Fix XALNS7015: run FixAbstractMethodsStep before R2R in non-trimmed CoreCLR builds

[simonrozsival]

Summary

Fixes #11025

In non-trimmed CoreCLR Release builds, PublishReadyToRun=true causes the inner build to compile IL assemblies to R2R images (via crossgen2) before _LinkAssembliesNoShrink runs. R2R assemblies have the ILONLY PE flag cleared, so when FixAbstractMethodsStep detects a missing abstract method and marks the assembly modified, SaveChangedAssemblyStep calls assembly.Write() via Mono.Cecil, which throws:

NotSupportedException: Writing mixed-mode assemblies is not supported

Changes

Fix: move _LinkAssembliesNoShrink to run before R2R compilation in the inner build

The previous _LinkAssembliesNoShrink target ran in the outer build using @(ResolvedAssemblies) and copied modified assemblies to an intermediate directory. This happened too late — the inner build had already produced R2R images by that point.

The fix restructures _LinkAssembliesNoShrink to run inside the inner build (BeforeTargets="_PrepareForReadyToRunCompilation"):

• Xamarin.Android.Common.targets: Rewrite _LinkAssembliesNoShrink to run in the inner build when trimming is disabled. It operates on @(ResolvedFileToPublish) (available in the inner build) instead of @(ResolvedAssemblies) (outer build only). Assemblies are copied to an intermediate directory (linked-noshrink/) rather than modified in-place, since ResolvedFileToPublish items may point to shared locations (NuGet cache, runtime packs). After the task runs, ResolvedFileToPublish is updated so R2R and publish consume the modified copies. ABI metadata is computed via RuntimeIdentifierToAbi since _ResolveAndroidTooling does not run in the inner build.
• Microsoft.Android.Sdk.AssemblyResolution.targets: In _PrepareAssemblies, populate _ResolvedAssemblies etc. unconditionally from @(ResolvedAssemblies) (no longer redirecting through the intermediate directory for non-trimmed builds).

Tests

• Remove Assert.Ignore workarounds in BuildTest.SimilarAndroidXAssemblyNames and LinkerTests.AndroidAddKeepAlives for the CoreCLR Release non-trimmed case — these scenarios now pass.

Related

• PR Run AssemblyModifierPipeline before R2R #10891 addresses the same class of issue for the trimmed build path and also handles sidecar XML file copying for R2R/NativeAOT output directories.

[Copilot]

Pull request overview

Fixes the CoreCLR Release (non-trimmed) PublishReadyToRun=true build failure where FixAbstractMethodsStep tries to rewrite an already-R2R’d assembly and Mono.Cecil throws NotSupportedException for mixed-mode output.

Changes:

• Run PostTrimmingPipeline before crossgen2/ReadyToRun for both trimmed and non-trimmed PublishReadyToRun builds, enabling FixAbstractMethods only in the non-trimmed path.
• Update PostTrimmingPipeline to optionally include FixAbstractMethodsStep and mark processed assemblies as Android/user assemblies in StepContext.
• Re-enable previously ignored CoreCLR test cases and add a unit regression test for FixAbstractMethods behavior via PostTrimmingPipeline.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/LinkerTests.cs	Adds a regression unit test validating PostTrimmingPipeline can inject a missing abstract-method stub.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs	Removes the CoreCLR non-trimmed ignore workaround now that the pipeline should run pre-R2R.
src/Xamarin.Android.Build.Tasks/Tasks/PostTrimmingPipeline.cs	Adds FixAbstractMethods support and updates StepContext flags so FixAbstractMethodsStep can run.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.LlvmIr.targets	Extends _PostTrimmingPipeline to also run for non-trimmed R2R builds and wires FixAbstractMethods into the task call.

[simonrozsival]

Replying to the Copilot review comments — the implementation was reworked in commits 3-4, so all four comments are now outdated. Resolving them below.