[robobun]

Problem

• With \r\n line ends, an error at the end of a file is at 3:0 with an empty lineText, where \n gives 2:19. bun build prints error: Unexpected end of file and no code frame. The dev server overlay throws RangeError: Invalid count value: -1.
• clamp_error_offset (src/ast/lib.rs) pulls that Loc to the last byte, the \n of the pair. The scan has then read the \r: one more line, column 0. A lone \r lowers every later column by one.
• No user report: a fuzz note found it.

Fix

• error_stop_offset replaces the clamp. It moves each Loc to the first byte of its \r\n pair or UTF-8 sequence.
• The \r arm starts a line at column 1. LineColumnTracker drops its rescan for a continuation byte.
• Verified: test/js/bun/transpiler/parse-error-column.test.ts (17 new tests that fail on main), test/bake/dev/bundle.test.ts (1), unit tests (Miri).
• Self-reviewed: 18 concerns raised, 18 addressed (Notes).

Background

• Question for a maintainer: the logger puts an end-of-file error on the last line. bun fmt, bun lint, esbuild and tsc say line N+1. This PR keeps the logger's answer. Is it the contract?
• A Loc is a byte offset.
• Considered: step back only for Loc >= len. The lexer also logs at len - 1 (lexer.rs:2969).

Downsides

• 118 of 228 probe rows change position. LF-only ASCII: 0 of 48.
• A \r\n end-of-file error prints its last line, as LF does: 4,120 writes for 4,096 bytes.
• Release .text +256 B. Instructions per diagnostic: LF 20,803 to 20,796, CRLF end 1,184 to 1,212.

Notes

Question for a maintainer: where is an error at the end of a file? Two pins on main disagree.

• test/js/bun/resolve/resolve-error.test.ts:26-29: the logger puts Unexpected end of file on the last line, with the caret, at 1:29. test-test.test.ts (1:3) and transpiler.test.js (1:15) pin the same.
• test/cli/format/format.test.ts:709 and :3717: bun fmt puts it at line N+1, column 1.

For const port = 3000; / function start() { with a final line break, on this branch: bun build prints 2:19 for LF, CR LF and lone CR. bun fmt --check and bun lint print 3:1 for all three. main prints 2:19, 3:0 and 2:18 from bun build. The report says that esbuild, tsc and node also give the line after the end.

• If the last line is the contract, this PR is complete.
• If line N+1 is wanted, that is a second change: the bound in error_stop_offset becomes min(start, len), the three pins and the 18 new tests move, and Data::write_format must decide what to draw for an empty line. The \r arm and the unit step stay, because the lexer also logs at len - 1.

The result of this PR is one line and column for every line break, and no column below 1. The frame it adds shows the last line of the file, which is not always the line with the mistake.

Where the request comes from. A fuzz note that a maintainer forwarded. There is no GitHub issue and no user report. The note compares the \n and \r\n spelling of one file. That is 30 of the 118 changed probe rows. The other 88 come from a read of the scanner: lone \r (64), a Loc inside a UTF-8 sequence (24). They are here so that no Location from the scanner has a column below 1 or a position that depends on the line break.

Repro.

import fs from "node:fs";
for (const [name, eol] of [["LF  ", "\n"], ["CRLF", "\r\n"]]) {
  fs.writeFileSync("in.js", ["const port = 3000;", "function start() {"].join(eol) + eol);
  const p = (await Bun.build({ entrypoints: ["./in.js"], throw: false })).logs[0].position;
  console.log(name, p.line, p.column, JSON.stringify(p.lineText));
}
// main: LF 2 19 "function start() {"   CRLF 3 0 ""
// here: LF 2 19 "function start() {"   CRLF 2 19 "function start() {"

What changes, by class. Bun.build logs[0].position.

