Add regression test for tuple swap with implicit indexer references

[sbomer]

Summary

Adds a regression test for #117122, a crash (Debug.Assert failure in VisitImplicitIndexerReference) hit by the ILLink Roslyn analyzer when analyzing a tuple-swap deconstruction assignment involving Span<T> elements accessed via the ^ (Index) operator, e.g.:

Span<int> span = stackalloc int[4];
(span[^1], span[^2]) = (span[^2], span[^1]);

Root cause

GetValueUsageInfo returns ValueUsageInfo.Write (without .Reference) for operations on the left side of a deconstruction assignment. VisitImplicitIndexerReference previously asserted that any Write-flagged implicit indexer reference must also be Reference, an assumption that didn't hold for deconstruction-assignment targets.

This is already fixed on main by #131624 ("Add dataflow support for deconstruction assignments"), which added proper VisitDeconstructionAssignment handling that routes deconstruction targets through dedicated assignment-processing logic instead of the generic operation visitor that could trip the old assert. That change closes #123767, and as a side effect also fixes this crash — verified by testing with only that fix applied (no separate patch needed here).

Testing

Folded the new test case (TestTupleSwap) into the existing ImplicitIndexerAccess nested test class in PropertyDataFlow.cs, since it already covers directly related ^-indexer and Span<T> write scenarios. Verified the test:

• Reproduces the crash against code prior to Add dataflow support for deconstruction assignments #131624.
• Passes with Add dataflow support for deconstruction assignments #131624 applied.
• Passes across all three tools that share this test-case file: the Roslyn analyzer (ILLink.RoslynAnalyzer.Tests), the IL trimmer (Mono.Linker.Tests), and NativeAOT (ILCompiler.Trimming.Tests).

Note

This content was created with assistance from AI.

Fixes #117122

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[Copilot]

Pull request overview

Adds regression coverage to prevent an ILLink Roslyn analyzer crash when analyzing tuple-swap deconstruction assignments that target Span<T> elements accessed via the ^ (Index) operator, matching the scenario from #117122.

Changes:

• Add a new TestTupleSwap case covering (span[^1], span[^2]) = (span[^2], span[^1]) within the existing ImplicitIndexerAccess coverage.
• Invoke TestTupleSwap from the nested ImplicitIndexerAccess.Test() entrypoint to keep it exercised consistently across tool suites using this file.

Show a summary per file

File	Description
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs	Adds a regression test case for tuple-swap deconstruction with implicit indexer (^) access on Span<T>.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

Review details

Suppressed comments (2)

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs:788

• The comment is slightly inaccurate: ^ is the C# index-from-end operator (which produces a System.Index), not an operator on System.Index itself. Rewording avoids confusion for future maintainers.

            // Tuple-swapping elements accessed through an implicit indexer (using the
            // System.Index '^' operator) used to crash the analyzer, because Roslyn
            // marks an implicit indexer reference that is a deconstruction assignment
            // target as a write without also marking it as a reference.

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs:791

• TestTupleSwap reads from span[^1]/span[^2] on the RHS, but the stackalloc int[4] buffer is left uninitialized. Initializing avoids reading uninitialized stack data while still exercising the same implicit-indexer/deconstruction pattern.

                Span<int> span = stackalloc int[4];

• Files reviewed: 1/1 changed files
• Comments generated: 0 new
• Review effort level: Lite

[agocke]

LGTM