[TrimmableTypeMap] Fix NativeAOT interface listener callbacks: encode JNI boolean as System.Boolean in n_* MemberRefs

[simonrozsival]

Fixes #11773.

Problem

Under the trimmable typemap (NativeAOT), binding/AndroidX interface listener callbacks that have a boolean parameter (e.g. IOnBackStackChangedListener.OnBackStackChangeStarted) don't work. ILC reports:

ILC: Method '..._IOnBackStackChangedListener_Proxy.n_OnBackStackChangeStarted_uco_*' will always throw
     because: Missing method 'Void ...IOnBackStackChangedListener.n_OnBackStackChangeStarted_...(..., SByte)'

…and replaces the generated UCO forwarder body with a throw.

Root cause — a signature mismatch, not trimming

The generator emits, for each marshal method, a UCO forwarder that calls the MCW-generated n_* callback via a cross-assembly MemberRef. That MemberRef signature is built by JniSignatureHelper.EncodeClrTypeForCallback, which encoded a JNI boolean (Z) as System.SByte.

But the real MCW n_* callbacks declare their JNI boolean as System.Boolean. Decompiled from the shipping Xamarin.AndroidX.Fragment:

.method ... static void n_OnBackStackChangeStarted_Landroidx_fragment_app_Fragment_Z (
    native int jnienv, native int native__this, native int native_fragment, bool pop) cil managed

Since System.SByte != System.Boolean in metadata, the emitted MemberRef can't bind to the real method → "Missing method" → "will always throw". This only affects callbacks with a boolean parameter (or boolean return), which is exactly the observed failure set.

This was previously misdiagnosed as trimming

A rooted cross-assembly call to a non-public static method — including one declared on an interface — is preserved by ILC. Verified with a minimal 2-project NativeAOT repro: the call survives and runs. Re-running ilc with an sbyte MemberRef against a bool method reproduces the exact issue diagnostic; switching to bool resolves and runs. So the earlier direct-dispatch attempt (which caused the reverted infinite-recursion regression) was treating the wrong problem.

Fix

Encode JNI boolean as System.Boolean for the callback MemberRef. The UCO entry point keeps byte (blittable, unsigned) for the [UnmanagedCallersOnly] ABI — only the managed n_* MemberRef changes. This matches the constructor path, which already references managed boolean parameters as System.Boolean. Passing the UCO byte argument into the bool parameter is valid IL (both are stack-int32; JNI jboolean is always 0/1), and mirrors the legacy LLVM-IR/marshal-methods path, which also targets the real bool n_* callbacks.

-		case JniParamKind.Boolean: encoder.SByte (); break;  // MCW n_* callbacks use sbyte for JNI boolean
+		case JniParamKind.Boolean: encoder.Boolean (); break; // MCW n_* callbacks declare JNI boolean as System.Boolean

Tests

• Updated the 4 unit tests that had codified the incorrect sbyte expectation; they now assert System.Boolean for the callback signature while keeping System.Byte for the UCO ABI.
• All TrimmableTypeMap generator unit tests pass (564/564).

Remaining / acceptance criteria

This resolves the ILC "will always throw" diagnostic and the binding-interface-callback breakage at the generator level. The issue's device-test criterion (IOnBackStackChangedListener firing under NativeAOT, no regression for built-in listeners like GlobalLayoutEvent_ShouldRegisterAndFire_OnActivityLaunch) should be validated by the device test legs.

[Copilot]

Pull request overview

This PR fixes NativeAOT failures in the TrimmableTypeMap generator where interface listener callbacks involving JNI boolean (Z) could not bind to the MCW-generated n_* methods due to a MemberRef signature mismatch (System.SByte vs System.Boolean). By encoding callback booleans as System.Boolean (while keeping the UCO ABI as byte), the generated cross-assembly MemberRefs correctly resolve and avoid the ILC “will always throw / Missing method … (…, SByte)” diagnostics.

Changes:

• Update JniSignatureHelper.EncodeClrTypeForCallback to encode JNI boolean as System.Boolean instead of System.SByte.
• Update unit tests to assert the corrected callback signature encoding while preserving byte for the UCO ABI.
• Strengthen/clarify regression-test comments around the prior failure mode (unresolvable MemberRef → ILC diagnostic).

Show a summary per file

File	Description
src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/JniSignatureHelper.cs	Fixes callback MemberRef boolean encoding to System.Boolean, matching MCW n_* signatures.
tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs	Updates assertions and regression tests to validate byte UCO ABI vs bool callback MemberRef encoding.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0

[jonathanpeppers]

/review

[simonrozsival]

/review