Allow ReadyToRun-only references in BuildValidator

[mwiemer-microsoft]

AI-generated:

Summary

Isolates the BuildValidator reference-resolution change discussed in #85776 from the xUnit v3 migration. This PR is based on main and retains xUnit 2.9.2; no test-framework packages or migration commits are included.

main skips every ReadyToRun (R2R) candidate. Consequently, BuildValidator cannot resolve a reference when its only available candidate is an R2R image, even when its MVID matches. Cache IL candidates first, then consider R2R candidates only for MVIDs still missing. If an IL image is discovered later under a different filename, replace the cached R2R fallback for its MVID. This preserves IL preference across filename lookup order while allowing R2R-only references.

An IL assembly and its R2R image can share an MVID but differ in PE header values recorded in the PDB. This change retains the existing IL preference; it does not introduce timestamp/image-size matching or otherwise redesign the MVID cache.

Regression coverage

Adds a focused BuildValidator.UnitTests project to the main and BuildValidator solutions. Cases cover IL-only, R2R-only, both candidate orders, different-MVID IL/R2R candidates, an unknown MVID, and both cross-filename lookup orders. The fixture emits a tiny assembly and marks a copy with synthetic R2R header fields, preserving its MVID; it does not require crossgen2 or execute native code.

Before the behavior change, the R2R-only case failed; before the cross-filename cache follow-up, its IL-preference ordering case failed. After both fixes, all eight focused cases pass under xUnit v2.

Validation

• dotnet test src\Tools\BuildValidator.UnitTests\BuildValidator.UnitTests.csproj --no-restore -p:RunAnalyzersDuringBuild=true -v:q — 8 passed.
• dotnet build src\Tools\BuildValidator\BuildValidator.csproj --no-restore -p:RunAnalyzersDuringBuild=true -v:q — 0 warnings/errors.
• Scoped dotnet format whitespace --verify-no-changes and git diff --check — passed.
• Confirmed the resolved test framework remains xUnit 2.9.2. The VSTest adapter version is unchanged.

Full crossgen2 publish/rebuild pipeline validation was not run. #85776 and its review-thread resolution states were not modified.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation matches the resolver contract and has comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Enables BuildValidator to resolve ReadyToRun-only references while retaining IL preference for matching MVIDs.

Changes:

• Uses R2R images only when no matching IL candidate exists.
• Adds six focused regression cases.
• Integrates and documents the new test project.

File	Description
LocalReferenceResolver.cs	Adds IL-first, R2R-fallback caching.
BuildValidator.csproj	Exposes internals to tests.
BuildValidator.sln	Includes the test project.
LocalReferenceResolverTests.cs	Tests candidate resolution and ordering.
BuildValidator.UnitTests.csproj	Defines the test project.
Roslyn.slnx	Adds tests to the main solution.
.github/​memory/​testing/​compiler.md	Documents behavior and test command.

[mwiemer-microsoft]

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