[TrimmableTypeMap] Stop generating dead Java.Interop.__TypeRegistrations class

[simonrozsival]

Part of #10958.

The binding generator still emits a Java.Interop.__TypeRegistrations class into every binding assembly (including Mono.Android.dll itself), but the runtime code that consumed it was removed in #9471 (commit 6ac69bee2, 2024-11-08). The generated class is now pure dead code — it is never invoked and the dictionary it populates is never read. We should stop generating it.

Background: how it used to work

__TypeRegistrations.RegisterPackages() called Java.Interop.TypeManager.RegisterPackages(string[] packages, Converter<string, Type?>[] lookups), which populated a packageLookup dictionary. The Java→managed type resolver (GetJavaToManagedTypeCore) had a TypeRegistrationFallback(class_name) path that lazily invoked __TypeRegistrations.RegisterPackages() and then read packageLookup to resolve a managed Type by package + class name.

This fallback was:

• made opt-out behind an AppContext switch in [illink] Type registrations fallback switch #5629 (commit 5b3d8cdd1, "[illink] Type registrations fallback switch"), with a linker substitution stubbing RegisterPackages out for linked apps,
• further trimmed in [Microsoft.Android.Sdk.ILLink] remove more linker sources #8572 ("[Microsoft.Android.Sdk.ILLink] remove more linker sources"),
• and fully removed in [Mono.Android] remove Xamarin.Android.VSAndroidDesigner.IsSupported feature switch #9471 (commit 6ac69bee2) — that diff deletes TypeRegistrationFallback, the __TypeRegistrations.RegisterPackages () call, and the packageLookup.TryGetValue(...) read.

Today, GetJavaToManagedTypeCore resolves types exclusively via the native typemap (monovm_typemap_java_to_managed / clr_typemap_java_to_managed) or, on the trimmable path, TrimmableTypeMapTypeManager. The [TrimmableTypeMap] work makes this generated class even more clearly obsolete.

Evidence it is dead code (verified on main)

• No caller of the no-arg __TypeRegistrations.RegisterPackages () exists anywhere in source. The generated class has no [ModuleInitializer]/static-ctor hook, so it is never auto-invoked.
• Java.Interop.TypeManager.packageLookup is write-only: RegisterPackage / RegisterPackages populate it (src/Mono.Android/Java.Interop/TypeManager.cs:465-497); nothing reads it.
• The Lookup(...) helper and per-package lookup_<pkg>_package converters in the generated file are unreferenced.
• Mono.Android's own emitted copy is 2448 lines / 174 converters (src/Mono.Android/obj/.../mcw/Java.Interop.__TypeRegistrations.cs) of dead code; every binding assembly carries a similar file.

Current locations

• Generator (primary target): external/Java.Interop/tools/generator/Java.Interop.Tools.Generator.ObjectModel/ClassGen.cs
  • GenerateTypeRegistrations(opt, gen_info) (line ~172) writes the partial class __TypeRegistrations with RegisterPackages(), Lookup(...), and the per-package converters. It already early-returns for CodeGenerationTarget.JavaInterop1.
• Generator call site: external/Java.Interop/tools/generator/CodeGenerator.cs:251 (ClassGen.GenerateTypeRegistrations(opt, gen_info);).
• Runtime plumbing now unused: src/Mono.Android/Java.Interop/TypeManager.cs
  • packageLookup field + LazyInitPackageLookup() (lines ~456-463)
  • RegisterPackage(string, Converter<string, Type>) (line ~465)
  • RegisterPackages(string[], Converter<string, Type?>[]) (line ~476)
  • LookupTypeMapping(string[], string) (line ~90) — only used by the generated Lookup.
• Public shim: src/Mono.Android/Android.Runtime/TypeManager.cs (the [Obsolete] wrappers forwarding to the above).
• Generator test baselines: external/Java.Interop/tests/generator-Tests/expected.xaji/*/Java.Interop.__TypeRegistrations.cs (one per fixture) plus the generator-Tests support TypeManager.cs.

Suggested implementation

1. Stop generating __TypeRegistrations for the XAJavaInterop1 target as well (the method already no-ops for JavaInterop1). Simplest: make GenerateTypeRegistrations return early for all targets, or remove the method and its call in CodeGenerator.cs.
2. Update / delete the expected.xaji generator-test baselines accordingly so generator-Tests stay green.
3. Remove the now-unused runtime plumbing in Java.Interop/TypeManager.cs (packageLookup, LazyInitPackageLookup, RegisterPackage, the internal RegisterPackages body, and LookupTypeMapping if it has no remaining callers).

Public API consideration

Java.Interop.TypeManager.RegisterPackages(string[], Converter<string, Type?>[]), RegisterPackage(...), LookupTypeMapping(...) and the [Obsolete] Android.Runtime.TypeManager shims are shipped public API (present in src/Mono.Android/PublicAPI/*/PublicAPI.Shipped.txt). Removing the generation is safe and self-contained; removing the public members must go through the normal API-removal process (or they can be kept as no-op stubs). This issue can be split into two PRs: (a) stop generating __TypeRegistrations, (b) retire the public API.

Acceptance criteria

• The binding generator no longer emits Java.Interop.__TypeRegistrations for any codegen target.
• generator-Tests baselines updated; generator-Tests pass.
• Mono.Android.dll and customer binding assemblies no longer contain the dead class (verify the file disappears from obj/.../mcw/ and from a built binding).
• No behavioral change: Java→managed type resolution continues to go through the native typemap / TrimmableTypeMapTypeManager.
• Existing Mono.Android and device tests pass.

References

• [Mono.Android] remove Xamarin.Android.VSAndroidDesigner.IsSupported feature switch #9471 — removed the runtime fallback that was the sole consumer (commit 6ac69bee2)
• [illink] Type registrations fallback switch #5629 — earlier "[illink] Type registrations fallback switch" that put the fallback behind a switch (commit 5b3d8cdd1)
• [Microsoft.Android.Sdk.ILLink] remove more linker sources #8572 — "[Microsoft.Android.Sdk.ILLink] remove more linker sources"

[simonrozsival]

Addressed by a coordinated pair of PRs:

• [generator] Stop generating dead Java.Interop.__TypeRegistrations java-interop#1470 — stops the generator from emitting the dead Java.Interop.__TypeRegistrations class (+ updates generator-Tests baselines; 455 passed, 0 failed).
• [Mono.Android] Remove dead Java.Interop.__TypeRegistrations plumbing #11667 — bumps the external/Java.Interop submodule to the above and removes the now-dead packageLookup runtime plumbing in Mono.Android.

The android PR's submodule bump points at the java-interop PR branch; it should be updated to the merge commit once #1470 lands.

[simonrozsival]

Update: the two PRs are now decoupled (no submodule bump pinning them together), which also fixed the confusing merge conflicts on the java-interop PR.

• [generator] Stop generating dead Java.Interop.__TypeRegistrations java-interop#1470 — generator-side: stops emitting the dead class. Rebased onto java-interop main (the earlier conflicts were because it had been branched from the older android-pinned submodule SHA). generator-Tests: 455 passed, 0 failed; now cleanly mergeable.
• [Mono.Android] Remove dead Java.Interop.__TypeRegistrations plumbing #11667 — consumer-side: removes the dead packageLookup plumbing in Mono.Android (now a single-file change, no submodule bump).

Each is independently mergeable. #1470's effect reaches Mono.Android via the normal external/Java.Interop bump after it merges (bundled with the other java-interop main changes such as #1441), rather than through #11667.