fix(build): emit import attributes with the with key

[agantelin]

🔗 Linked issue

#4518

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)

📚 Description

Rollup 4 re-prints the import attributes of external modules with its default key, assert, which
Node.js removed in 22.0.0 (nodejs/node#52104). Rollup itself flips the default in v5
(rollup/rollup#6248, merged to rollup-5), and documents output.importAttributesKey as the
workaround until then; no 5.x is published.

This only bites the rollup paths — the default rolldown builder emits with already — so it's
builder: "rollup" and the rollup branch of the vite builder:

• src/build/rollup/config.ts — the default in the builder's output config; it sits in the object
  defu merges under nitro.options.rollupConfig, so user config still wins.
• src/build/vite/bundler.ts — same default, applied with ??= after the merge, because in that
  branch commonConfig is merged last and a literal would outrank user config.

Symptom: the build is green, the server starts, and the route that imports the module answers 500
with TypeError [ERR_IMPORT_ATTRIBUTE_MISSING]. I hit it through @nuxt/icon's
serverBundle.externalizeIconsJson, which generates a correct with { type: "json" } import.
Setting the key costs nothing on the supported matrix: the runtimes that take assert but reject
with are all below engines.node.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly. — importAttributesKey isn't documented; after
  this the assets docs from feat(build): support bytes and text import attributes #4431 describe what actually ships on the rollup path too.

Verification

• test/unit/import-attributes.test.ts covers three things, each checked by reverting the
  corresponding line one at a time: the rollup builder's default, the vite builder's rollup branch,
  and that an explicit rollupConfig.output.importAttributesKey from user config survives both.
• End-to-end on nitro@3.0.260610-beta with builder: "rollup": assert → HTTP 500
  ERR_IMPORT_ATTRIBUTE_MISSING, with → 200. Case 4 in
  https://github.com/agantelin/nitro-import-attributes-repro
• A test/fixture/ regression route can't express this: the case needs an external JSON import,
  which the worker and edge presets can't load, so the fixture suite would fail for other reasons.
• What I could not check end to end is a real vite build — builder: "vite" doesn't get that far
  on Windows for me (ERR_UNSUPPORTED_ESM_URL_SCHEME … Received protocol 'c:', unrelated to this
  change, but you may want to know). The emitted config is pinned by the test above.

[vercel]

@agantelin is attempting to deploy a commit to the Nitro Team on Vercel.

A member of the Team first needs to authorize it.

[coderabbitai]

📝 Walkthrough

Walkthrough

Rollup output configuration now sets importAttributesKey to "with" by default. Tests cover direct Rollup and Vite builder output, including explicit "assert" configuration.

Changes

Import attributes configuration

Layer / File(s)	Summary
Rollup output configuration src/build/rollup/config.ts, src/build/vite/bundler.ts	Rollup uses "with" for import attributes. The Vite bundler applies "with" when no value is configured.
Configuration validation test/unit/import-attributes.test.ts	Tests verify default "with" output and preserve explicit "assert" configuration for both output paths.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related issues

• Import attributes don't survive the rollup builder: emitted with the removed assert key, and dropped in .ts sources on v2 #4518 — The change addresses the output.importAttributesKey defect in Rollup configuration and the Vite bundler.

Possibly related PRs

• nitrojs/nitro#4431 — It adds related import-attributes handling in the build pipeline.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The changes do not address linked issue #39, which requires adding a Layer0 preset and related build outputs.	Link the correct issue for the import-attribute fix, or implement the Layer0 preset requirements from #39.
Out of Scope Changes check	⚠️ Warning	The import-attribute changes are unrelated to linked issue #39, whose scope is the Layer0 preset.	Use a linked issue covering the Rollup import-attribute fix, or limit this pull request to Layer0 preset changes.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title follows Conventional Commits format and clearly describes the import-attribute output fix.
Description check	✅ Passed	The description directly explains the Rollup fix, affected paths, user overrides, tests, and verification results.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (2)

src/build/rollup/config.ts (1)

61-62: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Remove the added explanatory source comments.

Both changed source segments explain compatibility details for a configuration line. Remove these comments or move the rationale to documentation.

• src/build/rollup/config.ts#L61-L62: remove the two-line comment.
• src/build/vite/bundler.ts#L100-L100: remove the duplicate comment.

As per coding guidelines: “Do not add comments explaining what the line does unless prompted.”

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/build/rollup/config.ts` around lines 61 - 62, Remove the two-line
explanatory comment at src/build/rollup/config.ts lines 61-62 and the duplicate
comment at src/build/vite/bundler.ts line 100, leaving the associated
configuration lines unchanged.

Source: Coding guidelines

test/unit/import-attributes.test.ts (1)

9-14: 🗄️ Data Integrity & Integration | 🔵 Trivial | ⚡ Quick win

Cover both configuration paths and the override contract.

This test invokes getRollupConfig only. It does not exercise the Rollup branch of src/build/vite/bundler.ts (Line 70-106) or verify that an explicit importAttributesKey: "assert" remains unchanged. Add default and override cases for both builders.

Based on PR objectives: the change covers both the Rollup builder and the Vite Rollup branch while preserving user overrides.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@test/unit/import-attributes.test.ts` around lines 9 - 14, Expand the
import-attributes tests around getRollupConfig and the Rollup branch in
bundler.ts to cover both builders: verify each defaults importAttributesKey to
"with", and verify an explicit "assert" configuration remains unchanged for each
path. Reuse the existing Nitro/Vite setup and preserve the current default
assertion.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/build/rollup/config.ts`:
- Around line 61-62: Remove the two-line explanatory comment at
src/build/rollup/config.ts lines 61-62 and the duplicate comment at
src/build/vite/bundler.ts line 100, leaving the associated configuration lines
unchanged.

In `@test/unit/import-attributes.test.ts`:
- Around line 9-14: Expand the import-attributes tests around getRollupConfig
and the Rollup branch in bundler.ts to cover both builders: verify each defaults
importAttributesKey to "with", and verify an explicit "assert" configuration
remains unchanged for each path. Reuse the existing Nitro/Vite setup and
preserve the current default assertion.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d68b8388-75d2-46b7-96cf-1df50ca62b85

📥 Commits

Reviewing files that changed from the base of the PR and between 52abde8 and fb8c898.

📒 Files selected for processing (3)

• src/build/rollup/config.ts
• src/build/vite/bundler.ts
• test/unit/import-attributes.test.ts

[agantelin]

Pushed an update (force-push, same two source lines):

• The test now also covers the vite builder's rollup branch and the user-override contract, so both
  changed lines are pinned rather than just the rollup one. Each of the three assertions was checked
  by reverting its line individually.
• Trimmed the rationale comments to one line each.

Nothing else changed. Still unverified on my side: an actual vite build, which doesn't start on
Windows (ERR_UNSUPPORTED_ESM_URL_SCHEME, unrelated to this PR).