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The CSS tokenizer takes &[u8] and hands out raw sub-slices of the source as token values, so a byte sequence that is not valid UTF-8 inside a string, ident or url() was copied into the emitted stylesheet, and an @import or url() specifier holding one reached the resolve-error formatter and hit 'panic: unreachable' in BabyString::in. The escape decoder also advanced by the encoded width of U+FFFD after such a byte and dropped the source bytes that followed. Add strings::replace_invalid_utf8(bytes, arena), which returns the input when it is already valid and otherwise an arena copy with each maximal ill-formed subsequence replaced by U+FFFD, and call it where CSS source enters the parser: the bundler ParseTask (before Source is built, so offsets, diagnostics and source maps share one buffer) and the --no-bundle transpile path. bun pm diff falls back to a text diff for such files. The tokenizer now debug-asserts the precondition, and BabyString::in reports an error with no specifier instead of aborting when the formatted message does not embed it.
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Reproduced on 1.4.2 and canary python3 -c 'open("s.css","wb").write(b".a::before{content:\"caf\xe9\xff\";font-family:\"F\xf8nt\"}\n")'
bun build ./s.css --outfile=out.css && python3 -c 'open("out.css","rb").read().decode()' # UnicodeDecodeError: 0xe9
python3 -c 'open("u.css","wb").write(b".a{background:url(\"im\xe9.png\")}\n")' && bun build ./u.css # panic: unreachableWith this branch the first emits Since df8cb80 the build also warns when it replaced bytes, once per file, at the first replaced sequence: A sheet that starts with a
A one-line sheet prints its line and a caret line with this warning, as every diagnostic does: 2,097,276 bytes of stderr for a 1 MB sheet with the bad byte at its end. With #43313 merged into this branch the same build prints 368 bytes. 4175d77 removes a cost that 8720fcb had on JS and JSON builds (+0.36 % and +1.3 % instructions). The cause and the comparison of the release binaries are in the PR notes. CI on 4175d77 (build 121031): 176 of 181 jobs passed. Both test files of this PR pass on every lane, and
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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review. WalkthroughCSS paths now replace malformed UTF-8 before parsing and report the first invalid location. CSS normalization rejects invalid UTF-8. Regression tests cover repaired output and diagnostics. ChangesCSS UTF-8 handling
AST string behavior
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to CSS builds use the repaired input as intended, but direct parser callers can still trigger a debug assertion with malformed bytes. This bounded issue should be tracked before merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@test/js/bun/css/invalid-utf8.test.ts`:
- Line 164: Refactor the related invalid-UTF8 column tests into a single
parameterized test body using describe.each(), with each case supplying its byte
input and expected location. Preserve the existing assertions and test behavior
for all cases, including the lone 0xF0 case.
- Around line 42-45: Update the subprocess handling around the build invocation
so its result, including exitCode and stderr, is checked before reading
generated files from the out directory. Move the readdirSync and Bun.file
output-reading logic to run only after the success assertion, and do not let the
current empty catch block hide build or output-directory failures.
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Updated 2:09 PM PT - Sep 26th, 2026
❌ @robobun, your commit 4175d77 has 2 failures in
🧪 To try this PR locally: bunx bun-pr 41801That installs a local version of the PR into your bun-41801 --bun |
…decode-invalid-utf8
…laced replace_invalid_utf8 now also returns the offset of the first ill-formed sequence. Both CSS load sites pass it to bun_css::warn_invalid_utf8, which logs one warning per file at that position. When the sheet starts with a `@charset "...";` that is not UTF-8, the warning names the label, because a Latin-1 sheet used to have its bytes copied through unchanged. A sheet that is already valid UTF-8 gets no warning and takes the same path as before.
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- 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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Inline comments:
In `@src/css/css_parser.rs`:
- Around line 4101-4104: Update ParserInput::new to pass input bytes through
strings::replace_invalid_utf8 using the provided arena before initializing the
tokenizer, so malformed UTF-8 is repaired at the parser boundary.
