceDesignerStep : LinkDesignerBase
-#if !ILLINK
-		, Xamarin.Android.Tasks.IAssemblyModifierPipelineStep
-#endif  // !ILLINK
+	public class FixLegacyResourceDesignerStep : LinkDesignerBase, Xamarin.Android.Tasks.IAssemblyModifierPipelineStep
 	{
 		internal const string DesignerAssemblyName = "_Microsoft.Android.Resource.Designer";
 		internal const string DesignerAssemblyNamespace = "_Microsoft.Android.Resource.Designer";
@@ -36,14 +29,6 @@ public class FixLegacyResourceDesignerStep : LinkDesignerBase
 		Dictionary<string, MethodDefinition> lookup;
 		Dictionary<string, MethodDefinition> lookupCaseInsensitive;
 
-		protected override void EndProcess ()
-		{
-			if (designerAssembly != null) {
-				LogMessage ($"  Setting Action on {designerAssembly.Name} to Link.");
-				Annotations.SetAction (designerAssembly, AssemblyAction.Link);
-			}
-		}
-
 		protected override void LoadDesigner ()
 		{
 			if (designerLoaded)
@@ -72,7 +57,6 @@ protected override void LoadDesigner ()
 			}
 		}
 
-#if !ILLINK
 		public void ProcessAssembly (AssemblyDefinition assembly, Xamarin.Android.Tasks.StepContext context)
 		{
 			// Only run this step on non-main user Android assemblies
@@ -81,7 +65,6 @@ public void ProcessAssembly (AssemblyDefinition assembly, Xamarin.Android.Tasks.
 
 			context.IsAssemblyModified |= ProcessAssemblyDesigner (assembly);
 		}
-#endif  // !ILLINK
 
 		internal override bool ProcessAssemblyDesigner (AssemblyDefinition assembly)
 		{

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/LinkDesignerBase.cs

@@ -4,43 +4,27 @@
 using Mono.Linker;
 using Mono.Linker.Steps;
 using System;
-using System.Linq;
 using Xamarin.Android.Tasks;
 using System.Collections.Generic;
 using System.Globalization;
 using Mono.Cecil.Cil;
 using System.Text.RegularExpressions;
 using Mono.Collections.Generic;
-#if ILLINK
-using Microsoft.Android.Sdk.ILLink;
-#endif
 
 
 namespace MonoDroid.Tuner  {
 	public abstract class LinkDesignerBase : BaseStep
 	{
 		protected IMetadataResolver Cache => Context;
 
-		public
-#if !ILLINK
-		override
-#endif
-		void LogMessage (string message)
+		public override void LogMessage (string message)
 		{
 			Context.LogMessage (message);
 		}
 
-		public
-#if !ILLINK
-		override
-#endif
-		void LogError (int code, string error)
+		public override void LogError (int code, string error)
 		{
-#if ILLINK
-			Context.LogMessage (MessageContainer.CreateCustomErrorMessage (error, code, origin: new MessageOrigin ()));
-#else   // !ILLINK
 			Context.LogError ($"XA{code}", error);
-#endif  // !ILLINK
 		}
 
 		public virtual AssemblyDefinition Resolve (AssemblyNameReference name)

