mix volt.build: --no-sourcemap ignored; empty/CSS-only entries fail with Rolldown source map error

[mtarnovan]

Summary

When bundling an entry that emits effectively empty JS (for example, a JS entry that only imports CSS), mix volt.build fails with:

Build failed: [%{message: "Rolldown did not produce a source map"}]

Also, --no-sourcemap currently does not disable sourcemaps, so this failure is hard to avoid unless using --sourcemap false explicitly.

Environment

• volt 0.8.2
• oxc 0.10.0 (via volt deps)
• Elixir 1.19.4 / OTP 28
• macOS Darwin 25.4.0

Repro (Volt)

1. Create a JS entry with only a CSS import:

// assets/pdf/js/pdf.js
import "../css/pdf.css"

2. Run:

mix volt.build --entry assets/pdf/js/pdf.js --outdir priv/static/pdf/assets --target es2025 --resolve-dir deps --no-hash --no-minify --no-sourcemap

Actual

Build failed: [%{message: "Rolldown did not produce a source map"}]

Expected

--no-sourcemap should disable sourcemap generation and the build should succeed.

Additional findings

• --sourcemap false works as a workaround:

mix volt.build --entry assets/pdf/js/pdf.js --outdir priv/static/pdf/assets --target es2025 --resolve-dir deps --no-hash --no-minify --sourcemap false

• --no-sourcemap appears to be ignored in Mix.Tasks.Volt.Build because :sourcemap is parsed as :string; OptionParser treats --no-sourcemap as invalid and it is dropped.

Quick proof:

elixir -e 'IO.inspect OptionParser.parse(["--no-sourcemap"], strict: [sourcemap: :string])'
# => {[], [], [{"--no-sourcemap", nil}]}

• There also seems to be an upstream edge case in oxc sourcemap handling for empty output:

mix run -e 'files=[{"main.js", ""}]; IO.inspect(OXC.bundle(files, entry: "main.js", sourcemap: true, format: :iife))'
# => {:error, [%{message: "Rolldown did not produce a source map"}]}

That suggests chunk.map can be absent for empty bundles, and the current wrapper turns that into a hard error.

Happy to open a separate oxc issue too if useful.