example of rslib issues in React Spectrum

[davidferguson]

We are seeing issues in React Spectrum / unplugin-parcel-macros when used in Rslib.

There seems to be a race condition so that the more macros that are used (and the more complex the macros that are used are), the more likely a rslib failure is, resulting in a CSS file not being generated in the virtual filesystem in time, and the build failing as it cannot find it.

Reproduction steps

Success case

1. cd examples/s2-rslib-semiworking/
2. yarn install
3. yarn build
4. Observe that the build (likely) succeeded. There is only one simple macro use in this project.

Failure case

1. cd examples/s2-rslib-notworking/
2. yarn install
3. yarn build
4. Observe the build (likely) failed. There are lots of files that use macros here, and they use larger/more complex macros.

Debugging notes from AI

TL;DR: It's a race condition in unplugin-parcel-macros + unplugin's virtual module system within rspack. No config-level fix found yet — likely needs an upstream fix or inlining styles.

---

🔍 The Problem
pnpm nx build out-package-name fails ~50% of the time with:

Module not found: Can't resolve 'macro-<hash>.css'

---

🕵️ Root Cause
The S2 style() macro generates CSS at build time. That CSS lives in:
• A module-level assets Map inside unplugin-parcel-macros (shared across plugin instances)
• Empty placeholder files in node_modules/.virtual/ written by unplugin's FakeVirtualModulesPlugin

The transform hook generates CSS → adds to assets Map → appends import "macro-<hash>.css" to the source. Then resolveId maps it to a .virtual/ file, and the load hook serves the real CSS from the Map.

The race happens within rspack's internal concurrent module processing. Even with a single lib entry and a single rspack compiler, the async transform → resolveId → load pipeline has a timing window where virtual CSS module resolution fails.

---

🧪 What I Tested

Approach	Result
Normal build (ESM + CJS parallel)	~40% pass
Patch: remove assets.delete() cleanup	Still fails
Patch: prevent .virtual/ dir deletion on shutdown	Still fails
Both patches combined	Still fails
Patch: per-instance assets Map	Worse
Sequential builds (separate processes)	ESM passes, CJS still flaky
Single lib entry only	Still flaky (3/10)
Disable DTS	Still fails
Clean .virtual/ before every build	Still fails
cssModules: { namedExport: false }	Still fails

---

💡 Key Takeaways
• Not caused by parallel ESM+CJS — fails with a single lib entry too
• Not caused by stale .virtual/ files — cleaning doesn't help
• Not caused by the asset cleanup code — removing it doesn't fix it
• The working repo (pandora-tooling-demo) has fewer macro files and simpler inline style() calls — smaller race window
• Importing style files with { type: 'macro' } doesn't work — S2 throws an error since the exports are already-expanded values, not macro functions