Import illink.targets for ref projects

[sbomer]

This lets the logic from the ILLink targets add AssemblyMetadata("IsTrimmable", "true") to ref projects which have trimming enabled.

Also moves the trimming opt-out into Directory.Build.props when it should be shared between src and corresponding ref projects.

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[Copilot]

Pull Request Overview

This PR enables ILLink targets to add AssemblyMetadata("IsTrimmable", "true") to reference (ref) projects that have trimming enabled by importing the illink.targets for ref projects. The main change consolidates trimming opt-out configuration by moving IsTrimmable>false</IsTrimmable> settings from individual source project files to their corresponding Directory.Build.props files, ensuring the setting is shared between source and reference projects.

Key changes:

• Imports illink.targets for reference assembly projects in addition to source projects
• Moves trimming opt-out declarations from individual .csproj files to Directory.Build.props files
• Maintains existing trimming behavior while enabling proper metadata for ref projects

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/Directory.Build.targets	Updates condition to import illink.targets for reference assembly projects
src/libraries/System.Speech/src/System.Speech.csproj	Removes IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.Speech/Directory.Build.props	Adds IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.Configuration.ConfigurationManager/src/System.Configuration.ConfigurationManager.csproj	Removes trimming opt-out and comment
src/libraries/System.Configuration.ConfigurationManager/Directory.Build.props	Adds trimming opt-out with comment
src/libraries/System.ComponentModel.Composition/src/System.ComponentModel.Composition.csproj	Removes IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.ComponentModel.Composition/Directory.Build.props	Adds IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.ComponentModel.Composition.Registration/src/System.ComponentModel.Composition.Registration.csproj	Removes IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.ComponentModel.Composition.Registration/Directory.Build.props	Adds IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.CodeDom/src/System.CodeDom.csproj	Removes IsTrimmable>false</IsTrimmable> declaration
src/libraries/System.CodeDom/Directory.Build.props	Adds IsTrimmable>false</IsTrimmable> declaration

[ViktorHofer]

Can you please provide context on why this is necessary now? Reference assemblies never had that assembly attribute on them. Just for my own education.

[ViktorHofer]

Assuming that the import of illink.targets by ref projects don't trigger any additional work. LGTM.

[sbomer]

Should've mentioned in the PR description - this is for #117712. We want to have an option for the Roslyn analyzer to warn if there are references to assemblies that aren't marked IsTrimmable.

[ViktorHofer]

This implies that the contract (the reference assembly) now governs whether an implementation must be AOT compatible. We should probably document that. cc @ericstj @bartonjs

[sbomer]

I'm planning to add docs for the trim/aot behavior. Is there another doc that should be updated for ref assemblies in general?

[ericstj]

Why do we want to trim the reference assemblies? Seems like that can hide bugs in the linker like @tannergooding recently discovered. We don't have anything in the reference projects that we need the linker to trim, I'm not certain what the motivation for this change is.

[MichalStrehovsky]

Why do we want to trim the reference assemblies? Seems like that can hide bugs in the linker like @tannergooding recently discovered. We don't have anything in the reference projects that we need the linker to trim, I'm not certain what the motivation for this change is.

We don't want to trim them, just mark them as trimmable (including ILLink.targets generates the attribute based on IsTrimmable property). Context is in the above mentioned #117712.