Source	main	this PR
function start() { + \r\n at the end	3:0 "", no frame	2:19 "function start() {"
unclosed ${ + \r\n, second diagnostic (Loc == len - 1)	3:0 ""	2:37, as the first
]; first on line 2, lone \r line ends	2:0	2:1
const host = ]; on line 2, lone \r line ends	2:13	2:14
function start() { + U+2028 at the end	2:21, line text ends in a cut sequence	2:19 "function start() {"
foo(中, no final line break	1:7	1:5, the last character, as for ASCII
LF-only ASCII source		no change

TOML with lone \r is the one row that still differs from its LF answer: the parser reports Bare carriage return is not allowed at the \r.

Why every Loc, not only Loc >= len. rescan_close_brace_as_template_token (src/js_parser/lexer.rs:2961) does self.end -= 1 and scans again, so an unclosed ${ logs Unterminated string literal at len - 1. In a \r\n file that is the \n of the pair. In a file that ends in a multi-byte character it is a continuation byte.

Why the tracker fallback can go. CodepointIterator reads a well-formed sequence as one unit and every other byte as a unit of one byte. wtf8_codepoint_start moves a stop back only when a well-formed sequence covers it. So each stop is the start of a unit, and a scan that resumes from it reads the same units as a scan from byte 0. line_column_tracker_matches_full_scan checks this for every Loc of each corpus entry, with three ill-formed sequences added. With the step removed and the fallback kept (main's shape), locs_in_order_are_located_in_one_pass fails.

wtf8_codepoint_start. It is code_point_start of the lint's regex subject (src/lint/regex/wtf8.rs), moved to bun_core::strings next to wtf8_codepoint_at. bun_lint depends on bun_ast, so the logger could not call it where it was. test/cli/lint/conformance.test.ts passes.

The dev server overlay. What was run on main with a \r\n file and a brace left open:

• A tab is open and the file is saved broken: the overlay client exits on RangeError: Invalid count value: -1 from "_".repeat(location.column - 1) (src/runtime/bake/client/overlay.ts:623).
• A fresh page load: the HMR client never connects.

This PR removes the scanner as a source of column 0. The reader has no guard on main. A guard (location.column > 0) and the line <= 0 rule of write_log_data are in open #39488 (from closed #37902 and #37885). A location that no scan builds, for example one from a plugin, still needs that guard.

Measurements. Release builds of main 5f5799ed16 and of this commit on it, Linux x64. perf, valgrind, strace and bloaty are not installed here, so the counts come from llvm-nm, size and gdb.

• Scanner code (init_or_null_impl + advance + scan_line_end, with the stop-offset step inlined): 2698 B -> 2946 B (llvm-nm -S). Release .text: +256 B (size).
• Instructions inside Location::init_or_null_impl (gdb stepi): LF mid-file 20,803 -> 20,796, LF end of file 1,186 -> 1,184, CRLF end of file 1,184 -> 1,212. Each cell ran twice with the same count.
• In-order walk of every Loc: 1,342 bytes scanned over a 1,344-byte CR LF source and 285 over a 288-byte source of 3-byte characters. The bound is the file length (670 and 141 at half the size). Rescans from byte 0: 0 and 0. On main the second source takes 193 rescans for 289 Locs and 28,220 bytes (97 of 145 and 7,196 bytes at half the size).
• Allocations inside Location::init_or_null_impl: LF mid-file 1 -> 1, LF end of file 1 -> 1, CRLF end of file 0 -> 1 (the copy of the line text). ErrorPositionState / ScanCursor / LineColumnTracker: 32 / 56 / 256 B, unchanged.
• CRLF end-of-file error, 18-byte last line: lineText 0 -> 18 bytes, stderr 66 -> 114 bytes, write(2) 15 -> 42 (LF file on main: 112 bytes, 42 writes). 4,096-byte last line: 4,120 writes and 8,277 bytes (main: 15 and 71, LF on both builds: 4,120 and 8,275).
• Positions changed vs main: 118 of 228 probe rows (CR LF end 30, lone CR 64, U+2028/9 end 12, multi-byte last character 10, Loc inside a sequence 2). LF-only ASCII rows changed: 0 of 48. Existing assertions edited: 0. Rows with a column below 1: 50 -> 0.
• bun run rust:miri -p bun_ast: 20 tests in 11.3 s -> 23 tests in 16.0 s.

