[TrimmableTypeMap] Implement alias support in codegen and runtime

[simonrozsival]

Summary

Implements trimmable typemap alias support end-to-end, covering both generated metadata/code and runtime lookup/activation.

Fixes #11103
Part of #10788

Changes

New: IJavaPeerAliases interface (JavaPeerProxy.cs)

• string[] Aliases { get; } — lists the exact TypeMap keys for an alias group

Build-time (ModelBuilder + Emitter)

For alias groups (≥2 .NET types sharing a JNI name, e.g., JavaCollection / JavaCollection<T>):

• Generates an alias holder class extending JavaPeerProxy + IJavaPeerAliases
• Base JNI name entry → alias holder (self-referencing trim target)
• Each type gets [0]-based indexed entry: "jni/Name[0]", "jni/Name[1]", ...
• TypeMapAssociation links each type to the alias holder (keeps holder alive when any alias survives trimming)
• TargetType / CreateInstance throw NotSupportedException (alias holder is never used for peer creation)
• Also adds self-application (AddCustomAttribute on typeDefHandle) for all proxy types — required for GetCustomAttribute<JavaPeerProxy>() to work at runtime

Runtime (TrimmableTypeMap.cs)

• GetProxyForManagedType(): if (proxy is IJavaPeerAliases aliases) → iterate exact keys from Aliases property
• GetAllTypesForJniName(): detect alias holder, enumerate listed keys
• ActivateInstance(): same alias holder detection
• Zero cost for non-alias types — single is check that's false, no dictionary probing

Generated output for an alias group

// TypeMap entries:
[assembly: TypeMap<Object>("some/JavaType", typeof(JavaType_Aliases), typeof(JavaType_Aliases))]
[assembly: TypeMap<Object>("some/JavaType[0]", typeof(A_Proxy), typeof(A))]
[assembly: TypeMap<Object>("some/JavaType[1]", typeof(B_Proxy), typeof(B))]

// Keep alias holder alive when any alias type survives trimming:
[assembly: TypeMapAssociation<Object>(typeof(A), typeof(JavaType_Aliases))]
[assembly: TypeMapAssociation<Object>(typeof(B), typeof(JavaType_Aliases))]

// Alias holder — IS a JavaPeerProxy, implements IJavaPeerAliases:
[JavaType_Aliases]
class JavaType_Aliases : JavaPeerProxy, IJavaPeerAliases
{
    public override Type TargetType => throw new NotSupportedException();
    public override IJavaPeerable? CreateInstance(...) => throw new NotSupportedException();
    public string[] Aliases => new[] { "some/JavaType[0]", "some/JavaType[1]" };
}

Tests

331 passing (+9 new):

• 3 alias fixture types (3-way alias group mirroring JavaCollection/JavaCollection<T> pattern)
• Model builder tests for alias holders, 3-way aliases, mixed activation
• Fixture scanner tests verifying alias detection from real assemblies
• Integration test verifying alias holder class + IJavaPeerAliases in emitted PE

[Copilot]

Pull request overview

Implements end-to-end alias support for the Trimmable TypeMap pipeline (codegen + runtime), enabling multiple managed types to share a single JNI name via an alias-holder indirection and indexed alias keys.

Changes:

• Add IJavaPeerAliases and generate alias-holder proxy types + indexed TypeMap entries + TypeMapAssociation records for alias groups.
• Update runtime lookup/activation paths (TrimmableTypeMap*) to expand alias holders into their concrete alias entries.
• Add/expand unit tests and fixture types covering 2-way/3-way aliases and emission round-trips.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/TestFixtures/TestTypes.cs	Adds fixture types that intentionally share a JNI name to form a 3-way alias group.
tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapModelBuilderTests.cs	Updates/extends model-builder tests for indexed alias entries, alias holders, and associations.
tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs	Adds a round-trip emission test asserting alias-related typemap artifacts are present in emitted metadata.
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapTypeManager.cs	Switches to enumerating all types for a JNI name (to support alias expansion).
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMap.cs	Adds alias-aware type enumeration and alias resolution for proxy lookup and activation.
src/Mono.Android/Java.Interop/JavaPeerProxy.cs	Introduces IJavaPeerAliases used by generated alias-holder proxy types.
src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs	Emits alias-holder types and self-applies proxy/holder attributes for AOT-safe GetCustomAttribute instantiation.
src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/ModelBuilder.cs	Generates alias holders, base-entry indirection, indexed alias keys, and associations for duplicate JNI-name groups.
src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/Model/TypeMapAssemblyData.cs	Extends the model to carry alias-holder emission data.