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A lossy decode maps different ill-formed bytes to the same U+FFFD, so two files that differ only in such a byte would re-print equal and read as 'formatting only'. The text fallback keeps the change visible.
…decode-invalid-utf8
…fset in the size pass A label of UTF-8 (utf8, unicode-1-1-utf-8, ...) or an empty label declares nothing that was ignored, so those sheets get the plain message. Any other label is reported as ignored, which is what happens to every @charset. replace_invalid_utf8 takes the offset of the first ill-formed sequence from the pass that sizes the output. It no longer scans the valid prefix once more.
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Beyond the inline finding, I also checked the remaining CSS parse entry points not touched by this diff — StyleSheet::parse and StyleAttribute::parse in src/css_jsc/css_internals.rs — for the same missing decode: both take a JS string already transcoded to UTF-8, so the new debug_assert!(is_valid_utf8) in Tokenizer::init_with_arena cannot trip there and no fourth decode site is needed.
Extended reasoning...
The change routes CSS bytes through a new lossy UTF-8 decode at the three raw-byte parse sites (ParseTask, transpiler --no-bundle, pm diff), adds a debug assertion in the tokenizer, and hardens BabyString::in; it touches no auth, crypto, or injection surface. The only other callers of the CSS parser live in the internal-for-testing css_jsc module and receive UTF-8 from JS strings, so the entry-point sweep is complete. The inline finding about the acknowledged Latin-1 @ charset behaviour change is a product decision the PR description already surfaces, which a human should weigh.
…<u8> Release builds share generic instances across crates. An ArenaVec<u8> in bun_core made that crate the owner of BabyVec<u8>::extend_from_slice, which bun_ast owned before and inlined into EString::flatten_rope. With the instance gone from bun_ast, flatten_rope was inlined into resolve_rope_if_needed and flattened instead, and those two became too large to inline into the printer, the parser and the linker. Every string literal paid a call: +28 instructions per line of JS and +101 per key of a JSON import in bun build. The copy is now written into a slice from alloc_slice_fill_copy, which is inline and instantiates nothing that another crate shares.
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Problem
bun build x.csscopies bytes that are not valid UTF-8 from a CSS string, ident orurl()into the output. In an@importspecifier the same byte aborts withpanic: unreachable(BabyString::in,src/ast/lib.rs:1108).&[u8]and returns raw sub-slices of the source as token values. Nothing decodes the source first.Fix
strings::replace_invalid_utf8(bytes, arena): one simdutf pass. Only on failure, an arena copy with U+FFFD for each ill-formed sequence, and the first offset.ParseTask(beforeSourceis built) and--no-bundle.@charset.test/js/bun/css/invalid-utf8.test.ts(9 of 14 cases fail on stock bun), one newbun-pm-diff.test.tscase.Background
&str.Downsides
@charsetis not honoured, as in esbuild.Notes
JS and JSON builds: 8720fcb cost
bun build+28 instructions per line of JS and +101 per key of a JSON import (+0.36 %, +1.3 %, from the pre-merge check). The diff has no work per line or per key. The cause was where a generic instance lives. Release builds pass-Zshare-generics=y, so a generic function that is not#[inline]has one instance, in the upstream-most crate that uses it.BabyVec::extend_from_sliceis such a function. On main itsu8instance belongs to bun_ast, which inlines it intoEString::flatten_rope. TheArenaVec<u8>inreplace_invalid_utf8moved that instance to bun_core. bun_ast then inlinedflatten_ropeintoresolve_rope_if_needed(307 bytes) andflattened(376 bytes), and those two were no longer inlined intoprint_expr,print_property,print_binding,Expr::Data::eql,scan_imports_and_exportsandvisit_expr_in_out. Each string literal paid a call. 4175d77 writes the copy into a slice fromarena.alloc_slice_fill_copy, which is#[inline].Checked on release builds (ThinLTO) of main 36cd151, 8720fcb and 4175d77 with
