Set CoreCLR runtime flavor for WASM Webcil conversion

[LoopedBard3]

Summary

Set RuntimeFlavor=CoreCLR when UseMonoRuntime=false in MicroBenchmarks.Wasm.targets, independently of the ReadyToRun opt-in. BenchmarkDotNet imports .Wasm.props before its CoreCLR override and .Wasm.targets afterward, so the setting must be applied in the latter.

After dotnet/runtime#134312, CoreCLR requires Webcil wrapper version 2. The WebAssembly SDK selects Webcil payload version 1 (which produces wrapper version 2) only when RuntimeFlavor=CoreCLR; the pinned BenchmarkDotNet generator sets UseMonoRuntime=false but leaves RuntimeFlavor unset. This can produce wrapper version 0 for converted assemblies, causing errors such as Webcil asset '/Dia2Lib.dll' has unsupported Webcil wrapper version 0; expected 2.

Add MSBuild-backed regression coverage for CoreCLR with and without R2R, unchanged Mono/unset behavior, and the generated project import order. Document the configuration in the WASM workflow. No runtime changes or private Webcil-version override.

Validation

• python -m pytest scripts\tests\test_wasm_coreclr_r2r.py -q: 47 passed, including 12 new MSBuild evaluation cases. The new tests reproduced 4 CoreCLR failures / 8 passes before the fix.
• A local MSBuild harness importing the real runtime Microsoft.NET.Sdk.WebAssembly.Browser.targets at b965265e2454e3469580c09012e424e46608e7d7 executed _ResolveWasmConfiguration: baseline CoreCLR selected payload 0; fixed CoreCLR selected payload 1 with R2R off and on; Mono remained payload 0.
• git diff --check passed.
• Internal WASM-only sanity run 3090983 completed against this PR's exact commit 87daace271a6b571add2744f6e834b74838f6088, with onlySanityCheck=true, reusing post-Webcil-v2 runtime bundles from build 3090494. Actual benchmark work items, not just submission/controller tasks, completed with exit code 0:
  • CoreCLR R2R: 34 benchmarks executed. Command includes --wasm-runtime-flavor CoreCLR --wasm-ready-to-run; console contains no unsupported Webcil or No Workload Results errors.
  • CoreCLR interpreter: 34 benchmarks executed.
  • Mono interpreter: 34 benchmarks executed.
• The overall pipeline is failed, not green: Mono WASM AOT failed before benchmark submission because existing runtime-perf-job.yml download conditions also request LinuxMonoAOTx64 from the current build. That native artifact is absent from this artifact-reuse/WASM-only validation run. The Webcil property change does not modify those conditions. Mono AOT remains unvalidated; no CI was rerun.

This was the requested sanity subset (System.Tests.Perf_*, partition 0), not the full microbenchmark suite. No benchmark execution or full WASM publish was performed locally.

---

AI disclosure: This change and its regression coverage were prepared with GitHub Copilot.

[Copilot]

Copilot review overview

🟢 Approval recommended

The targeted configuration change is correct, documented, and covered across CoreCLR, Mono, and ReadyToRun combinations.

Review effort: Balanced
Findings: None

What changed in this PR

Ensures CoreCLR WASM builds select Webcil wrapper version 2 regardless of ReadyToRun.

Changes:

• Sets RuntimeFlavor=CoreCLR when Mono is disabled.
• Adds MSBuild regression coverage.
• Documents the configuration behavior.

File	Description
src/​benchmarks/​micro/​MicroBenchmarks.Wasm.targets	Sets the CoreCLR runtime flavor.
scripts/​tests/​test_wasm_coreclr_r2r.py	Tests flavor evaluation and import order.
docs/​benchmarking-workflow-dotnet-runtime.md	Documents CoreCLR Webcil selection.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[LoopedBard3]

This specific failure is being seen as part of wasm_coreclr wasm coreclr_r2r_v8 runs, such as in https://dev.azure.com/dnceng/internal/_build/results?buildId=3090494&view=results.

[DrewScoggins]

LGTM