Tests.

• test/js/bun/transpiler/parse-error-column.test.ts: 25 pass with the debug build. With main's release build the 17 new tests fail.
• test/bake/dev/bundle.test.ts: 55 pass. On main the new test fails: the client never acknowledges the page load.
• AOT_PROGRAMS=parse-error-column bun test test/aot/buns-own-tests-as-compiled-code.test.ts (release build): pass. That suite compiles this test file with --aot and compares each test with plain Bun.
• Unchanged and passing: resolve-error.test.ts, test-test.test.ts, transpiler.test.js, runtime-transpiler.test.ts, jsonc.test.ts, json5.test.ts, json5-test-suite.test.ts, toml.test.ts, toml-test-suite.test.ts, bundler_defer.test.ts, test/bake/dev/css.test.ts, BunFrontendDevServer.test.ts.
• test/cli/format/format.test.ts: every position test passes. 40 other tests in that file time out at 5 s in the debug build in this container.

Self-review. A review of the diff before the PR raised 18 concerns. What changed because of it: the helper moved to bun_core (one implementation with the lint's), a comment-only hunk in serialized_failure.rs is gone, the dev server test also covers a fresh page load, the unit tests are smaller (Miri time), each added comment in src/ is one line, and this body names the request's source, the reader-side PRs and the landing order. Two suggestions are not taken. The body keeps its section order, so the question for a maintainer is the first Background bullet and the first note, not the first line. An optional unit test that checks the scanner against Source::line_col_to_byte_offset is not added.

Not in this PR.

• to_error_position starts the line text one byte early and trims \n and \r. After a U+2028 or U+2029 line end the text starts with a cut sequence. logger: indent the caret relative to the windowed line excerpt #41658 carries the fix.
• The line text is not windowed when the error is in the last 80 bytes of a long line, and the caret padding costs one write(2) per byte. logger: search for the end of a line only as far as the excerpt of a diagnostic goes #44811, logger: indent the caret relative to the windowed line excerpt #41658 and logger: keep an excerpt of a line over 1,000 bytes wherever the error is #43313 work on this.
• CSS diagnostics and the code frame of a thrown error do not use this scanner. They use ErrorLocation::to_location and get_lines_in_text, which have their own line break rules. css: report the diagnostic's line without its line breaks in position.lineText #38342, strings: drop the '\r' from line 1 of CRLF files in error code frames #38338 and strings: include unterminated final line in error code-frame preview #36683 carry those.
• position.offset stays the raw Loc. That is the contract today.

Order. This PR is meant to land first among the open logger PRs. #44811 and #35550 merge cleanly into main today and conflict with this branch. #41658, #43313, #43316 and #38825 conflict with main already. #45787 is stacked on this PR.

---

[human-review] gate passed · iteration 0 · 7 files touched

fails on main (without fix)

ASAN without fix: BUILD FAILED (no junit output)
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bake/dev/bundle.test.ts test/js/bun/transpiler/parse-error-column.test.ts
error: Source list 'zigGeneratedClasses' matched nothing
  at: file:///workspace/bun/scripts/glob-sources.ts
  hint: Patterns: src/**/*.classes.ts
error: script "bd" exited with code 1
__F:-1:S:0

release without fix: all passed
bun test v1.4.4-canary.1 (80bfab5c2)

