[TrimmableTypeMap] Emit empty stubs for trimmed per-assembly typemaps

[simonrozsival]

Problem

The root _Microsoft.Android.TypeMaps assembly is generated before trimming with an [assembly: TypeMapAssemblyTarget<T>("_X.TypeMap")] entry for every per-assembly typemap. ILLink can trim individual _X.TypeMap assemblies when their target Java binding is unused, but those attributes reference the assembly by an opaque name string (not a metadata reference), so ILLink neither keeps the target nor prunes the attribute.

At startup, CoreCLR's TypeMapping enumerates every TypeMapAssemblyTarget attribute and Assembly.Loads the named assembly, throwing for the trimmed ones:

System.IO.FileNotFoundException: Could not load file or assembly '_GoogleGson.TypeMap, Culture=neutral, PublicKeyToken=null'.
   at System.Runtime.InteropServices.TypeMapLazyDictionary.ProcessAttributes(...)
   at System.Runtime.InteropServices.TypeMapping.GetOrCreateExternalTypeMapping[TTypeMapGroup]()
   at Microsoft.Android.Runtime.TypeMapLoader.Initialize()
   at Android.Runtime.JNIEnvInit.InitializeTrimmableTypeMapData()

Found while running a real .NET MAUI app on CoreCLR + trimmable typemap (Release, android-arm64): six unused bindings — GoogleGson, Jsr305Binding, Xamarin.AndroidX.Print, Xamarin.JavaX.Inject, Xamarin.JSpecify, Xamarin.Kotlin.StdLib — were trimmed together with their _X.TypeMap assemblies, but the dangling TypeMapAssemblyTarget attributes on the root remained, so the app crashed on launch.

Fix

After ILLink, emit an empty (entry-free) stub assembly for each per-assembly typemap that was trimmed away, so Assembly.Load succeeds and contributes no mappings. Stubs are written into the linked/ output before the typemap DLLs flow into the publish/ReadyToRun pipeline.

• New task GenerateMissingTypeMapStubs — compares the pre-trim typemap set against the surviving linked/ set and emits a stub for each missing one.
• New public helper TypeMapAssemblyGenerator.GenerateEmpty(...) — uses the existing PEAssemblyBuilder (System.Reflection.Metadata; no Mono.Cecil).
• Wired into Microsoft.Android.Sdk.TypeMap.Trimmable.CoreCLR.targets (AfterTargets="ILLink").

Why stubs, and not "regenerate the root without the dangling entries"?

Regenerating the root re-introduces references to the System.Runtime.InteropServices facade (which ILLink had reconciled to System.Private.CoreLib in the linked root). The shipped, trimmed facade no longer forwards TypeMapping, so a regenerated root fails at runtime with TypeLoadException: Could not load type 'System.Runtime.InteropServices.TypeMapping'. Emitting stubs leaves ILLink's reconciled root untouched.

Testing

• Existing trimmable-typemap unit tests: 616 passed.
• Manual end-to-end: the MAUI app (CoreCLR, trimmable, Release) now builds, installs, and launches to its first page with no crash — previously it crashed at startup. The six dangling TypeMapAssemblyTargets now resolve to empty stubs.
• APK size impact is negligible (6 stubs ≈ 12 KB uncompressed).

Notes / follow-ups

• Scope: wired into the CoreCLR trimmable path (where the runtime Assembly.Load crash occurs). The NativeAOT trimmable path resolves typemaps at ILC time; happy to extend if equivalent handling is wanted there.
• Can add unit / build-integration tests for GenerateMissingTypeMapStubs / GenerateEmpty as a follow-up.

[Copilot]

Pull request overview

This PR prevents CoreCLR + trimmable typemap apps from crashing at startup when ILLink trims away per-assembly _X.TypeMap.dll assemblies that are still referenced (by name string) from the root _Microsoft.Android.TypeMaps assembly. The fix emits empty “stub” typemap assemblies post-ILLink so Assembly.Load succeeds and contributes no mappings.

Changes:

• Add GenerateMissingTypeMapStubs MSBuild task to emit empty stub assemblies for missing per-assembly typemaps in the linked/ output.
• Add TypeMapAssemblyGenerator.GenerateEmpty(...) helper to generate an entry-free typemap assembly via the existing PEAssemblyBuilder.
• Wire stub generation into the CoreCLR trimmable typemap ILLink pipeline (AfterTargets="ILLink", before publish/R2R inputs are computed).

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tasks/GenerateMissingTypeMapStubs.cs	New task that detects missing _*.TypeMap.dll after trimming and writes empty stubs into linked/.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.CoreCLR.targets	Registers and schedules the new task after ILLink to ensure missing typemap assemblies exist before publish/R2R.
src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyGenerator.cs	Adds GenerateEmpty API to emit a minimal typemap assembly with no entries.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2