Enable R2R compilation for CoreCLR Debug builds

Co-authored-by: simonrozsival <374616+simonrozsival@users.noreply.github.com>

Author: Copilot

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.CoreCLR.targets

@@ -13,9 +13,15 @@ This file contains the CoreCLR-specific MSBuild logic for .NET for Android.
   </PropertyGroup>
   <!-- Properties for $(OutputType)=Exe (Android Applications) -->
   <PropertyGroup Condition=" '$(AndroidApplication)' == 'true' ">
-    <!-- Default to R2R Composite for CoreCLR Release mode -->
-    <PublishReadyToRun Condition=" '$(PublishReadyToRun)' == '' and '$(Configuration)' == 'Release' ">true</PublishReadyToRun>
-    <PublishReadyToRunComposite Condition=" '$(PublishReadyToRunComposite)' == '' and '$(PublishReadyToRun)' == 'true' ">true</PublishReadyToRunComposite>
+    <!--
+      Enable R2R by default for CoreCLR applications:
+      - Release builds: Use composite R2R for best startup performance
+      - Debug builds: Use individual R2R to improve runtime performance while debugging
+      Set PublishReadyToRun=false to disable R2R compilation.
+    -->
+    <PublishReadyToRun Condition=" '$(PublishReadyToRun)' == '' ">true</PublishReadyToRun>
+    <!-- Composite R2R only for Release builds - it's slower to compile but faster at runtime -->
+    <PublishReadyToRunComposite Condition=" '$(PublishReadyToRunComposite)' == '' and '$(PublishReadyToRun)' == 'true' and '$(Configuration)' == 'Release' ">true</PublishReadyToRunComposite>
     <_IsPublishing Condition=" '$(_IsPublishing)' == '' and '$(PublishReadyToRun)' == 'true' ">true</_IsPublishing>
     <AllowReadyToRunWithoutRuntimeIdentifier Condition=" '$(PublishReadyToRun)' == 'true' and '$(RuntimeIdentifiers)' != '' ">true</AllowReadyToRunWithoutRuntimeIdentifier>
   </PropertyGroup>

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest2.cs

@@ -150,6 +150,37 @@ public void BasicApplicationPublishReadyToRun ([Values] bool isComposite, [Value
 				$"ReadyToRun image not found in {assemblyName}.dll! ManagedNativeHeaderDirectory should not be empty!");
 		}
 
+		[Test]
+		public void DebugBuildPublishReadyToRun ([Values ("android-x64", "android-arm64")] string rid)
+		{
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = false, // Debug build - R2R is now enabled by default for CoreCLR
+			};
+
+			proj.SetRuntime (AndroidRuntime.CoreCLR);
+			proj.SetProperty ("RuntimeIdentifier", rid);
+			proj.SetProperty ("AndroidEnableAssemblyCompression", "false");
+			// Composite R2R should NOT be enabled by default for Debug builds
+			proj.SetProperty ("PublishReadyToRunComposite", "false");
+
+			var b = CreateApkBuilder ();
+			Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+
+			var assemblyName = proj.ProjectName;
+			var apk = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, rid, $"{proj.PackageName}-Signed.apk");
+			FileAssert.Exists (apk);
+
+			var helper = new ArchiveAssemblyHelper (apk, true);
+			var abi = MonoAndroidHelper.RidToAbi (rid);
+			Assert.IsTrue (helper.Exists ($"assemblies/{abi}/{assemblyName}.dll"), $"{assemblyName}.dll should exist in apk!");
+
+			using var stream = helper.ReadEntry ($"assemblies/{assemblyName}.dll");
+			stream.Position = 0;
+			using var peReader = new System.Reflection.PortableExecutable.PEReader (stream);
+			Assert.IsTrue (peReader.PEHeaders.CorHeader.ManagedNativeHeaderDirectory.Size > 0,
+				$"ReadyToRun image not found in Debug build {assemblyName}.dll! ManagedNativeHeaderDirectory should not be empty!");
+		}
+
 		[Test]
 		public void NativeAOT ()
 		{