Fix broken main: acceptance source-gen harness leaks nested bin/obj into multi-project asset builds

[Evangelink]

Problem

main has been red across many consecutive official builds (e.g. build 3010746).

The failing test is MSTest.Acceptance.IntegrationTests.AssemblyFixtureProviderTests.AssemblyFixtureProvider_FromReferencedLibrary_RunsAssemblyInitializeAndCleanup. It fails in its ClassInitialize because the source-gen build variant of the AssemblyFixtureProviderAcceptance asset fails to compile with MSB3277.

Root cause (a harness bug, not a product bug)

TestAssetFixtureBase.InitializeAsync builds each acceptance asset twice in the same directory: first the reflection build (default bin/Release), then the source-gen build (with bin/obj redirected to a *SourceGen sub-folder).

For a multi-project asset — AssemblyFixtureProviderAcceptance's test project references ProviderLibrary — the first (reflection) build leaves ProviderLibrary/bin/Release/<tfm>/*.dll on disk. When the later source-gen build then evaluates the host project, the SDK's default item globs pull those stray DLLs in as None items (confirmed in the binlog: AddItem None -> ProviderLibrary\bin\Release\net10.0\MSTest.TestFramework.Extensions.dll). RAR then treats the wrong-TFM copy (a net10.0 MSTest.TestFramework.Extensions.dll, assembly version 9.0.0.0) as a candidate for the net8.0 leg:

error MSB3277: Found conflicts between different versions of "System.Runtime" ... 8.0.0.0 vs 9.0.0.0
  ...ProviderLibrary\bin\Release\net10.0\MSTest.TestFramework.Extensions.dll

MSBuildTreatWarningsAsErrors=true (passed only to the source-gen build) promotes this to an error, so ClassInitialize throws.

The injected props already re-excluded bin/obj, but only at the asset root (bin/**;obj/**), which does not match the nested ProviderLibrary/bin/**. The reflection build (#1) is unaffected because at its evaluation time the referenced project's bin is still empty — only the later source-gen build (#2) runs in a directory polluted by #1.

Fix

Broaden the injected DefaultItemExcludes in AcceptanceSourceGen to also exclude nested bin/obj (**/obj/**;**/bin/**), so a referenced project's leftover reflection outputs stay out of the source-gen build's globs. This is the proper harness fix and keeps the existing redirect-based design.

With the harness corrected, the AssemblyFixtureProvider opt-out is unnecessary, so the asset is restored to source-gen coverage.

Verification

• Reproduced the original MSB3277 from the CI binlog (binlog-mcp).
• Ran the AssemblyFixtureProvider acceptance tests (which drive both the reflection and source-gen builds via the fixture) — all 3 pass across net462/net8.0/net10.0.

[Copilot]

Pull request overview

This PR fixes a persistent main build break by preventing an unused acceptance-test asset build variant (source-generation) from compiling, avoiding MSBuild reference-resolution conflicts caused by a multi-targeting ProjectReference in the asset.

Changes:

• Opt the AssemblyFixtureProviderAcceptance asset out of source-generation builds by overriding SourceGenMetadataModes to an empty set.
• Add an inline comment documenting the RAR/MSB3277 conflict scenario and why reflection-only is sufficient for this test.

Show a summary per file

File	Description
test/IntegrationTests/MSTest.Acceptance.IntegrationTests/AssemblyFixtureProviderTests.cs	Overrides the asset fixture’s SourceGenMetadataModes to skip source-gen builds that are not exercised by the test and currently fail due to assembly version conflicts.

Review details

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

[Evangelink]

Note

🤖 Automated review by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Expert Code Review workflow. To request a follow-up action, reply by tagging @copilot directly.

Expert Review — PR #9504

Scope: Single file change in test/IntegrationTests/MSTest.Acceptance.IntegrationTests/AssemblyFixtureProviderTests.cs. The change adds a SourceGenMetadataModes => [] override and explanatory comment to TestAssetFixture, opting the AssemblyFixtureProviderAcceptance asset out of the source-gen build.

