Warn on field keyword references in trim analyzer

[jtschuster]

Depends on #117034

Adds warning when a 'field' keyword is the source in a dataflow assignment.

Does not warn when it is the target of an assignment yet.

[Copilot]

Pull Request Overview

This PR introduces a new warning for dataflow assignments that use the literal 'field' keyword as a source and updates many test cases and diagnostics accordingly. Key changes include:

• Updating ExpectedWarning attributes in numerous test files to include GitHub URLs (e.g. Consider detecting unnecessary annotations linker#1971, Trim analyzer has better control flow graph than illink and ilc #102830) and "NativeAOT-specific warning" strings.
• Modifying the trim analysis visitor and related classes to better handle backing field access and property dataflow.
• Adding new overloads and adjustments related to generic argument dataflow and backing field annotations.

Reviewed Changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/TrimAnalysisVisitor.cs	Added VisitInvalid override to detect a 'field' keyword and updated GetFieldTargetValue to use a new helper for backing fields.
src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/FieldValue.cs	Introduced a constructor overload for IPropertySymbol to support backing field analysis.
src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/FlowAnnotations.cs	Added a branch for backing field annotation that currently throws NotImplementedException.
src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalDataFlowVisitor.cs	Added abstract GetBackingFieldTargetValue to support property backing field dataflow.
Multiple test files under src/tools/illink/test/Mono.Linker.Tests.Cases/…	Updated warning attributes to align with new diagnostic messages and include GitHub issue URLs.

Comments suppressed due to low confidence (3)

src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/TrimAnalysisVisitor.cs:97

• Consider documenting the rationale for checking if operation.Syntax.ToString() equals 'field' in VisitInvalid, to ensure that this string-based check remains robust against potential syntax changes.

        public override MultiValue VisitInvalid(IInvalidOperation invalidOperation, StateValue argument)

src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/FieldValue.cs:19

• The addition of a constructor overload in FieldValue to support IPropertySymbol is clear; please ensure that treating property backing fields similar to regular fields in dataflow analysis is intended and sufficiently documented.

        public FieldValue(IPropertySymbol propertySymbol)

src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalDataFlowVisitor.cs:134

• The introduction of GetBackingFieldTargetValue improves handling of property backing fields in dataflow analysis. Please verify that all call sites invoking field dataflow now account for property references appropriately.

        public abstract TValue GetBackingFieldTargetValue(IPropertyReferenceOperation propertyReference, in TContext context);

[Copilot]

The branch in GetFieldAnnotation for fields that have an associated symbol (i.e. backing fields for properties) currently throws a NotImplementedException. Ensure that this scenario is properly handled before finalizing the change or add a detailed TODO comment for future implementation.

Suggested change

	throw new NotImplementedException($"Field {field.GetDisplayName()} is not implemented in FlowAnnotations.GetFieldAnnotation. This should never happen, please report a bug.");
	// return property.GetDynamicallyAccessedMemberTypes ();
	if (field.AssociatedSymbol is IPropertySymbol property)
	{
	return GetBackingFieldAnnotation(property);
	}
	else
	{
	// TODO: Handle other cases of associated symbols if necessary
	Debug.Fail($"Unexpected associated symbol type for field {field.GetDisplayName()} in FlowAnnotations.GetFieldAnnotation.");
	return DynamicallyAccessedMemberTypes.None;
	}

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