Trimmer removes `InternalsVisibleTo` attributes for assemblies which are trimmed

[vitek-karas]

Trimmer has special code to handle InternalsVisibleToAttribute. Specifically, it tries to remove those attribute instances which point to assemblies which either can't be resolved, or which are not kept in the app after trimming.

https://github.com/dotnet/runtime/blob/main/src/tools/illink/src/linker/Linker.Steps/MarkStep.cs#L295-L307

This creates an observable difference in behavior between trimmed and non-trimmed apps without getting a trim analysis warning. The app can use reflection to inspect the InternalsVisibleTo attributes and it will see a different set after trimming.

The trimmer should preserve all InternalsVisibleTo attributes, the size savings are very small.

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Issue Details

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Trimmer has special code to handle InternalsVisibleToAttribute. Specifically, it tries to remove those attribute instances which point to assemblies which either can't be resolved, or which are not kept in the app after trimming.

https://github.com/dotnet/runtime/blob/main/src/tools/illink/src/linker/Linker.Steps/MarkStep.cs#L295-L307

This creates an observable difference in behavior between trimmed and non-trimmed apps without getting a trim analysis warning. The app can use reflection to inspect the InternalsVisibleTo attributes and it will see a different set after trimming.

The trimmer should preserve all InternalsVisibleTo attributes, the size savings are very small.

Author:	vitek-karas
Assignees:	-
Labels:	area-Tools-ILLink
Milestone:	-

[vitek-karas]

There are existing tests covering this functionality https://github.com/dotnet/runtime/blob/main/src/tools/illink/test/Mono.Linker.Tests.Cases/Attributes/IVTUnused.cs.