test/bake/dev/bundle.test.ts:
Dev server testing directory: /tmp/bun-dev-test-TcelkZ
^[[0;30mdev|^[[0m Started development server: http://localhost:35639
^[[0;30mdev|^[[0m ^[[32mBundled page in 15ms^[[0m^[[2m:^[[0m routes/index.ts ^[[2m+ 1 more^[[0m
^[[0;30mdev|^[[0m [Bun] Hot update was not accepted because it or its importers do not call `import.meta.hot.accept`. To prevent full page reloads, call `import.meta.hot.accept` in one of the following files to handle the update:
^[[0;30mdev|^[[0m 
^[[0;30mdev|^[[0m Module "db.ts" is not accepted by routes/index.ts,
^[[0;30mdev|^[[0m ^[[32mReloaded in 1ms^[[0m^[[2m:^[[0m db.ts
^[[0;30mdev|^[[0m [Bun] Hot update was not accepted because it or its importers do not call `import.meta.hot.accept`. To prevent full page reloads, call `import.meta.hot.accept` in one of the following files to handle the update:
^[[0;30mdev|^[[0m 
^[[0;30mdev|^[[0m Module "routes/index.ts" is a root module that does not self-accept.
^[[0;30mdev|^[[0m ^[[36m[x2]^[[0m ^[[32mReloaded in 1ms^[[0m^[[2m:^[[0m routes/index.ts
(pass)  DEV:bundle-1: import identifier doesnt get renamed [266.83ms]
^[[0;30mdev|^[[0m Started development server: http://localho
... (truncated)

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bake/dev/bundle.test.ts test/js/bun/transpiler/parse-error-column.test.ts
bun test v1.4.4 (367d939d9)

test/bake/dev/bundle.test.ts:
Dev server testing directory: /tmp/bun-dev-test-b2O6zx
^[[0;30mdev|^[[0m Started development server: http://localhost:42745
^[[0;30mdev|^[[0m ^[[32mBundled page in 102ms^[[0m^[[2m:^[[0m routes/index.ts ^[[2m+ 1 more^[[0m
^[[0;30mdev|^[[0m [Bun] Hot update was not accepted because it or its importers do not call `import.meta.hot.accept`. To prevent full page reloads, call `import.meta.hot.accept` in one of the following files to handle the update:
^[[0;30mdev|^[[0m 
^[[0;30mdev|^[[0m Module "db.ts" is not accepted by routes/index.ts,
^[[0;30mdev|^[[0m ^[[32mReloaded in 31ms^[[0m^[[2m:^[[0m db.ts
^[[0;30mdev|^[[0m [Bun] Hot update was not accepted because it or its importers do not call `import.meta.hot.accept`. To prevent full page reloads, call `import.meta.hot.accept` in one of the following files to handle the update:
^[[0;30mdev|^[[0m 
^[[0;30mdev|^[[0m Module "routes/index.ts" is a root module that does not self-accept.
^[[0;
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 1224ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[0/2] cargo plan → /workspace/bun/build/release/rust-target/plan.json
270 units: 194 lib, 16 proc-macro (host), 21 custom-build (host), 17 run custom-build, 17 lib (host), 4 run custom-build (host), 1 rlib
[1/290] gen ErrorCode+*.h
[2/290] gen NodeModuleModule.lut.h
Generating /workspace/bun/build/release/codegen/NodeModuleModule.lut.h from /workspace/bun/src/jsc/modules/NodeModuleModule.cpp
[3/290] gen generated_host_exports.rs
generated_host_exports.rs: 116 exports (host=5, lazy=4, generic=107, rust=0); 252 extern-C blocks audited
[4/290] gen ZigGeneratedClasses.{cpp,h,rs}
Found 2 classes from /workspace/bun/src/jsc/resolve_message.classes.ts
  - ResolveMessage (15 fields)
  - BuildMessage (10 fields)
Found 1 classes from /workspace/bun/src/runtime/api/Archive.classes.ts
  - Archive (4 fields, 1 class fields)
Found 1 classes from /workspace/bun/src/runtime/api/BunObject.classes.ts
  - Subprocess (20 fields)
Found 1 classes from /workspace/bun/src/runtime/api/filesystem_router.classes.ts
  - FrameworkF
... (truncated)

