[NativeAOT] Preserve GC bridge temporary peers through R8

src/Xamarin.Android.Build.Tasks/Resources/proguard_trimmable_nativeaot.cfg

@@ -6,6 +6,12 @@
 -keep class net.dot.android.crypto.** { *; <init>(...); }
 # NativeAOT resolves these interface methods through JNI during startup.
 -keep class mono.android.IGCUserPeer { *; }
+# The GC bridge constructs temporary peers and calls these methods through JNI.
+-keep class mono.android.GCUserPeer {
+   <init>();
+   public void monodroidAddReference(java.lang.Object);
+   public void monodroidClearReferences();
+}
 # Native hosts resolve these package-private fields by name during startup.
 -keepclassmembers class mono.android.Runtime {
    static java.lang.Class *;

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

@@ -1640,6 +1640,38 @@ public void Build_WithTrimmableTypeMap_KeepsNativeAotRuntimeHostAcws ()
 				$"`{dexFile}` should include the UncaughtExceptionMarshaler runtime ACW.");
 		}
 
+		[Test]
+		public void Build_WithTrimmableTypeMap_KeepsNativeAotGcBridgeTemporaryPeer ()
+		{
+			const bool isRelease = true;
+			if (IgnoreUnsupportedConfiguration (AndroidRuntime.NativeAOT, release: isRelease)) {
+				return;
+			}
+
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = isRelease,
+				LinkTool = "r8",
+			};
+			proj.SetRuntime (AndroidRuntime.NativeAOT);
+			proj.SetProperty ("AndroidTypeMapImplementation", "trimmable");
+
+			using var builder = CreateApkBuilder ();
+			Assert.IsTrue (builder.Build (proj), "Build should have succeeded.");
+
+			var dexDirectory = builder.Output.GetIntermediaryPath (Path.Combine ("android", "bin"));
+			var dexFiles = Directory.GetFiles (dexDirectory, "classes*.dex");
+			Assert.IsNotEmpty (dexFiles, "R8 should produce DEX files.");
+			foreach (var (method, signature) in new [] {
+				("<init>", "()V"),
+				("monodroidAddReference", "(Ljava/lang/Object;)V"),
+				("monodroidClearReferences", "()V"),
+			}) {
+				Assert.IsTrue (dexFiles.Any (dex => DexUtils.ContainsClassWithMethod (
+					"Lmono/android/GCUserPeer;", method, signature, dex, AndroidSdkPath)),
+					$"R8 must preserve GCUserPeer.{method}{signature} for the native GC bridge.");
+			}
+		}
+
 		[Test]
 		public void Build_WithTrimmableTypeMap_DeletesStaleGeneratedJavaSources ()
 		{