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Resource.Designer.targets

@@ -226,18 +226,43 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     In additon we MUST set the `PostprocessAssembly` metadata to `true` so that the file
     is processed by the ILLink step. If we do not do this then the reference to
     `netstandard.dll` is not replaced with `System.Private.CoreLib` and the app crashes.
+
+    We use a TrimmerRootDescriptor (not TrimmerRootAssembly) to prevent ILLink from
+    trimming the designer's resource properties. FixLegacyResourceDesignerStep runs
+    after ILLink and needs all properties present when rewriting library assemblies.
+    A descriptor (-x) preserves types without making the assembly an entry point,
+    which avoids pulling netstandard.dll into the output.
+    See https://github.com/dotnet/runtime/issues/126518
+
+    TODO: Once dotnet/runtime#126518 is fixed and flows to dotnet/android,
+    simplify this to use TrimmerRootAssembly instead of the XML descriptor.
 -->
 <Target Name="_AddResourceDesignerToPublishFiles"
     Condition=" '$(AndroidUseDesignerAssembly)' == 'True' "
     AfterTargets="ComputeResolvedFilesToPublishList"
     DependsOnTargets="_SetupDesignerProperties">
+  <PropertyGroup>
+    <_DesignerLinkerDescriptor>$(IntermediateOutputPath)_Microsoft.Android.Resource.Designer.xml</_DesignerLinkerDescriptor>
+  </PropertyGroup>
+  <WriteLinesToFile
+      File="$(_DesignerLinkerDescriptor)"
+      Overwrite="true"
+      WriteOnlyWhenDifferent="true"
+      Lines="&lt;linker&gt;&lt;assembly fullname=&quot;$(_DesignerAssemblyName)&quot;&gt;&lt;type fullname=&quot;*&quot; preserve=&quot;all&quot; /&gt;&lt;/assembly&gt;&lt;/linker&gt;"
+  />
   <ItemGroup>
     <ResolvedFileToPublish Include="$(_GenerateResourceDesignerAssemblyOutput)">
       <RelativePath>$(_DesignerAssemblyName).dll</RelativePath>
       <CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
       <PostprocessAssembly>true</PostprocessAssembly>
       <IsTrimmable>true</IsTrimmable>
     </ResolvedFileToPublish>
+    <!-- Preserve all designer types/members via descriptor so ILLink keeps resource
+         properties. Using a descriptor (not TrimmerRootAssembly) avoids making the
+         assembly an entry point, which would pull netstandard.dll into the output.
+         See https://github.com/dotnet/runtime/issues/126518 -->
+    <TrimmerRootDescriptor Include="$(_DesignerLinkerDescriptor)" />
+    <FileWrites Include="$(_DesignerLinkerDescriptor)" />
   </ItemGroup>
 </Target>
 

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.LlvmIr.targets

@@ -167,7 +167,7 @@
   <Target Name="_PrepareLinking"
       Condition=" '$(PublishTrimmed)' == 'true' "
       AfterTargets="ComputeResolvedFilesToPublishList"
-      DependsOnTargets="GetReferenceAssemblyPaths;_CreatePropertiesCache">
+      DependsOnTargets="GetReferenceAssemblyPaths;_CreatePropertiesCache;_AddResourceDesignerToPublishFiles">
     <PropertyGroup>
       <TrimmerRemoveSymbols Condition=" '$(AndroidIncludeDebugSymbols)' != 'true' ">true</TrimmerRemoveSymbols>
       <_ExtraTrimmerArgs Condition=" '$(_EnableSerializationDiscovery)' != 'false' ">--enable-serialization-discovery $(_ExtraTrimmerArgs)</_ExtraTrimmerArgs>
@@ -204,12 +204,6 @@
       <_TrimmerCustomSteps Include="$(_AndroidLinkerCustomStepAssembly)" Type="Microsoft.Android.Sdk.ILLink.PreserveJavaInterfaces" />
       <!-- Custom steps that run after MarkStep -->
       <!-- Custom steps that run after CleanStep -->
-      <_TrimmerCustomSteps
-          Condition=" '$(AndroidUseDesignerAssembly)' == 'true' "
-          Include="$(_AndroidLinkerCustomStepAssembly)"
-          BeforeStep="MarkStep"
-          Type="MonoDroid.Tuner.FixLegacyResourceDesignerStep"
-      />
       <_TrimmerCustomSteps
           Condition=" '$(_AndroidTypeMapImplementation)' == 'managed' "
           Include="$(_AndroidLinkerCustomStepAssembly)"
@@ -254,7 +248,8 @@
         Assemblies="@(_PostTrimmingAssembly)"
         AddKeepAlives="$(AndroidAddKeepAlives)"
         AndroidLinkResources="$(AndroidLinkResources)"
-        Deterministic="$(Deterministic)" />
+        Deterministic="$(Deterministic)"
+        UseDesignerAssembly="$(AndroidUseDesignerAssembly)" />
   </Target>
 
