[TrimmableTypeMap] MSBuild task + targets wiring + incremental builds

[simonrozsival]

Part of #10789
Depends on: Generator sub-issue (PR2)

Overview

Wire the scanner and generators into the MSBuild pipeline: implement the GenerateTypeMapAssembly task and fill in the trimmable .targets stubs.

MSBuild Task (GenerateTypeMapAssembly)

Thin orchestrator extending AndroidTask:

Inputs:

• [Required] ResolvedAssemblies (ITaskItem[]) — all resolved assemblies to scan
• [Required] OutputDirectory (string) — output dir for generated assemblies
• [Required] JavaSourceOutputDirectory (string) — output dir for JCW .java files
• CustomViewMapFile (string) — custom view types from layout XML
• ErrorOnCustomJavaObject (bool)

Outputs:

• [Output] GeneratedAssemblies (ITaskItem[]) — per-assembly typemaps + root assembly
• [Output] GeneratedJavaFiles (ITaskItem[]) — generated .java files
• [Output] TypeMapEntryAssemblyName (string)
• [Output] UpdatedResolvedAssemblies (ITaskItem[])

RunTask() flow:

1. Scan assemblies → List<JavaPeerInfo> (per-assembly, with incremental caching)
2. Load custom view map → mark matching types unconditional
3. Generate per-assembly typemap assemblies (only for changed inputs)
4. Generate root assembly (alias detection, cross-ref unconditional forcing)
5. Generate JCW .java files for ACW types
6. Set output properties

Targets Wiring

Microsoft.Android.Sdk.TypeMap.Trimmable.targets

• UsingTask declaration for GenerateTypeMapAssembly (from Microsoft.Android.Build.TypeMap.dll)
• Define _TypeMapAssemblyName = _Microsoft.Android.TypeMaps
• RuntimeHostConfigurationOption for System.Runtime.InteropServices.TypeMappingEntryAssembly

Microsoft.Android.Sdk.TypeMap.Trimmable.CoreCLR.targets

• _GeneratePerAssemblyTypeMaps target with per-assembly Inputs/Outputs for incrementality
• _GenerateRootTypeMap target for root assembly generation
• Adds generated assemblies to resolved assemblies list for ILLink with TrimmerRootAssembly metadata
• Adds --typemap-entry-assembly:$(_TypeMapAssemblyName) to ILLink args
• Override _PrepareNativeAssemblySources as empty (no native typemap)

Microsoft.Android.Sdk.TypeMap.Trimmable.NativeAOT.targets

• Same generation targets
• Adds assemblies to IlcReference, UnmanagedEntryPointsAssembly, and TrimmerRootAssembly

Incremental Build Support

Per-assembly typemap generation with MSBuild Inputs/Outputs:

<Target Name="_GeneratePerAssemblyTypeMaps"
        Inputs="@(_ResolvedUserMonoAndroidAssemblies)"
        Outputs="@(_ResolvedUserMonoAndroidAssemblies->'$(IntermediateOutputPath)typemaps/%(Filename).TypeMap.dll')">

Typical incremental build: only app assembly changed → rescan app, regenerate app's typemap + root assembly. Cost: <0.5s.

Tests

• Task integration tests with MockBuildEngine
• Full build with _AndroidTypeMapImplementation=trimmable succeeds
• Artifact validation: no .o typemap objects, no legacy mono.android.jar copy
• Incremental build: second build with no changes skips generation
• Incremental build: app assembly change only regenerates app typemap