printer: print ill-formed UTF-8 in 8-bit strings as U+FFFD instead of NUL

[robobun]

Problem

• A byte that is not valid UTF-8 in a text import, or in a string of a .json, .jsonc or .yaml file under bun build, prints as \x00 and the bytes after it are lost: the file A E9 B imports as "A\x00". A stray 80..BF or F8..FF byte becomes U+0080..U+00FF, and --target=browser writes it raw, so the bundle is not valid UTF-8.
• Cause: the UTF-8 arm of write_pre_quoted_string_inner (src/js_printer/lib.rs:1093) decodes a bad sequence with decode_wtf8_rune_t(.., 0), prints the 0, and skips the width the lead byte implies. fmt: escape lone surrogates and malformed UTF-8 in the JSON string formatters #40718 fixed only the bun_core copy of this loop.

Fix

• Same treatment as the bun_core copy: a failed decode, or a width-1 byte >= 0x80, is malformed. It prints as U+FFFD (\uFFFD when ascii_only) and the loop continues at the next byte.
• WTF-8 surrogates (ED A0 80) still decode, so "\ud800" in a JSON file still prints as \uD800. Valid input takes the same branches as before.
• Verified: new cases in test/bundler/bundler_loader.test.ts and test/js/bun/import-attributes/import-attributes.test.ts, both fail on 1.4.3. More suites in Notes.
• Self-reviewed: 2 concerns raised, 1 addressed (the tests now say why their byte sequences make every decoder agree). Not taken: the title says "8-bit strings" while raw template literals keep their own path, which is out of scope here.

Background

• The text and data-file loaders wrap file bytes in an E::String node, and the printer turns it back into a JS string literal. Its 8-bit payload is WTF-8 by contract (UTF-8 plus 3-byte lone surrogates), but file bytes reach it unvalidated.
• Related: printer: decode ill-formed UTF-8 as U+FFFD when quoting byte strings #35306 (closed for this one) used the strict WHATWG decoder here, which also turns WTF-8 surrogates into U+FFFD. Decode text and md imports as UTF-8 instead of passing raw file bytes to the printer #38253 decodes text and md files at the loader and does not reach JSON or YAML strings. Deduplicate bundler dispatch and output glue, the JS string escaper, and the exports-map retry #31998 folds both copies of this loop into one.

Notes

• One U+FFFD per undecodable byte, as in bun_core::printer::write_pre_quoted_string, strings::write_wtf8_as_utf16le, CodepointIterator::next and esbuild. TextDecoder emits one U+FFFD per maximal subpart, so E2 82 41 gives two U+FFFD here and one there. The tests use sequences where both agree. Decode text and md imports as UTF-8 instead of passing raw file bytes to the printer #38253 gives text and md imports the exact TextDecoder result on top of this.

• wtf8_byte_sequence_length_with_invalid returns the sequence length for a lead byte and 1 for a byte that cannot start one, so the loop can always advance.

• Before and after, with printf 'A\xe9B' > t2.txt; printf '{"k":"A\xe9B"}' > j2.json; printf 'hi \xe9\xff w' > t.txt:

  bun 1.4.3:
    bun build ./t2.txt     var t2_default = "A\x00";
    bun build ./j2.json    var k = "A\x00";
    import t.txt + j2.json at runtime: ["hi \^@w","A�B"]
  this PR:
    bun build ./t2.txt     var t2_default = "A�B";   (EF BF BD in the output)
    bun build ./j2.json    var k = "A�B";
    runtime:               ["hi �� w","A�B"]
  

• Runtime imports of JSON files were already correct (they do not print through this path). Text imports at runtime and every loader under bun build were not.

• The other callers of this function (quote_for_json, write_json_string for console, dev server and snapshot output) get the same change: no NUL and no raw invalid byte in their output.

• Also ran: bundler_string, transpiler, text-loader, yaml, md-edge-cases, internal-sourcemap-roundtrip, snapshot, metafile.

