Add (commented) test case that crashes LibraryImportGenerator

[jtschuster]

Ran into this while testing some some sample code. When a collection marshaller does not conform to the correct shape and an element marshaller is specified, we crash the generator with a NullRef.

Filed #117448 to track this.

[Copilot]

Pull Request Overview

This PR introduces a new test snippet that triggers a generator crash when a collection marshaller is mis-shaped, but leaves the test commented out until the issue is resolved.

• Added ImproperCollectionWithMarshalUsingOnElements snippet in CodeSnippets.cs.
• Added commented-out test entry in CompileFails.cs for the new snippet.

Reviewed Changes

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

File	Description
CompileFails.cs	Commented new test case for improper collection marshalling that currently crashes the generator
CodeSnippets.cs	Defined ImproperCollectionWithMarshalUsingOnElements snippet to reproduce the crash

Comments suppressed due to low confidence (3)

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/CompileFails.cs:60

• [nitpick] Add a TODO with a reference to the GitHub issue tracking this crash so it’s clear when to re-enable the test (e.g., // TODO: re-enable once dotnet/runtime#12345 is fixed).

            // Bug: crashes

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/CompileFails.cs:61

• The test for the improper collection scenario is currently disabled, leaving this crash case unverified; consider adding a placeholder assertion or a skipping mechanism that documents expected behavior until the bug is fixed.

            // yield return new[] { ID(), CodeSnippets.ImproperCollectionWithMarshalUsingOnElements };

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/CompileFails.cs:61

• The commented test uses new[] without specifying the expected DiagnosticResult array or matching the object[] signature used elsewhere. Use new object[] { ID(), CodeSnippets.ImproperCollectionWithMarshalUsingOnElements, Array.Empty<DiagnosticResult>() } to match the helper method signature.

            // yield return new[] { ID(), CodeSnippets.ImproperCollectionWithMarshalUsingOnElements };

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