diff hotspot

src/ast/lib.rs                                    | 210 +++++++++++++++++-----
 src/bun_core/string/immutable.rs                  |   5 +-
 src/bun_core/string/immutable/wtf8_text.rs        |   8 +
 src/lint/regex/subject.rs                         |   2 +-
 src/lint/regex/wtf8.rs                            |   8 -
 test/bake/dev/bundle.test.ts                      |  20 +++
 test/js/bun/transpiler/parse-error-column.test.ts | 158 +++++++++++++++-
 7 files changed, 357 insertions(+), 54 deletions(-)

gate history · 2 passed · 0 rejected · iteration 0

evidence per changed file

file                                               reads  edits  tests
src/ast/lib.rs                                         6      6     45
src/bun_core/string/immutable.rs                       1      0     43
src/bun_core/string/immutable/wtf8_text.rs             0      0     43
src/lint/regex/subject.rs                              0      0     44
src/lint/regex/wtf8.rs                                 0      0     44
test/bake/dev/bundle.test.ts                           0      0     30
test/js/bun/transpiler/parse-error-column.test.ts      1      0     23

[robobun]

^{Updated 2:37 AM PT - Oct 29th, 2026}

❌ @robobun, your commit 3fc5135 has 6 failures in Build #134918 (All Failures):

• test/aot/values-that-may-be-something-else-15.test.ts - code 1 on 🐧 13 x64 - test-aot-drawn
• test/aot/perf/generated-code-of-idioms-5.test.ts - code 1 on 🐧 13 x64 - test-aot-drawn
• test/aot/programs-that-ask-the-shell-3.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
• test/aot/perf/generated-code-of-idioms-7.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
• test/aot/perf/generated-code-of-operations-1.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
• test/aot/perf/generated-code-of-idioms-11.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn

---

🧪   To try this PR locally:

bunx bun-pr 45554

That installs a local version of the PR into your bun-45554 executable, so you can run:

bun-45554 --bun

[robobun]

Status

• Reproduced on main with the script in the PR body. A file with \r\n line ends and a brace left open reports 3:0 and an empty lineText, and bun build prints no code frame. The same file with \n reports 2:19.
• The dev server case was run with the new test in test/bake/dev/bundle.test.ts. On main the client never acknowledges the page load.
• The fix and its tests are in this PR (logger: locate a diagnostic at the first byte of its CR LF pair or UTF-8 sequence, and count columns from 1 after a CR #45554).

CI on 3fc5135f82 (Buildkite build 134918)

• The diff is green. 188 jobs passed. No test of this PR failed. cargo miri test, cargo clippy, the format check and the source lints pass.
• The 6 failed jobs are test-aot-drawn lanes. The failed files are test/aot/perf/generated-code-of-idioms-5, -7 and -11, generated-code-of-operations-1, programs-that-ask-the-shell-3 and values-that-may-be-something-else-15. They check the code that the ahead-of-time compiler generates, and this diff does not touch that compiler. Those lanes failed in 13 of the 16 most recent builds that have them, on PRs with no relation to this one.
• Eight other test files failed one time and passed on the retry. None of them is in this diff.
• The 2 jobs that remain are the macOS x64 test jobs. They run at this time.

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• src/ast/lib.rs

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Walkthrough

Diagnostic offsets are normalized before position scans. Lone-CR columns start at 1. Tests cover tracker scans and reported positions across line endings, Unicode text, parser APIs, CLI commands, and DevServer behavior.