nm -Sandobjdump, absolute addresses masked. Of the printer, parser, lexer, JSON, linker andEStringfunctions, 30 of 92 differ from main on 8720fcb and 0 of 93 on 4175d77. What still differs from main on 4175d77:replace_invalid_utf8andwarn_invalid_utf8(new),Transpiler::build_css_output+335 bytes,pm_diff_normalize::normalize+89,parse_worker::run_from_thread_pool-51,Log::add_resolve_error_with_level-24,do_resolve-24,resolve_maybe_needs_trailing_slash+12,NestedRuleParser::parse_block-6, one more instance ofalloc_print, and an out-of-lineStyleSheet::to_css. The instruction count of 4175d77 is not measured here: this container deniesperf_event_openand has novalgrind.The warning, CLI form (
Bun.buildgets the same text as awarnlevelBuildMessagewithposition.offset):Without a
@charset, with an empty label, or with a label of UTF-8 (utf-8,utf8,unicode-1-1-utf-8,unicode11utf8,unicode20utf8,x-unicode20utf8, in any case) the text isThis file is not valid UTF-8, each invalid byte sequence was replaced with U+FFFD. Any other label is named as ignored, also one that names no encoding. The@charsetlabel is matched with the byte pattern of css-syntax-3 §3.2 (@charset "...";at offset 0, within 1024 bytes). The check only runs after the validation pass failed, so a valid sheet with@charset "ISO-8859-1"gets no warning and no new work.The tradeoff for a sheet that declares
@charset "ISO-8859-1"and holds Latin-1 bytes: main drops the rule and copies the bytes (content: "caf<E9>"), this PR drops the rule and emitscontent: "caf<EF BF BD>"plus the warning. Neither follows §3.2 step 2 (take the encoding from the@charsetbytes). esbuild 0.21.5 makes the same choice: it warns"UTF-8" will be used instead of unsupported charset "ISO-8859-1"and emitscaf\fffd. Honouring the label also changes sheets that work today: a sheet saved as UTF-8 with a stale@charset "ISO-8859-1";line printscontent: "café"on main, on this branch and in esbuild, and would printcaféwhen decoded as windows-1252. To honour it, the WHATWG label table andencoding_rsglue (EncodingLabelinsrc/runtime/webcore/) have to move belowbun_bundlerin the crate graph. That is a separate change and needs a decision.bun pm diff: a CSS file that is not valid UTF-8 now diffs as text with thenot parsedbadge. Before, the raw bytes went through the parser, so a reformat of a sheet with a Latin-1 byte in a comment read asformatting only. A lossy decode there would be wrong: it maps0xE9and0xE8to the same U+FFFD, the two re-prints come out equal, andpm_diff_command.rs:1207would call a real byte changeformatting only. The new pm diff test pins that case.Metafile:
inputs[path].bytesis the length of the parsed text, so a decoded sheet reports 2 bytes more per replaced byte than its size on disk. This matches how a BOM is already handled on main (a 19-bytebom.jsreports 16).Pre-existing, not changed here: the dev server prints no CSS warnings at all (the
@nestdeprecation warning is also silent there), and a file whose@importfails to resolve loses its warnings (same for@nest). The build fails in that second case, and the error shows the U+FFFD specifier.Cost numbers (instructions, one core, from the two pre-merge checks of this PR). Valid input, on 6c0f45b: a 1-rule sheet +0.01 %, a 7 MB ASCII sheet +0.09 %, a 6 MB non-ASCII sheet +0.17 %, 200 small sheets +0.20 %,