   <!-- Inject _TypeMapKind into the property cache -->

src/Xamarin.Android.Build.Tasks/Tasks/PostTrimmingPipeline.cs

@@ -6,6 +6,7 @@
 using Microsoft.Android.Build.Tasks;
 using Microsoft.Build.Framework;
 using Mono.Cecil;
+using Mono.Linker;
 using MonoDroid.Tuner;
 
 namespace Xamarin.Android.Tasks;
@@ -16,7 +17,7 @@ namespace Xamarin.Android.Tasks;
 /// This opens each assembly once (via DirectoryAssemblyResolver with ReadWrite) and
 /// runs all registered steps on it, then writes modified assemblies in-place. Currently
 /// runs CheckForObsoletePreserveAttributeStep, StripEmbeddedLibrariesStep and
-/// (optionally) AddKeepAlivesStep.
+/// (optionally) AddKeepAlivesStep and FixLegacyResourceDesignerStep.
 ///
 /// Runs in the inner build after ILLink but before ReadyToRun/crossgen2 compilation,
 /// so that R2R images are generated from the already-modified assemblies.
@@ -34,6 +35,8 @@ public class PostTrimmingPipeline : AndroidTask
 
 	public bool Deterministic { get; set; }
 
+	public bool UseDesignerAssembly { get; set; }
+
 	public override bool RunTask ()
 	{
 		using var resolver = new DirectoryAssemblyResolver (
@@ -100,6 +103,15 @@ public override bool RunTask ()
 				},
 				(msg) => Log.LogDebugMessage (msg)));
 		}
+		if (UseDesignerAssembly) {
+			// Create an MSBuildLinkContext so FixLegacyResourceDesignerStep can resolve assemblies
+			// and log messages. The resolver is owned by the outer 'using' block, so we intentionally
+			// do not dispose this context (LinkContext.Dispose would double-dispose the resolver).
+			var linkContext = new MSBuildLinkContext (resolver, Log);
+			var fixLegacyStep = new FixLegacyResourceDesignerStep ();
+			fixLegacyStep.Initialize (linkContext);
+			steps.Add (new PostTrimmingFixLegacyResourceDesignerStep (fixLegacyStep));
+		}
 
 		foreach (var (item, assembly) in loadedAssemblies) {
 			var context = new StepContext (item, item);
@@ -118,3 +130,24 @@ public override bool RunTask ()
 		return !Log.HasLoggedErrors;
 	}
 }
+
+/// <summary>
+/// Thin wrapper around <see cref="FixLegacyResourceDesignerStep"/> for the post-trimming pipeline.
+/// Calls <see cref="FixLegacyResourceDesignerStep.ProcessAssemblyDesigner"/> directly, matching the
+/// behavior of the former ILLink path which processed all assemblies without StepContext flag filtering.
+/// Assemblies without a resource designer are skipped internally by ProcessAssemblyDesigner.
+/// </summary>
+class PostTrimmingFixLegacyResourceDesignerStep : IAssemblyModifierPipelineStep
+{
+	readonly FixLegacyResourceDesignerStep _inner;
+
+	public PostTrimmingFixLegacyResourceDesignerStep (FixLegacyResourceDesignerStep inner)
+	{
+		_inner = inner;
+	}
+
+	public void ProcessAssembly (AssemblyDefinition assembly, StepContext context)
+	{
+		context.IsAssemblyModified |= _inner.ProcessAssemblyDesigner (assembly);
+	}
+}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.CoreCLR.apkdesc

