fix(build): observe failOnWarn warnings independently of logLevel

[spokodev]

Problem

failOnWarn: true is documented as a way to fail a build whenever a warning is emitted. In practice it relied on rolldown's defaultHandler to escalate 'warn' into a build error, but when the caller passes logLevel: 'silent' rolldown's defaultHandler becomes a no-op. So warnings such as unresolved imports get silently externalised and the build succeeds when the user explicitly opted into failing.

A common shape where this bites is CI scripts that pass logLevel: 'silent' to keep their own logs clean and failOnWarn: true to fail builds with unresolved imports. The current behaviour means those builds silently pass.

Repro

import { mkdtempSync, writeFileSync } from 'node:fs'
import { tmpdir } from 'node:os'
import { join } from 'node:path'
import { build } from 'tsdown'

const dir = mkdtempSync(join(tmpdir(), 'tsdown-failonwarn-'))
writeFileSync(
  join(dir, 'package.json'),
  JSON.stringify({ type: 'module', private: true }),
)
writeFileSync(
  join(dir, 'index.ts'),
  `import express from 'express'\nexport const handler = () => express()\n`,
)

await build({
  entry: { index: join(dir, 'index.ts') },
  outDir: join(dir, 'dist'),
  format: 'esm',
  platform: 'node',
  dts: false,
  config: false,
  logLevel: 'silent',
  failOnWarn: true,
  deps: { alwaysBundle: () => true, onlyBundle: false },
})
// before: resolves, unresolved import was silently externalised
// after:  throws "Build failed with 1 error" as expected

The same call with logLevel: 'warn' already throws, so the bug only surfaces under silent.

Fix

Decouple rolldown's logLevel from tsdown's logger.level whenever failOnWarn is enabled and the user-facing log level would otherwise suppress warnings:

logLevel: logger.options?.failOnWarn
  ? logger.level === 'silent' || logger.level === 'error'
    ? 'warn'
    : logger.level
  : logger.level === 'error'
    ? 'silent'
    : logger.level,

This keeps existing behaviour for callers who do not set failOnWarn and for callers whose logLevel already allows warnings. The escalation path inside the onLog handler is unchanged.

Tests

Four direct-build tests added to tests/e2e.test.ts that bypass the testBuild helper, which is necessary because testBuild's inputOptions override forces rolldown's logLevel to 'info' and therefore could never reproduce the silent-mode path. The scenarios cover all four axes:

• Positive: silent + failOnWarn + warning => build() throws.
• Negative: silent + failOnWarn + clean build (no warnings) => build() succeeds.
• Negative: silent + failOnWarn: false + warning => build() succeeds (opt-out respected).
• Regression preservation: warn + failOnWarn + warning => build() still throws.

Without the fix the positive scenario fails on main (the build resolves silently); the other three pass on baseline already and continue to pass with the patch, confirming the fix narrowly targets the broken path.

pnpm lint clean, pnpm typecheck clean, full e2e suite green for all 5 failOnWarn cases (1 pre-existing + 4 new). The 3 pre-existing exe.test.ts failures on main are unrelated to this change.

Closes #952

Note on disclosure

Bug surfacing and the repro are from @tbeseda in the issue body. I traced the coupling in src/features/rolldown.ts, reproduced it via the script above, applied the minimal log-level decoupling, and added the tests. AI assistance was used while drafting parts of the test boilerplate and this PR body; the fix and tests reflect my own engineering decisions.

[netlify]

✅ Deploy Preview for tsdown-main ready!

Name	Link
🔨 Latest commit	b13025d
🔍 Latest deploy log	https://app.netlify.com/projects/tsdown-main/deploys/6a146ffb6b16fa00087905e7
😎 Deploy Preview	https://deploy-preview-960--tsdown-main.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pkg-pr-new]

Open in StackBlitz

tsdown

pnpm add https://pkg.pr.new/tsdown@960 -D

npm i https://pkg.pr.new/tsdown@960 -D

yarn add https://pkg.pr.new/tsdown@960.tgz -D

create-tsdown

pnpm add https://pkg.pr.new/create-tsdown@960 -D

npm i https://pkg.pr.new/create-tsdown@960 -D

yarn add https://pkg.pr.new/create-tsdown@960.tgz -D

@tsdown/css

pnpm add https://pkg.pr.new/@tsdown/css@960 -D

npm i https://pkg.pr.new/@tsdown/css@960 -D

yarn add https://pkg.pr.new/@tsdown/css@960.tgz -D

@tsdown/exe

pnpm add https://pkg.pr.new/@tsdown/exe@960 -D

npm i https://pkg.pr.new/@tsdown/exe@960 -D

yarn add https://pkg.pr.new/@tsdown/exe@960.tgz -D

tsdown-migrate

pnpm add https://pkg.pr.new/tsdown-migrate@960 -D

npm i https://pkg.pr.new/tsdown-migrate@960 -D

yarn add https://pkg.pr.new/tsdown-migrate@960.tgz -D

commit: b13025d