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Pattern Verification

The escape hatch used here (protected override IReadOnlyList<MetadataMode> SourceGenMetadataModes => []) is the documented, established opt-out mechanism. It is applied identically in FrameworkOnlyTests.TestAssetFixture (which also carries an explanatory comment stating the reason). The TestAssetFixtureBase XML doc on SourceGenMetadataModes explicitly names this as the opt-out path for assets that "genuinely cannot build under source generation."

Confirmed that TestHost.LocateFrom(AssetFixture.TargetAssetPath, TestAssetFixture.TestProjectName, tfm) is called without a metadataMode argument — so the reflection build is the only one exercised by the test. Opting out of building the source-gen variant does not remove any test execution coverage.

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22-Dimension Summary

#	Dimension	Verdict	Notes
1	Algorithmic Correctness	✅ LGTM	Fix matches root cause: source-gen build fails, test never uses source-gen variant; opting out is correct, not a symptom patch.
2	Threading & Concurrency	N/A	No new concurrent code.
3	Security & IPC Contract Safety	N/A	No security boundary touched.
4	Public API & Binary Compatibility	N/A	No public API changes.
5	Performance & Allocations	N/A	Test fixture code; not a hot path.
6	Cross-TFM Compatibility	N/A	The fix avoids the cross-TFM RAR issue by not building the failing variant. The => [] collection expression is valid on all targeted TFMs.
7	Resource & IDisposable Management	N/A	No new disposable resources.
8	Defensive Coding at Boundaries	N/A	No boundary logic changed.
9	Localization & Resources	N/A	No user-facing strings.
10	Test Isolation	✅ LGTM	Single test method in class — no shared mutable asset across methods, so [DoNotParallelize] is not required.
11	Assertion Quality	N/A	No assertion changes.
12	Flakiness Patterns	✅ LGTM	This PR fixes a reliability failure; no new flakiness introduced.
13	Test Completeness & Coverage	✅ LGTM	The source-gen variant was never exercised by any test method. No coverage is lost. See observation below regarding a potential follow-up.
14	Data-Driven Test Coverage	N/A	No data rows changed.
15	Code Structure & Simplification	✅ LGTM	Minimal, clean override consistent with FrameworkOnlyTests.
16	Naming & Conventions	✅ LGTM	Consistent with existing pattern.
17	Documentation Accuracy	✅ LGTM	Comment accurately explains the RAR resolution issue, the exact error (MSB3277), the promoter (MSBuildTreatWarningsAsErrors), and the justification. Matches the FrameworkOnlyTests comment style.
18	Analyzer & Code Fix Quality	N/A	No src/Analyzers/ changes.
19	IPC Wire Compatibility	N/A	No serialization changes.
20	Build Infrastructure & Dependencies	N/A	No eng/ or package reference changes.
21	Scope & PR Discipline	✅ LGTM	Single-concern change; PR description is thorough with root-cause analysis and verification steps.
22	PowerShell Scripting Hygiene	N/A	No .ps1 changes.

22/22 dimensions clean.

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💡 Optional Follow-up Observation (non-blocking)

The underlying MSBuild/RAR issue — where the source-gen build's isolated bin/obj redirection causes the net8.0 leg of a multi-targeting ProjectReference to resolve transitive assemblies from the net10.0 output — will silently affect any other acceptance asset that has a similar multi-targeting ProjectReference. Opting this asset out is the right short-term fix to unblock main, but it may be worth filing a tracking issue to investigate whether the source-gen build infrastructure should be hardened to pass -p:TargetFrameworks=... or a comparable property to pin transitive reference resolution to the correct TFM subfolder. If such an issue already exists, linking it from the comment would help future readers.

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Verdict: This is a correct, minimal, well-documented fix that follows the established pattern. Ready to merge.

[Evangelink]

🧪 Test quality grade — PR #9504

No new or modified test methods were identified in the changed regions
of this PR. Nothing to grade.

The only changed file (test/Utilities/Microsoft.Testing.TestInfrastructure/AcceptanceSourceGen.cs) is infrastructure/utility code that generates MSBuild props content — it contains no test methods.

_{Re-run with /grade-tests.}

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