[robobun]

^{Updated 8:05 PM PT - Sep 7th, 2026}

❌ @robobun, your commit c2d5fe2 has 1 failures in Build #112321 (All Failures):

• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 41789

That installs a local version of the PR into your bun-41789 executable, so you can run:

bun-41789 --bun

[robobun]

Reproduced on bun 1.4.3 (Linux x64 and Windows x64):

printf 'A\xe9B' > t2.txt && bun build ./t2.txt          # var t2_default = "A\x00";
printf '{"k":"A\xe9B"}' > j2.json && bun build ./j2.json # var k = "A\x00";
printf 'import t from "./t2.txt" with {type: "text"}; console.log(JSON.stringify(t));' > rt.mjs && bun rt.mjs   # "A\u0000"

With this branch all three print A\uFFFDB (A, EF BF BD, B in non-ASCII output). The new cases in test/bundler/bundler_loader.test.ts and test/js/bun/import-attributes/import-attributes.test.ts fail on 1.4.3 and pass on a debug ASAN build of this branch.

CI (build 112321, c2d5fe2): the new tests pass on every lane. The one red job is test/js/node/test/parallel/test-crypto-dh-leak.js on debian 13 x64-asan, which fails on main as well and does not touch the printer. Ready for review.

[coderabbitai]

Walkthrough

Changes

Malformed UTF-8 handling

Layer / File(s)	Summary
Decode and emit replacement characters src/js_printer/lib.rs, src/bun_core/string/mod.rs	The JavaScript printer now detects invalid and truncated UTF-8, emits U+FFFD or an ASCII escape, and processes malformed bytes one at a time. The string documentation reflects the behavior.
Validate loader behavior test/bundler/bundler_loader.test.ts, test/js/bun/import-attributes/import-attributes.test.ts	Tests cover malformed UTF-8 in text, JSON, JSONC, and YAML loaders. They verify replacement characters, preserved valid characters, valid UTF-8 output, and errors for unsupported source types.

Suggested reviewers: jarred-sumner, dylan-conway

Merge Risk: 🔵 Low · up to c10b7

Malformed UTF-8 now produces replacement characters without dropping following bytes. The remaining risk is limited to missing test coverage for the target-specific emitted representation, which could allow an ascii-only output regression to pass unnoticed.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: printing ill-formed UTF-8 as U+FFFD instead of NUL in the printer.
Description check	✅ Passed	The description explains the problem, cause, fix, scope, compatibility behavior, and verification. It does not use the exact template headings, but it provides the required information and is substant…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/bundler/bundler_loader.test.ts`:
- Line 465: Update the assertion around TextDecoder in the bundler loader test
to inspect the emitted replacement representation: require escaped \uFFFD bytes
for the bun target and literal U+FFFD bytes for the browser target, preserving
the existing valid-UTF-8 check.

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📥 Commits

Reviewing files that changed from the base of the PR and between d316760 and c10b77a.

📒 Files selected for processing (4)

• src/bun_core/string/mod.rs
• src/js_printer/lib.rs
• test/bundler/bundler_loader.test.ts
• test/js/bun/import-attributes/import-attributes.test.ts

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

This change also fixes bun build --sourcemap for source files that are not UTF-8. LinkerContext::compute_quoted_source_contents (src/bundler/LinkerContext.rs:1694) quotes the raw file bytes for sourcesContent through the same Encoding::Utf8 arm with json = true, so a Latin-1 .js file made the .map (external, linked and inline) invalid UTF-8 while the emitted .js was fine:

printf '// \xa9 2020 Soci\xe9t\xe9, legacy latin-1 file\nexport const cafe = "caf\xe9";\n' > legacy.js
printf 'import {cafe} from "./legacy.js"; console.log(cafe);\n' > entry.js
bun build ./entry.js --outdir=out --sourcemap=external --target=node
python3 -c 'import json; json.load(open("out/entry.js.map", encoding="utf-8"))'
# bun 1.4.3: UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa9 in position 94
#            (sourcesContent holds the raw 0xA9, and "Soci\xe9t\xe9," prints as "Soci\u0000,")
# this diff: loads; sourcesContent[0] == "// \ufffd 2020 Soci\ufffdt\ufffd, legacy latin-1 file\nexport const cafe = \"caf\ufffd\";\n"
#            which is the string esbuild writes for the same input

I had the same write_pre_quoted_string_inner change on robobun/72f91ee1/printer-ill-formed-utf8-fffd for that report and will not open it as a second PR. The source map test from that branch is below if you want to fold it in here (it goes after sourcemap sourcesContent is valid JSON when source contains C0 control chars in test/bundler/bun-build-api.test.ts, fails on 1.4.3 with ERR_ENCODING_INVALID_ENCODED_DATA, passes with this diff). It may also be worth naming sourcesContent in the Problem section so the source map symptom is searchable.

