[ILLink] Fix crash when array types are used as TypeMap trim targets with PublishTrimmed

[Copilot]

PublishTrimmed crashes with NotSupportedException: TypeDefinition cannot be resolved from 'Mono.Cecil.ArrayType' when a TypeMap attribute specifies an array type as its trim target (e.g., typeof(Foo[])). PublishAot is unaffected (handled in #123031).

Root cause: LinkContext.Resolve throws for any TypeSpecification that isn't a GenericInstanceType. Array types, pointer types, and byref types all fall into this category.

Description

• TypeMapHandler.AddExternalTypeMapEntry: Strip non-resolvable TypeSpecification wrappers (array, pointer, byref, etc.) from the trim target before passing to RecordTypeMapEntry, using the element type instead. Loop terminates at the first TypeDefinition or GenericInstanceType (both resolvable).
• TypeMapHandler.AddProxyTypeMapEntry: Same fix applied symmetrically to the source type parameter. Corrected the comment to accurately state that TypeMapAssociationAttribute has two constructor type arguments (source and proxy).

// Before fix: crashes with NotSupportedException
[assembly: TypeMap<TestTypeMap>("Key", typeof(SomeTarget), typeof(Foo[]))]

// After fix: strips Foo[] → Foo, uses Foo as the trim dependency
// The TypeMap entry is included iff Foo (the element type) is trimmer-reachable

The loop handles nested arrays (Foo[][] → Foo), pointer types, and mixed cases like List<Foo>[] (strips to List<Foo>, which is a GenericInstanceType and resolvable).

Customer Impact

PublishTrimmed builds fail with an unhandled NotSupportedException / fatal ILLink error when any assembly attribute uses an array type as a TypeMap trim target. No workaround short of removing the array type.

Regression

No — this is a new feature (TypeMap) that never handled array trim targets correctly in ILLink.

Testing

Added test cases to TypeMap.cs:

• TrimTargetIsUsedArrayType: array trim target whose element type is reachable — attribute must be preserved; verified with KeptAttributeAttribute.
• TrimTargetIsUnusedArrayType: array trim target whose element type is unreachable — attribute and target types are removed; verified with explicit RemovedTypeInAssembly("test", ...) assertions on both ArrayTypeTrimTargetUnusedClass and ArrayTypeTrimTargetUnusedTarget.

Both existing Reflection.TypeMap and Reflection.TypeMapEntryAssembly tests pass.

Risk

Low. The change is localized to two private methods in TypeMapHandler. The stripping logic mirrors the existing TypeSpecification/GenericInstanceType guard already present in LinkContext.Resolve. No behavior change for existing valid (non-array) trim targets.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

[Copilot]

Pull request overview

Fixes an ILLink crash in PublishTrimmed when TypeMap attributes specify non-resolvable TypeSpecification types (notably arrays) as trim targets by stripping wrappers down to a resolvable element type before calling LinkContext.Resolve.

Changes:

• Normalize TypeMap trim targets and association source types by unwrapping TypeSpecification (array/pointer/byref/…) to an element type until reaching a TypeDefinition or GenericInstanceType.
• Add new linker test coverage for an array-type trim target scenario (kept case), and introduce a second unused-case scenario that should assert removal.

Reviewed changes

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

File	Description
src/tools/illink/src/linker/Linker/TypeMapHandler.cs	Unwraps TypeSpecification wrappers before resolving trim targets / proxy source types to avoid NotSupportedException crashes.
src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/TypeMap.cs	Adds test inputs for array-type trim targets and ensures at least the reachable-element case is preserved.

[jkoritzinsky]

@copilot please address the feedback from @copilot and validate test changes

[Copilot]

@copilot please address the feedback from @copilot and validate test changes

Done in af0ac93:

1. Added explicit RemovedTypeInAssembly("test", typeof(ArrayTypeTrimTargetUnusedClass)) and RemovedTypeInAssembly("test", typeof(ArrayTypeTrimTargetUnusedTarget)) assertions on the TypeMap class so the expected removal is explicitly verified by the test runner.
2. Fixed the misleading comment in AddProxyTypeMapEntry — it now says "two constructor type arguments (source and proxy)" instead of "single type argument".

Both Reflection.TypeMap and Reflection.TypeMapEntryAssembly tests still pass.