Changes

Diagnostic positions

Layer / File(s)	Summary
Shared WTF-8 codepoint lookup src/bun_core/string/immutable/wtf8_text.rs, src/bun_core/string/immutable.rs, src/lint/regex/subject.rs, src/lint/regex/wtf8.rs	A shared helper finds the start of the WTF-8 codepoint containing a byte offset. Regex subject handling uses it instead of the removed local helper.
Offset normalization and position scanning src/ast/lib.rs	Diagnostic offsets are normalized before tracker or full scans. Scanner APIs accept normalized byte offsets. Tests cover line endings, UTF-8 boundaries, tracker behavior, and ordered scans.
Parser diagnostic output coverage test/js/bun/transpiler/parse-error-column.test.ts, test/bake/dev/bundle.test.ts	Tests check positions and displayed diagnostics across line endings, Unicode text, parser and CLI paths, and DevServer reloads.

Suggested reviewers: jarred-sumner, dylan-conway

Priority: Normal

Merge Risk: Minimal · up to 4064a

No actionable merge-blocking risk was established.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title names the main change: diagnostic locations move to the first byte of a CR LF pair or UTF-8 sequence, and columns count from 1 after a lone CR. The changeset does this in src/ast/lib.rs (err…
Linked Issues check		Issue #37902 is closed and supplies historical context only. No active directly linked issue remains. Therefore, no linked-issue coding requirements apply.
Out of Scope Changes check		The changes stay within the current PR scope. The scanner normalization, line-ending handling, shared WTF-8 helper, parser tests, and CRLF dev-server regression test all support consistent diagnostic …
Description check		The description is complete and relevant. It explains the problem, implementation, scope, limitations, and verification results, although it uses Problem and Fix headings instead of the template headi…

Full details: Title check

Explanation

The title names the main change: diagnostic locations move to the first byte of a CR LF pair or UTF-8 sequence, and columns count from 1 after a lone CR. The changeset does this in src/ast/lib.rs (error_stop_offset replaces clamp_error_offset, ErrorPositionState::advance starts columns at 1 after CR) and adds supporting helper wtf8_codepoint_start. The title is specific and matches the primary change, though it is long (118 characters) and uses a semicolon-free compound phrasing that is somewhat dense.

Full details: Out of Scope Changes check

Explanation

The changes stay within the current PR scope. The scanner normalization, line-ending handling, shared WTF-8 helper, parser tests, and CRLF dev-server regression test all support consistent diagnostic positions and overlay behavior. No unrelated change is demonstrated.

Full details: Description check

Explanation

The description is complete and relevant. It explains the problem, implementation, scope, limitations, and verification results, although it uses Problem and Fix headings instead of the template headings.

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Actionable comments posted: 1

---

• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/ast/lib.rs:
- Line 3651: Update with_line_break to use
bun_core::strings::index_of_char_usize to locate newline bytes instead of
scanning each byte manually. Append the intervening slices and replacement line
breaks, then append the remaining suffix unchanged.

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📒 Files selected for processing: 7

• src/ast/lib.rs
• src/bun_core/string/immutable.rs
• src/bun_core/string/immutable/wtf8_text.rs
• src/lint/regex/subject.rs
• src/lint/regex/wtf8.rs
• test/bake/dev/bundle.test.ts
• test/js/bun/transpiler/parse-error-column.test.ts

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• src/lint/regex/wtf8.rs

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline findings, I also checked whether dropping the continuation-byte rescan in LineColumnTracker::error_position can leave the cursor mid-unit: wtf8_codepoint_at and CodepointIterator both decode through decode_wtf8_rune_t* and treat truncated or malformed sequences as 1-byte units, so every stop from error_stop_offset is a unit boundary for the scanner. The Loc to usize signature change covers every caller in src/ (including the tracker's own fallback and the lint regex subject), and wtf8_codepoint_start is a byte-for-byte move of the lint helper.

Extended reasoning...

The change touches the diagnostic line/column scanner in src/ast/lib.rs, moves a WTF-8 helper into bun_core::strings, and adds Rust unit tests plus Bun.build/CLI/dev-server tests for CRLF, lone CR, U+2028/9 and multi-byte EOF errors; no security-sensitive surface. Three inline findings were posted (two pre-existing conditions at lines 835 and 2590, one behavior consequence at 850), so a human should weigh them before merging.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.