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Vite 8 / Rolldown Migration Notes

This document summarizes the migration work, regressions found, fixes applied, and investigation outcomes while moving @fresh/plugin-vite from Vite 7/Rollup behavior to Vite 8/Rolldown behavior.

Current Status

  • deno test -A packages/plugin-vite/tests/build_test.ts
    • ✅ 30 passed / 0 failed
  • Previously failing areas during migration:
    • Bare/JSR island resolution (@marvinh-test/fresh-island)
    • .cjs parsing failures in SSR build
    • CSS Modules not applied for island/imported chunks

Root Causes Identified

1) Resolver contract differences in Rolldown path

Symptoms

  • Build error:
    • Could not load @marvinh-test/fresh-island - No such file or directory (os error 2)

Causes

  • resolveId result shape assumptions from previous behavior no longer held.
  • Returned string IDs were not always sufficient in this path.
  • resolveId metadata assumptions (resolvedBy) were invalid with current typings/runtime surface.
  • resolveId options import attributes (options.attributes) are not available in current Rolldown integration (see linked Rolldown issue below).

Fixes

  • Return object from resolver for deno namespaced IDs:
    • return { id: toDenoSpecifier(...) }
  • Removed usage of unsupported resolver provenance metadata (resolvedBy).
  • Stopped relying on unavailable import-attributes options in resolver:
    • Fallback to default type handling in plugin path.
  • Result: remote island resolution now works again.

2) .cjs files transformed to ESM, then parsed as CommonJS

Symptoms

  • SSR build errors such as:
    • Cannot use export statement outside a module
    • Cannot use import statement outside a module
  • Reported on transformed .cjs files (e.g. demo fixtures).

Cause

  • fresh:patches includes cjsPlugin, which rewrites CommonJS patterns to ESM-style syntax.
  • Under Vite 8 / Rolldown / OXC strictness, .cjs is still interpreted as CommonJS by extension semantics.
  • Result: transformed ESM syntax in .cjs became invalid for parser expectations.

Fix

  • SSR workaround in packages/plugin-vite/src/mod.ts:
    • Mark .cjs as external in rolldownOptions.external.
  • This bypasses problematic bundling/parsing path for .cjs in SSR.

Notes

  • This is a practical compatibility workaround.
  • Long-term ideal is upstream/toolchain alignment of module-kind and transformed output.

3) CSS Modules missing from island/imported chunks

Symptoms

  • vite build - css modules test failed (computed styles were incorrect).
  • Island route rendered, but expected island CSS was missing in output.

Cause

  • Island CSS collection logic relied on manifest assumptions that did not consistently hold after migration.
  • Shared/imported chunk CSS was not always associated with island entries.
  • Manifest lookup by a single key strategy was insufficient with current output shape.

Fix

  • In server_snapshot.ts, switched to robust CSS collection:
    • Build helper to resolve manifest chunk by reference (supports key/file matching).
    • Recursively walk imports graph.
    • Aggregate all reachable CSS into island entry CSS list.
  • Result: CSS modules tests passed.

Code-Level Changes (Summary)

packages/plugin-vite/src/plugins/deno.ts

  • Updated resolver behavior for Rolldown compatibility:
    • Return object form for deno specifier IDs.
  • Removed unsupported assumptions:
    • No resolvedBy checks.
    • No direct options.attributes usage (unavailable in current environment).
  • Preserved plugin-chain interoperability while keeping deno-first resolution intent.

packages/plugin-vite/src/mod.ts

  • Added SSR Rolldown external rule:
    • Externalize .cjs to avoid parser/module-kind mismatch in bundle phase.

packages/plugin-vite/src/plugins/server_snapshot.ts

  • Reworked island CSS gathering:
    • Added manifest reference resolution helper.
    • Added recursive import-graph CSS collection.
    • Ensured island registry receives full CSS set including transitive imports.

Related Upstream References

Migration Lessons

  1. Do not rely on non-standard resolver metadata unless guaranteed by current types/runtime.
  2. Treat .cjs carefully under stricter parser semantics; avoid producing ESM syntax in .cjs bundle path.
  3. Assume manifest shape variance across bundlers; use resilient chunk resolution and import-graph traversal.
  4. Prefer behavior-based guards (e.g., external, virtual IDs, imports graph) over provenance-based assumptions.

Remaining Follow-up (Optional)

  • Revisit .cjs external workaround when upstream parser/module-kind behavior improves.
  • Consider narrowing cjsPlugin application scope to avoid .cjs transformation hazards.
  • Add targeted regression tests around:
    • manifest key/file reference differences
    • transitive island CSS inclusion
    • deno resolver fallback behavior under Rolldown.