Move CheckForObsoletePreserveAttribute into PostTrimmingPipeline

[Copilot]

CheckForObsoletePreserveAttribute runs as a standalone MSBuild task on every incremental build, costing ~120ms to open and scan every user assembly with PEReader. This is unnecessary work since PostTrimmingPipeline already loads all assemblies via Mono.Cecil.

Changes

• New CheckForObsoletePreserveAttributeStep — implements IAssemblyModifierPipelineStep, checks for Android.Runtime.PreserveAttribute type references using the already-loaded AssemblyDefinition (zero additional I/O)
• Added step to PostTrimmingPipeline — runs before StripEmbeddedLibrariesStep, emits the same IL6001 warning
• Removed standalone infrastructure — deleted CheckForObsoletePreserveAttribute.cs task, its UsingTask declaration, and the _CheckForObsoletePreserveAttribute target from Microsoft.Android.Sdk.AssemblyResolution.targets

[jonathanpeppers]

@copilot there is a build error:

       (CoreCompile target) -> 
         /Users/builder/azdo/_work/2/s/android/src/Xamarin.Android.Build.Tasks/Tasks/PostTrimmingPipeline.cs(50,18): error CS0246: The type or namespace name 'CheckForObsoletePreserveAttributeStep' could not be found (are you missing a using directive or an assembly reference?) [/Users/builder/azdo/_work/2/s/android/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj]

    177 Warning(s)
    1 Error(s)

[Copilot]

Fixed in 3add14d. Two issues: (1) the Linker\** directory is excluded from compilation in the csproj, so the new file needed an explicit <Compile Include> entry, and (2) LogCodedWarning is an extension method from Microsoft.Android.Build.Tasks namespace which needed a using directive.

[Copilot]

Pull request overview

This PR aims to remove the incremental-build cost of scanning assemblies for Android.Runtime.PreserveAttribute references by moving the check into the existing post-trimming pipeline, reusing already-loaded Mono.Cecil assemblies.

Changes:

• Added a new CheckForObsoletePreserveAttributeStep implementing IAssemblyModifierPipelineStep to emit IL6001 without extra I/O.
• Registered the new step in PostTrimmingPipeline so it runs alongside other post-trim assembly modifications.
• Removed the standalone CheckForObsoletePreserveAttribute MSBuild task and its _CheckForObsoletePreserveAttribute target wiring.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj	Includes the new linker step source file in the explicit Linker compile list.
src/Xamarin.Android.Build.Tasks/Tasks/PostTrimmingPipeline.cs	Adds the new warning step into the post-trimming step list.
src/Xamarin.Android.Build.Tasks/Tasks/CheckForObsoletePreserveAttribute.cs	Removes the standalone PEReader-based task implementation.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets	Removes the UsingTask and target that ran the warning task after _PrepareAssemblies.
src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/CheckForObsoletePreserveAttributeStep.cs	New Cecil-based pipeline step that detects PreserveAttribute type references and logs IL6001.

[Copilot]

Removing the standalone _CheckForObsoletePreserveAttribute target drops the IL6001 warning from the normal _PrepareAssemblies path, and the replacement implementation currently only runs via PostTrimmingPipeline (which is conditioned on $(PublishTrimmed)). This likely changes behavior for standard Release linked builds (AndroidLinkMode/TrimMode) that don’t set PublishTrimmed—LinkerTests.WarnWithReferenceToPreserveAttribute currently covers this and will probably fail. Please ensure the warning is still emitted for the configurations where ILLink/linking runs today, or update the targets/conditions accordingly.

[Copilot]

Adding the warning check as a PostTrimmingPipeline step means it will only run when the PostTrimmingPipeline MSBuild target runs (currently gated on $(PublishTrimmed) in Microsoft.Android.Sdk.TypeMap.LlvmIr.targets). The existing build/test scenario LinkerTests.WarnWithReferenceToPreserveAttribute expects the IL6001 warning in a normal Release linked build without setting PublishTrimmed, so this change will likely stop emitting the warning (and break that test) for non-PublishTrimmed builds. Consider moving this check into the ILLink pipeline used for AndroidLinkMode/TrimMode release linking, or otherwise ensuring the step runs whenever linking/trimming is performed, not only for PublishTrimmed publishes.

Suggested change

steps.Add (new CheckForObsoletePreserveAttributeStep (Log));