[NativeAOT] Preserve GC bridge temporary peers through R8

[jonathanpeppers]

NativeAOT Jetchat aborted during GC bridge processing with Failed to create GC bridge temporary peer. The failed APK's DEX contains mono.android.GCUserPeer as an abstract class with no constructor or methods; the matching native source aborts when JNI NewObject() returns null. Keeping IGCUserPeer alone does not preserve this JNI-created implementation.

Preserve only GCUserPeer, its default constructor, and monodroidAddReference(Object) / monodroidClearReferences(). Add a Release NativeAOT/R8 build regression asserting that these methods survive in the final DEX. No GC algorithm changes, runtime fallback, or broad keep rules are introduced.

Validation

• Full Release product/native build and packaging succeeded, including NativeAOT arm, arm64, and x64 artifacts.
• Matched Jetchat builds using the same frozen Compose source and official RC1 toolchain reproduce the stripped abstract peer in the baseline and retain a concrete peer with all three methods in the candidate. Baseline DEX files match the original failing APK byte for byte.
• An isolated R8/JNI host probe confirms that the candidate can be constructed and its reference callbacks invoked. Host JVM behavior is not presented as Android runtime validation.
• The new build regression was attempted but blocked before compilation by local workload discovery (NETSDK1147). Device gesture validation remains pending; no device run was performed in this session. The original RC1 abort did not log the pending Java exception.

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Pull Request
title and
description
should follow the
commit-messages.md workflow documentation, and in particular should include:

• Useful description of why the change is necessary.
• Links to issues fixed - N/A; no linked issue.
• Unit tests - focused build/DEX regression added; execution limitation noted above.

[Copilot]

🟡 Changes recommended

The regression test must verify concreteness and reliably scope member checks to GCUserPeer.

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Pull request overview

Preserves the NativeAOT GC bridge peer through R8 and adds Release DEX regression coverage.

Changes:

• Adds targeted keep rules for GCUserPeer.
• Adds assertions for its constructor and reference-management methods.

File summaries

File	Review
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/TrimmableTypeMapBuildTests.cs	Adds regression coverage; must also verify the class is non-abstract and fix per-class helper state handling.
src/Xamarin.Android.Build.Tasks/Resources/proguard_trimmable_nativeaot.cfg	Adds targeted GCUserPeer keep rules.

Review details

Suppressed comments (1)

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/TrimmableTypeMapBuildTests.cs:1671

• ❌ Test validity — DexUtils.ContainsClassWithMethod does not reset its hasType flag when Class descriptor changes (it only resets hasName), so a matching method/signature in an earlier DEX class can make this assertion succeed even when GCUserPeer has no such member. Since these are common methods and <init>()V is ubiquitous, please fix the helper to reset the per-class match state (or use a class-scoped DEX parser) before relying on this regression test.

				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.");

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite