Make PrepareTrimConfiguration a public extension point

[sbomer]

Note

This PR was created with assistance from AI.

Summary

Rename _PrepareTrimConfiguration to PrepareTrimConfiguration so that external SDKs can hook BeforeTargets="PrepareTrimConfiguration" to add RuntimeHostConfigurationOption items before they are snapshotted into _TrimmerFeatureSettings.

Problem

PR #124801 extracted the RuntimeHostConfigurationOption → _TrimmerFeatureSettings conversion from PrepareForILLink into a new internal target _PrepareTrimConfiguration. This target runs as a transitive dependency of PrepareForILLink (via _ComputeManagedAssemblyToLink), so it executes before BeforeTargets="PrepareForILLink" hooks fire.

MAUI adds its feature switches (e.g. IsHybridWebViewSupported) via BeforeTargets="PrepareForILLink". After #124801, these items arrive after _PrepareTrimConfiguration has already snapshotted RuntimeHostConfigurationOption into _TrimmerFeatureSettings. As a result, ILC never receives --feature flags for MAUI's switches on Android NativeAOT (where RunILLink=true), causing spurious IL3050 warnings.

Fix

Make PrepareTrimConfiguration a public target so SDKs have a stable, intentional hook point for adding feature switch configuration that flows to both ILLink and ILC. MAUI (and other SDKs) can then use BeforeTargets="PrepareTrimConfiguration" to add their RuntimeHostConfigurationOption items before the snapshot.

Verified

Tested with a MAUI Android NativeAOT app:

• Before: --feature:Microsoft.Maui.RuntimeFeature.IsHybridWebViewSupported=false missing from ILC rsp → IL3050 emitted
• After: --feature flag present → IL3050 suppressed by FeatureGuard

Fixes #127017

cc @sbomer @simonrozsival

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[Copilot]

Pull request overview

This PR makes the trim configuration target a public MSBuild extension point so external SDKs can reliably add RuntimeHostConfigurationOption items before they’re snapshotted into _TrimmerFeatureSettings (and thus flow into both ILLink and ILC/NativeAOT). It also includes a linker marking behavior change around type-forwarders, with new tests covering the scenarios.

Changes:

• Rename _PrepareTrimConfiguration to PrepareTrimConfiguration and retarget dependencies to the new public name.
• Update NativeAOT publish targets to depend on / order around PrepareTrimConfiguration.
• Adjust ILLink MarkStep behavior for type-forwarding scenarios and add new test cases (plus ILTrim expected-failure entries).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets	Renames target to PrepareTrimConfiguration and documents it as an extension point; updates dependent target to use new name.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Publish.targets	Switches dependencies and ordering from _PrepareTrimConfiguration to PrepareTrimConfiguration.
src/tools/illink/src/linker/Linker.Steps/MarkStep.cs	Changes marking behavior for exported types/type references in rooted/copy scenarios.
src/tools/illink/test/Mono.Linker.Tests.Cases/TypeForwarding/RootedForwarderWithExportedTypesIsHandled.cs	New linker test for rooted forwarder assemblies with exported types.
src/tools/illink/test/Mono.Linker.Tests.Cases/TypeForwarding/UsedForwarderIsRemovedWhenReferencedByRootedAssembly.cs	New linker test ensuring forwarder removal even when the referencing assembly is rooted.
src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt	Adds the two new TypeForwarding tests to ILTrim’s expected-failure list.

[Copilot]

Pull request overview

This PR makes PrepareTrimConfiguration a public MSBuild extension point so external SDKs (e.g., MAUI) can reliably inject RuntimeHostConfigurationOption items before those settings are snapshotted into _TrimmerFeatureSettings and used by both ILLink and ILC (NativeAOT).

Changes:

• Renamed the internal target _PrepareTrimConfiguration to PrepareTrimConfiguration in ILLink targets and documented it as a public hook point.
• Updated NativeAOT publish target dependencies/orderings to reference PrepareTrimConfiguration instead of _PrepareTrimConfiguration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets	Exposes PrepareTrimConfiguration as a public target and wires _ComputeManagedAssemblyToLink to depend on it.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Publish.targets	Updates NativeAOT pipeline targets to depend on / schedule around the newly public PrepareTrimConfiguration.

[MichalStrehovsky]

Why is MAUI adding this in a target? This sounds like just asking for trouble. Is it guaranteed that this will end up in runtimeconfig.json in non-trimmed scenarios?

[sbomer]

This sounds like just asking for trouble. Is it guaranteed that this will end up in runtimeconfig.json in non-trimmed scenarios?

I believe the ordering is guaranteed by https://github.com/dotnet/maui/blob/fecaf3e041/src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.targets#L289 (relying on a private target). I agree it's asking for trouble.

I'm now thinking we should not guarantee that the trimmer settings pick up RuntimeHostConfigurationOption in any particular target - so anyone setting these items in a target is relying on an implementation detail. If that's unacceptable to maui folks we can revisit. Filed dotnet/maui#35163.