@@ -5,25 +5,25 @@
       "Size": 3036
     },
     "classes.dex": {
-      "Size": 397520
+      "Size": 400044
     },
     "lib/arm64-v8a/libassembly-store.so": {
-      "Size": 3091928
+      "Size": 3115312
     },
     "lib/arm64-v8a/libclrjit.so": {
-      "Size": 3141224
+      "Size": 3202072
     },
     "lib/arm64-v8a/libcoreclr.so": {
-      "Size": 5736320
+      "Size": 5766640
     },
     "lib/arm64-v8a/libmonodroid.so": {
-      "Size": 1375784
+      "Size": 1365104
     },
     "lib/arm64-v8a/libmscordaccore.so": {
-      "Size": 2442416
+      "Size": 2493552
     },
     "lib/arm64-v8a/libmscordbi.so": {
-      "Size": 1894280
+      "Size": 1902744
     },
     "lib/arm64-v8a/libSystem.Globalization.Native.so": {
       "Size": 71936
@@ -32,13 +32,13 @@
       "Size": 1281696
     },
     "lib/arm64-v8a/libSystem.Native.so": {
-      "Size": 105664
+      "Size": 107904
     },
     "lib/arm64-v8a/libSystem.Security.Cryptography.Native.Android.so": {
       "Size": 165536
     },
     "lib/arm64-v8a/libxamarin-app.so": {
-      "Size": 20128
+      "Size": 20520
     },
     "META-INF/BNDLTOOL.RSA": {
       "Size": 1221
@@ -74,5 +74,5 @@
       "Size": 1904
     }
   },
-  "PackageSize": 9176858
+  "PackageSize": 9258778
 }

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.MonoVM.apkdesc

@@ -8,16 +8,16 @@
       "Size": 22388
     },
     "lib/arm64-v8a/lib__Microsoft.Android.Resource.Designer.dll.so": {
-      "Size": 18288
+      "Size": 18696
     },
     "lib/arm64-v8a/lib_Java.Interop.dll.so": {
-      "Size": 88040
+      "Size": 88048
     },
     "lib/arm64-v8a/lib_Mono.Android.dll.so": {
-      "Size": 118024
+      "Size": 117928
     },
     "lib/arm64-v8a/lib_Mono.Android.Runtime.dll.so": {
-      "Size": 26544
+      "Size": 26464
     },
     "lib/arm64-v8a/lib_System.Console.dll.so": {
       "Size": 24432
@@ -35,7 +35,7 @@
       "Size": 21632
     },
     "lib/arm64-v8a/lib_UnnamedProject.dll.so": {
-      "Size": 20032
+      "Size": 20144
     },
     "lib/arm64-v8a/libarc.bin.so": {
       "Size": 19176
@@ -44,7 +44,7 @@
       "Size": 36616
     },
     "lib/arm64-v8a/libmonodroid.so": {
-      "Size": 1386512
+      "Size": 1386072
     },
     "lib/arm64-v8a/libmonosgen-2.0.so": {
       "Size": 3124368
@@ -62,7 +62,7 @@
       "Size": 165536
     },
     "lib/arm64-v8a/libxamarin-app.so": {
-      "Size": 19840
+      "Size": 19792
     },
     "META-INF/BNDLTOOL.RSA": {
       "Size": 1221

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.NativeAOT.apkdesc

@@ -5,13 +5,13 @@
       "Size": 3124
     },
     "classes.dex": {
-      "Size": 24224
+      "Size": 25400
     },
     "lib/arm64-v8a/libUnnamedProject.so": {
-      "Size": 4968680
+      "Size": 5056848
     },
     "META-INF/BNDLTOOL.RSA": {
-      "Size": 1211
+      "Size": 1221
     },
     "META-INF/BNDLTOOL.SF": {
       "Size": 1211
@@ -44,5 +44,5 @@
       "Size": 1904
     }
   },
-  "PackageSize": 2094050
+  "PackageSize": 2122722
 }