test/bundler/bun-build-api.test.ts

test("sourcemap sourcesContent is valid UTF-8 when a source file is not UTF-8", async () => {
  // A Latin-1 file: 0xA9 is the copyright sign, 0xE9 is e-acute. As UTF-8,
  // 0xA9 is a stray continuation byte and 0xE9 opens a 3-byte sequence that
  // the ASCII after it does not continue. The map used to carry those bytes
  // raw, so strict UTF-8 readers rejected the whole file.
  const latin1 = (s: string) => Buffer.from(s, "latin1");
  using dir = tempDir("sourcemap-latin1", {
    "legacy.js": latin1('// \xA9 2020 Soci\xE9t\xE9, legacy latin-1 file\nexport const cafe = "caf\xE9";\n'),
    "in.js": `import { cafe } from "./legacy.js";\nconsole.log(cafe);\n`,
  });

  for (const sourcemap of ["external", "inline"] as const) {
    const res = await Bun.build({
      entrypoints: [join(String(dir), "in.js")],
      sourcemap,
      outdir: join(String(dir), sourcemap),
    });
    expect(res.success).toBe(true);

    let bytes: Uint8Array;
    if (sourcemap === "external") {
      bytes = new Uint8Array(await res.outputs.find(o => o.kind === "sourcemap")!.arrayBuffer());
    } else {
      const js = await res.outputs.find(o => o.kind === "entry-point")!.text();
      const match = js.match(/\/\/# sourceMappingURL=data:application\/json;base64,([A-Za-z0-9+/=]+)/);
      expect(match).not.toBeNull();
      bytes = Buffer.from(match![1], "base64");
    }

    const text = new TextDecoder("utf-8", { fatal: true }).decode(bytes);
    const parsed = JSON.parse(text);
    const index = parsed.sources.findIndex((s: string) => s.endsWith("legacy.js"));
    expect(parsed.sourcesContent[index]).toBe(
      '// \uFFFD 2020 Soci\uFFFDt\uFFFD, legacy latin-1 file\nexport const cafe = "caf\uFFFD";\n',
    );
  }
});

[robobun]

A parallel run on the same report produced the branch robobun/daea8cb4/printer-ill-formed-utf8. It fixes the same printer arm, so it is not opened as a second PR. It covers two faces that this PR leaves open, in case you want to fold them in here:

1. Object keys. A key with a stray 80..BF / F8..FF byte ({"k\xFFy":1} in a .json, .jsonc or .yaml file) never reaches write_pre_quoted_string_inner. CodepointIterator::next reads the byte as U+00FF, is_identifier accepts it, and print_identifier writes the raw byte (--target=browser, --minify) or \u{ff} (--target=bun). The bundle is still not valid UTF-8, and the key differs from the k\uFFFDy that bun run reads. On the branch CodepointIterator::next decodes through the same helper, so such a key is quoted and prints as "k\uFFFDy".
2. One U+FFFD per maximal subpart instead of one per byte, from one helper (strings::decode_wtf8_with_fffd, WTF-8 aware, built on convert_utf8_bytes_into_utf16) that the JS printer, bun_core::printer::write_pre_quoted_string, CodepointIterator::next, write_wtf8_as_utf16le and decode_wtf8_one all call. a E2 82 41 is then a\uFFFDA in the bundle, in bun run of the .json (WebKit decoder), in bun run of the .jsonc / .yaml (wtf8_to_utf16_alloc), in node and in TextDecoder. With per-byte replacement the bundle and bun run of the same .json still disagree on truncated sequences.

Tests on the branch: test/bundler/bundler_loader.test.ts "ill-formed UTF-8 in data files" (json, jsonc, yaml, txt and .env / --env=PUBLIC_* values, an ill-formed key, bun and browser targets, output checked with TextDecoder("utf-8", { fatal: true }), plus a bun run parity test over the same files), and a decode_wtf8_with_fffd unit test in src/bun_core/string/immutable.rs. They fail on 1.4.3-canary and pass on a debug build on Linux and Windows. Commits: 2aaddd4 (fix and tests), ac53613 (unit test), 6678f7d (inline attribute).

[claude]

Code review found no issues

No high-confidence issues detected in this change.