[WIP] Run FixLegacyResourceDesigner before trimming

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

@@ -227,15 +227,10 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     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.
+    No TrimmerRootDescriptor is needed: PreTrimmingFixLegacyDesigner rewrites library
+    assemblies *before* ILLink, replacing designer field loads with calls to the designer
+    assembly's property getters. ILLink then naturally preserves only the referenced
+    properties through its mark step, allowing everything else to be trimmed.
 -->
 <Target Name="_AddResourceDesignerToPublishFiles"
     Condition=" '$(AndroidUseDesignerAssembly)' == 'True' "

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

@@ -8,6 +8,7 @@
   <UsingTask TaskName="Xamarin.Android.Tasks.RemoveRegisterAttribute" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.GenerateProguardConfiguration" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.PostTrimmingPipeline" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.PreTrimmingFixLegacyDesigner" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
 
   <PropertyGroup>
     <_RemoveRegisterFlag>$(MonoAndroidIntermediateAssemblyDir)shrunk\shrunk.flag</_RemoveRegisterFlag>
@@ -223,6 +224,60 @@
     </ItemGroup>
   </Target>
 
+  <!--
+    Fix legacy resource designer references *before* ILLink runs. This rewrites
+    library assemblies so their resource field accesses (ldsfld) become calls to the
+    designer assembly's property getters, and clears the per-library designer classes.
+    Because the rewritten IL no longer references the old designer fields, ILLink can
+    freely trim unused designer types without needing a root descriptor.
+
+    Modified assemblies are written to an intermediate directory (not in-place) to
+    avoid mutating files in the shared NuGet cache or shared intermediate output paths.
+  -->
+  <Target Name="_CollectPreTrimmingAssemblies"
+      BeforeTargets="_PreTrimmingFixLegacyDesigner"
+      Condition=" '$(PublishTrimmed)' == 'true' and '$(AndroidUseDesignerAssembly)' == 'True' ">
+    <ItemGroup>
+      <_PreTrimmingAssembly Include="@(ResolvedFileToPublish)" Condition=" '%(Extension)' == '.dll' and '%(ResolvedFileToPublish.PostprocessAssembly)' == 'true' " />
+    </ItemGroup>
+  </Target>
+
+  <Target Name="_PreTrimmingFixLegacyDesigner"
+      BeforeTargets="ILLink"
+      DependsOnTargets="_CollectPreTrimmingAssemblies"
+      Condition=" '$(PublishTrimmed)' == 'true' and '$(AndroidUseDesignerAssembly)' == 'True' "
+      Inputs="@(_PreTrimmingAssembly)"
+      Outputs="$(_AndroidStampDirectory)_PreTrimmingFixLegacyDesigner.stamp">
+    <PreTrimmingFixLegacyDesigner
+        Assemblies="@(_PreTrimmingAssembly)"
+        TargetName="$(TargetName)"
+        OutputDirectory="$(IntermediateOutputPath)prelink"
+        Deterministic="$(Deterministic)">
+      <Output TaskParameter="ModifiedAssemblies" ItemName="_PreTrimmingModifiedAssembly" />
+    </PreTrimmingFixLegacyDesigner>
+    <Touch Files="$(_AndroidStampDirectory)_PreTrimmingFixLegacyDesigner.stamp" AlwaysCreate="true" />
+    <ItemGroup>
+      <FileWrites Include="@(_PreTrimmingModifiedAssembly)" />
+    </ItemGroup>
+  </Target>
+
+  <!--
+    Replace ResolvedFileToPublish entries with the prelink copies so ILLink picks them up.
+    This must run even when _PreTrimmingFixLegacyDesigner is skipped (up-to-date) because
+    ResolvedFileToPublish is populated fresh each build.
+  -->
+  <Target Name="_PreTrimmingFixLegacyDesignerUpdateItems"
+      BeforeTargets="ILLink"
+      DependsOnTargets="_PreTrimmingFixLegacyDesigner"
+      Condition=" '$(PublishTrimmed)' == 'true' and '$(AndroidUseDesignerAssembly)' == 'True' ">
+    <ItemGroup>
+      <_PreTrimmingSwappableItem Include="@(ResolvedFileToPublish)"
+          Condition=" '%(Extension)' == '.dll' and Exists('$(IntermediateOutputPath)prelink/%(Filename)%(Extension)') " />
+      <ResolvedFileToPublish Remove="@(_PreTrimmingSwappableItem)" />
+      <ResolvedFileToPublish Include="@(_PreTrimmingSwappableItem->'$(IntermediateOutputPath)prelink/%(Filename)%(Extension)')" />
+    </ItemGroup>
+  </Target>
+
   <Target Name="_TouchAndroidLinkFlag"
       AfterTargets="ILLink"
       Condition=" '$(PublishTrimmed)' == 'true' and '$(RunILLink)' != 'false' and Exists('$(_LinkSemaphore)') "

