[wasm] Fail hard when the engine rejects a Webcil R2R image

[lewing]

Problem

Related to #132865 / #132855.

CoreCLR-on-wasm loads a per-assembly R2R image as a .wasm file sitting beside the assembly's .dll (src/coreclr/hosts/corerun/wasm/libCorerun.js, BrowserHost_ExternalAssemblyProbe). If that .wasm file exists but new WebAssembly.Module(...) throws - for example because crossgen2 emitted a function type exceeding the wasm engine's parameter-count limit (#132855) - the probe caught the exception and returned false, which is exactly what happens when there's simply no R2R image for that assembly at all.

(Note: the emitted wasm is well-formed; it's rejected because it exceeds an implementation-defined engine limit, not because it's malformed.)

The result: the runtime silently falls back to fully interpreting the whole assembly. The app still runs and tests still pass, with the only trace being a generic Ready to Run header not found: "<assembly>" log line - indistinguishable from the normal "never R2R compiled" case. A rejected R2R image can regress to full interpretation with zero visible signal.

Fix

Distinguish the two cases:

• The .wasm sibling doesn't exist -> legitimate, silent fallback (unchanged).
• The .wasm sibling exists but the engine rejects it during WebAssembly.Module construction -> this is not a normal fallback path, so it now throws instead of being swallowed.

Verification

Reproduced against an unfixed (pre-#132865) crossgen2 targeting browser-wasm, compiling System.Text.Json.Tests (which contains the 1003-param ClassWithManyConstructorParameters type from #132855) and running it under a rebuilt corerun.js/corerun.wasm:

Before this change:

Ready to Run header not found: "System.Text.Json.Tests".

...followed by Tests run: 346 Passed: 345 Failed: 1 - the whole assembly silently ran fully interpreted.

After this change:

Failed to construct WebAssembly module for Webcil image: {
  wasmPath: 'System.Text.Json.Tests.wasm',
  errorMessage: 'WebAssembly.Module(): param count of 1003 exceeds internal limit of 1000 @+1433'
}

process exits 1, no tests run at all - the rejection is immediately visible instead of silently degrading.

Note

This PR description and commit were drafted with GitHub Copilot assistance.

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[Copilot]

Pull request overview

This PR changes the CoreCLR-on-wasm corerun host’s Webcil (per-assembly R2R) probe so that a present-but-unloadable sibling .wasm image no longer looks identical to the “no R2R image exists” case, by throwing on WebAssembly.Module(...) construction failure instead of returning false.

Changes:

• Treat WebAssembly.Module(wasmBytes) failures as fatal by throwing, rather than silently falling back via return false.
• Update the logged message to reflect that the .wasm module is invalid for a Webcil image.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/coreclr/hosts/corerun/wasm/libCorerun.js:200

• The new throw replaces the original exception with a new Error, which drops the original error object/stack. Preserving the original exception as an error cause makes this much easier to diagnose (especially for engine-specific rejection messages).

            } catch (e) {
                const errorMessage = e instanceof Error ? e.message : String(e);
                console.error("Failed to construct WebAssembly module for Webcil image:", { wasmPath, errorMessage });
                throw new Error(`Failed to construct WebAssembly module for Webcil image '${wasmPath}': ${errorMessage}`);
            }