Move diagnostic generation from LibraryImportGenerator to LibraryImportDiagnosticsAnalyzer

[Copilot]

Description

Moves diagnostic generation from the LibraryImportGenerator source generator to a new LibraryImportDiagnosticsAnalyzer, per Roslyn team recommendation.

Changes:

• Added LibraryImportDiagnosticsAnalyzer.cs with infrastructure to run diagnostic logic
  • Uses SymbolEqualityComparer to compare attribute types instead of string matching
  • Returns early when ProcessLibraryImportAttribute returns null (invalid compilation)
  • GetDiagnosticIfInvalidMethodForGeneration is internal static so the generator can call it directly
  • LibraryImportGeneratorOptions is created once per compilation in RegisterCompilationStartAction
• Updated CSharpSourceGeneratorVerifier to accept a TAnalyzer type parameter for specifying which analyzer to run alongside a generator
  • Uses collection expression syntax for GetDiagnosticAnalyzers()
• Removed diagnostic reporting from LibraryImportGenerator:
  • Removed all DiagnosticOr usage from the pipeline
  • Generator calls LibraryImportDiagnosticsAnalyzer.GetDiagnosticIfInvalidMethodForGeneration directly
  • Methods are filtered in a single Where clause that combines null check and validity check
• Updated diagnostic tests to use CSharpAnalyzerVerifier instead of CSharpSourceGeneratorVerifier:
  • Tests now run only the analyzer without the generator
  • CSharpAnalyzerVerifier.Test ignores compiler diagnostics since partial methods won't have implementations without the generator
• Updated other test files to use two type parameters:
  • LibraryImportGenerator tests use LibraryImportDiagnosticsAnalyzer
  • Other generators use EmptyDiagnosticAnalyzer from Microsoft.CodeAnalysis.Testing as a placeholder

Current state: 672/703 tests pass (95%). The analyzer reports most diagnostics but doesn't yet handle all edge cases that the generator previously handled (e.g., InvalidStringMarshallingConfiguration validation, some MarshalAs field validations).

Next steps (future PRs):

• Complete analyzer to handle all diagnostic edge cases
• Apply pattern to ComInterfaceGenerator, JSImportGenerator, JSExportGenerator, DownlevelLibraryImportGenerator

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Mis-clicked approve

[jkoritzinsky]

@copilot address this review.

[jkoritzinsky]

@copilot please address the feedback in this review.