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

@@ -0,0 +1,94 @@
+#nullable enable
+
+using System.Collections.Generic;
+using System.IO;
+using Java.Interop.Tools.Cecil;
+using Microsoft.Android.Build.Tasks;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+using Mono.Cecil;
+using MonoDroid.Tuner;
+
+namespace Xamarin.Android.Tasks;
+
+/// <summary>
+/// Runs <see cref="FixLegacyResourceDesignerStep"/> on assemblies that are about to be
+/// trimmed by ILLink. This rewrites library assemblies so their resource field accesses
+/// (ldsfld) become calls to the designer assembly's property getters.
+///
+/// Running this *before* ILLink means the trimmer sees the rewritten IL and can freely
+/// trim unused designer types/fields. This avoids the need to root the entire designer
+/// assembly during trimming (which causes an APK size regression).
+///
+/// Modified assemblies are written to <see cref="OutputDirectory"/> rather than in-place,
+/// to avoid mutating files in the shared NuGet cache or shared intermediate output paths.
+/// </summary>
+public class PreTrimmingFixLegacyDesigner : AndroidTask
+{
+	public override string TaskPrefix => "PTD";
+
+	[Required]
+	public ITaskItem [] Assemblies { get; set; } = [];
+
+	[Required]
+	public string TargetName { get; set; } = "";
+
+	[Required]
+	public string OutputDirectory { get; set; } = "";
+
+	public bool Deterministic { get; set; }
+
+	[Output]
+	public ITaskItem []? ModifiedAssemblies { get; set; }
+
+	public override bool RunTask ()
+	{
+		Directory.CreateDirectory (OutputDirectory);
+
+		using var resolver = new DirectoryAssemblyResolver (
+			this.CreateTaskLogger (), loadDebugSymbols: true);
+
+		foreach (var assembly in Assemblies) {
+			var dir = Path.GetFullPath (Path.GetDirectoryName (assembly.ItemSpec) ?? "");
+			if (!resolver.SearchDirectories.Contains (dir)) {
+				resolver.SearchDirectories.Add (dir);
+			}
+		}
+
+		var linkContext = new MSBuildLinkContext (resolver, Log);
+		var fixLegacyStep = new FixLegacyResourceDesignerStep ();
+		fixLegacyStep.Initialize (linkContext);
+
+		var modified = new List<ITaskItem> ();
+
+		foreach (var item in Assemblies) {
+			// Match the filtering in FixLegacyResourceDesignerStep.ProcessAssembly:
+			// skip the main assembly and framework/BCL assemblies.
+			if (Path.GetFileNameWithoutExtension (item.ItemSpec) == TargetName) {
+				continue;
+			}
+			if (MonoAndroidHelper.IsFrameworkAssembly (item)) {
+				continue;
+			}
+
+			var assembly = resolver.GetAssembly (item.ItemSpec);
+			if (fixLegacyStep.ProcessAssemblyDesigner (assembly)) {
+				var outputPath = Path.Combine (OutputDirectory, Path.GetFileName (item.ItemSpec));
+				Log.LogDebugMessage ($"  Writing modified assembly: {outputPath}");
+				assembly.Write (outputPath, new WriterParameters {
+					WriteSymbols = assembly.MainModule.HasSymbols,
+					DeterministicMvid = Deterministic,
+				});
+
+				var outputItem = new TaskItem (outputPath);
+				item.CopyMetadataTo (outputItem);
+				outputItem.SetMetadata ("OriginalPath", item.ItemSpec);
+				modified.Add (outputItem);
+			}
+		}
+
+		ModifiedAssemblies = modified.ToArray ();
+
+		return !Log.HasLoggedErrors;
+	}
+}