--no-bundle+0.18 %. A 6 MB sheet with one bad byte, on 6c0f45b: +127 M instructions with the byte at the start, +264 M in the middle, +399 M at the end (+19.0 %, wall 711 to 738 ms). On 13abc7f, before the warning, that sheet paid +6.1 %. Two things grow with the offset of the first bad byte: the line and column lookup of the logger, and on 6c0f45b one scalar scan of the valid prefix to find that offset. 8720fcb takes the offset from the pass that sizes the output, so that scan is gone. Not re-measured after 8720fcb: this container deniesperf_event_openand has novalgrind.stderr in bytes for a 1 MB sheet with one bad byte, from a debug build of 8720fcb and from the same tree with logger: bound the excerpt printed for an error on a long line #43313 merged in:
The printer draws the line excerpt and pads the caret to the column for every diagnostic, so a CSS syntax error at the same place prints the same amount on main. logger: bound the excerpt printed for an error on a long line #43313 bounds the excerpt and the caret line in
Data::write_format, and logger: indent the caret relative to the windowed line excerpt #41658 places the caret inside a cut excerpt.BuildMessage.position.lineTextstill holds the whole line when the position is in the last 80 bytes of the line.The outputs affected before the fix: default,
--minify,--no-bundle, and the CSS chunk of an HTML entry.Tokenizer::consume_charalso steppedlen_utf8(U+FFFD) = 3bytes past a bad byte and dropped the 2 source bytes after it (an escaped bad byte ate a closing quote).Tokenizer::init_with_arenanow debug-asserts that its input is valid UTF-8.bun pm difffalls back to its text diff for a CSS file that is not valid UTF-8. esbuild also warns on a non-UTF-8@charset("UTF-8" will be used instead of unsupported charset).Supersedes Fix panic when bundling CSS that contains invalid UTF-8 #32795 (June), which made the same change against a tree 1960 commits back. Both review rounds there are reflected here: the helper lives in
bun_core::strings, takes the arena and returns the arena-lifetime slice, with no UTF-16 round trip. Ported rather than rebased becauseUNICODE_REPLACEMENT_STRand the publicArenaVec::leakit used are gone andbun pm diffadded a third parse site since.Repro on 1.4.2 / canary
d316760e8:BabyString::innow reports the error with an empty specifier instead of aborting when the formatted message does not embed the specifier bytes. After the decode no current producer hits this. It is hardening of an error path.U+FFFD is a valid ident code point, so a font-family like
"F<F8>nt"prints as the unquoted identF\u{FFFD}nt, the same way"Fønt"already prints asFønt.The JS lexer and the HTML scanner decode lossily on their own, and plugin or JS-string sources are transcoded to UTF-8 on the way in. Raw bytes reach the CSS parser from file reads and from
Uint8Arraycontents (pluginonLoad, thefilesoption), and all of those pass throughParseTask.Other shapes the decode covers, verified on the debug build: a continuation byte at a token start (
.a { color: red \xAF}used to trip theis_on_char_boundarydebug asserts),printf '.test{color:red;;;}\xbf\xbd"}'(same assert viaParser::state),Bun.build({throw:false})on the@importcase.Suites run on the debug build of 8720fcb (main at 36cd151 merged in):
test/js/bun/css/{invalid-utf8,css,doesnt_crash,css-loader}.test.ts,test/bundler/css/css-modules.test.ts,test/bundler/esbuild/css.test.ts,test/bake/dev/css.test.ts,test/internal/source-lints/.test/cli/install/bun-pm-diff.test.ts: 46 of 48 pass here, with the new case. "a scope too big for the LCS table" ran into its 5 s timeout in 3 of 3 local runs and "a tarball entry larger than 64 MiB" in 1 of 3, on a host with a load average near 500. Both diff JS or read a tarball, and both passed in CI on 6c0f45b, which has the samebun pm diffcode.Not changed here:
ResolveMessage.messagedecodes non-ASCII message text as Latin-1 (log.messageforimport "./café.js"is mis-decoded today with no invalid bytes involved), so theBun.buildresolve test asserts the ASCII around the replacement character. Thetextandjsonloaders have a separate bug with such bytes (they become\x00and the following bytes are dropped). That is tracked separately.no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/install/bun-pm-diff.test.ts