Add dataflow support for deconstruction assignments

[sbomer]

Fixes #123767.

The ILLink Roslyn analyzer's dataflow visitor did not handle IDeconstructionAssignmentOperation, so RequiresUnreferencedCode and DynamicallyAccessedMembers diagnostics were silently skipped for values flowing through deconstruction, including assignments and deconstruction in foreach.

Summary of changes

• Add dataflow support for tuple expressions, tuple-typed values, instance and extension Deconstruct methods, nested deconstruction, and user-defined conversions.
• Support local, field, property, parameter, discard, array-element, explicit-indexer, implicit-indexer, and nested tuple targets.
• Preserve C# evaluation order by evaluating target locations before source values and performing target writes only after all source values and nested Deconstruct calls have been evaluated.
• Preserve flow-captured target receivers and index arguments so target locations are evaluated once and retain the values selected before source-side mutations.
• Correctly map receivers for classic extension methods and C# 14 extension-block Deconstruct methods.
• Treat outer or nested [DoesNotReturn] Deconstruct calls as unreachable and suppress every target assignment on that path.

Testing

Added regression coverage for deconstruction sources, targets, evaluation ordering, flow captures, extension receivers, user-defined conversions, and direct and nested [DoesNotReturn] methods. The DataFlow suites pass across:

• ILLink.RoslynAnalyzer.Tests
• Mono.Linker.Tests
• ILCompiler.Trimming.Tests
• ILTrim.Tests

Two unsupported ILTrim scenarios are isolated as standalone expected failures instead of excluding the shared deconstruction coverage:

• DeconstructFieldTarget exposes an existing ILTrim.Core gap in static-field write dataflow.
• DeconstructUserDefinedConversion exposes an existing ILTrim.Core gap in tracking a user-defined conversion operator's return value.

The Roslyn analyzer, linker, and NativeAOT tests retain coverage for both cases. ILLink/ILTrim and NativeAOT also conservatively report the expected warning after [DoesNotReturn] calls because the IL-based tools do not treat that attribute as a reachability contract; those expectations are scoped to the IL-based tools while the Roslyn analyzer verifies the path is unreachable.

Note

This content was created with assistance from AI.

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[Copilot]

Pull request overview

This PR extends the ILLink Roslyn analyzer’s local dataflow analysis to understand deconstruction assignments (IDeconstructionAssignmentOperation), and updates trimming/dataflow test coverage to exercise the new behavior across multiple toolchains.

Changes:

• Implement deconstruction assignment evaluation + assignment sequencing in LocalDataFlowVisitor, including tuple element handling.
• Extend TrimAnalysisVisitor to model tuple-element values for tuple-typed sources.
• Add/adjust regression tests in ConstructedTypesDataFlow.cs, and mark the ILTrim suite as an expected failure.

Show a summary per file

File	Description
src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalDataFlowVisitor.cs	Adds VisitDeconstructionAssignment support and supporting helpers for deconstruction evaluation/assignment.
src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/TrimAnalysisVisitor.cs	Implements tuple-element value modeling needed by deconstruction-from-tuple-type sources.
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/ConstructedTypesDataFlow.cs	Adds deconstruction regression coverage across many target/source forms and adjusts expected diagnostics.
src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt	Marks the full DataFlow.ConstructedTypesDataFlow suite as an expected ILTrim failure.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

Suppressed comments (2)

src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalDataFlowVisitor.cs:769

• The synthesized Deconstruct() call is dispatched via HandleMethodCall directly, which bypasses the base visitor’s DoesNotReturn handling. If a user-defined Deconstruct method is annotated with [DoesNotReturn], the current implementation would still proceed to evaluate nested values and assign targets, potentially producing warnings/state changes after a non-returning call.

Consider explicitly honoring DoesNotReturn here by resetting state to Top and stopping deconstruction assignment processing when the attribute is present.

                HandleMethodCall(
                    deconstructMethod,
                    instanceValue,
                    arguments.MoveToImmutable(),
                    target,

src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt:87

• The PR description says only DeconstructFieldTarget is added to ILTrimExpectedFailures.txt due to an ILTrim limitation, but this change also adds DeconstructUserDefinedConversion. Either the description should be updated to explain why this additional case is expected to fail under ILTrim, or the extra entry should be removed if it’s unintended.

DataFlow.DeconstructFieldTarget
DataFlow.DeconstructUserDefinedConversion

• Files reviewed: 8/8 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

Suppressed comments (2)

src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalStateLattice.cs:80

• New public types were introduced into the analyzer's public surface (CapturedTargetKey, CapturedTargetValue<TValue>, CapturedTargetValueLattice<,>). If this code isn't intended to be a supported public API, these should be non-public (and/or the containing public state types reshaped) to avoid expanding API surface without approval.

    public readonly struct CapturedTargetKey : IEquatable<CapturedTargetKey>
    {
        private readonly IOperation Operation;

        public CapturedTargetKey(IOperation operation) => Operation = operation;

        public bool Equals(CapturedTargetKey other) => Operation == other.Operation;

        public override bool Equals(object obj)
            => obj is CapturedTargetKey inst && Equals(inst);

        public override int GetHashCode() => Operation.GetHashCode();
    }

    public readonly struct CapturedTargetValue<TValue> : IEquatable<CapturedTargetValue<TValue>>, IDeepCopyValue<CapturedTargetValue<TValue>>
        where TValue : IEquatable<TValue>
    {
        public readonly bool HasValue;

        public readonly TValue Value;

        public CapturedTargetValue(TValue value) => (HasValue, Value) = (true, value);

        public bool Equals(CapturedTargetValue<TValue> other) =>
            HasValue == other.HasValue &&
            (!HasValue || EqualityComparer<TValue>.Default.Equals(Value, other.Value));

        public override bool Equals(object obj)
            => obj is CapturedTargetValue<TValue> inst && Equals(inst);

        public override int GetHashCode() => HasValue ? EqualityComparer<TValue>.Default.GetHashCode(Value) : 0;

        public CapturedTargetValue<TValue> DeepCopy() =>
            HasValue
                ? new CapturedTargetValue<TValue>(
                    Value is IDeepCopyValue<TValue> copyValue ? copyValue.DeepCopy() : Value)
                : default;
    }

    public readonly struct CapturedTargetValueLattice<TValue, TValueLattice> : ILattice<CapturedTargetValue<TValue>>
        where TValue : IEquatable<TValue>
        where TValueLattice : ILattice<TValue>

src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt:87

• PR description says only DeconstructFieldTarget needed adding to ILTrimExpectedFailures.txt, but this file adds both DataFlow.DeconstructFieldTarget and DataFlow.DeconstructUserDefinedConversion. Either update the PR description/testing notes to reflect both expected failures, or drop the extra expected-failure entry if it isn't required.

DataFlow.DeconstructFieldTarget
DataFlow.DeconstructUserDefinedConversion

• Files reviewed: 9/9 changed files
• Comments generated: 1