[robobun]

Problem

• Four inputs of exports in Bun.Transpiler end the process. replace on a name of a re-export list: panic: index out of bounds: the len is 6 but the index is 10. eliminate on export let A: panic: called `Option::unwrap()` on a `None` value.
• eliminate on export class A {} or on export default: panic: unreachable. A string value of replace after another parse in the process: Segmentation fault.

Fix

• replace_reexport declares the exported name for both re-export forms. The name default goes through the visitor of export default.
• visit_decls drops the declaration that the entry removes. _parse moves only a part that append_part appended. The string node lives in the arena of the instance.
• A namespace member is not an export, so no entry applies to it.
• Verified: test/bundler/transpiler/transpiler.test.js, 10 new tests, all fail without the src/ change. Self-reviewed: 12 concerns raised, 12 addressed (re-export part).

Background

• exports.eliminate removes a named export. exports.replace gives it another value or, with [name, value], another name.
• This PR has the changes of js_parser: fix panic when exports.eliminate removes a hoistable export #33376, Bun.Transpiler: allocate exports.replace string values in the instance arena #38541 and js_parser: fix exports.eliminate/replace on a declaration with no initializer and on namespace members #44179, which are closed.
• Bun.Transpiler: exports.replace prints a module that does not parse when the module cannot declare the export name #44138 asks what a replaced export does when the module cannot declare its name.

Downsides

• Outputs that change: a replaced re-export throws when the module binds or imports its name, replace on export let A prints the export, and a namespace member stays as it is.
• A reserved word as the name prints export var class = 1 (Bun.Transpiler: exports.replace prints a module that does not parse when the module cannot declare the export name #44138).
• Release .text +3584 B. No exports option: +1 instruction for each export default, +5 for each part.

Notes

Found by fuzzing. There is no issue and no user report. #12892 and #2084 are about the same option. This PR does not change what they describe.

The parts of this PR

Part	Was	Commits
A name of a re-export	this PR	a3d8904, 1de33bf, 10f0370, e1527b3
A string value after another parse	#38541	59da676, e946a11
A declaration with no initializer, a member of a namespace	#44179	540701e, a219ce3, 0c94fd8, aa89336
A hoistable export	#33376	c67695a, 089fd53
An entry that injects from export default	new	36bb20b, 603fb7f

The crashes

Input	Option	1.4.2 and main a4f1429	this PR
export { ReadStream, WriteStream } from "node:fs"	replace: { ReadStream: "x" }	panic: index out of bounds: the len is 6 but the index is 10	export var ReadStream = "x"; and the list with WriteStream
export let A	eliminate: ["A"]	panic: called `Option::unwrap()` on a `None` value	empty module
export class A {} and export const keep = 1	eliminate: ["A"]	panic: unreachable	export const keep = 1;
export default 42 and export const keep = 1	replace: { default: ["N", 1] }	panic: unreachable	export var N = 1; and export const keep = 1;
export const ReadStream = 5, after require() of another file	replace: { ReadStream: "hello-value" }	Segmentation fault, or panic: capacity overflow	export const ReadStream = "hello-value";

Bun 1.3.13 prints the right module for the first row. Bun 1.3.14 prints it with bun -e and ends in Segmentation fault from a file. For the second row, 1.3.13 and 1.3.14 end in Segmentation fault at address 0x8. The Zig source had the same logic, so it was never correct, and the tests are not in test/regression/issue.

The causes (lines of main)

• s_export_from (src/js_parser/visit/visit_stmt.rs:293) passes the parse-time ref of the list item to inject_replacement_export. The index of that ref is the length of the imported name, and visit_binding uses it as an index into the symbol table.
• The arm of visit_decls for a declaration with no value (src/js_parser/visit/mod.rs:601) reads the result of replace_decl_and_possibly_remove the wrong way round. It visits the declaration that it must drop, and visit_decl unwraps the missing value.
• _parse (src/js_parser/parse/parse_entry.rs:1164 and :1176) moves the part of a hoistable statement to the top with parts.pop().expect("unreachable"). append_part appends nothing when the visit removed each statement.
• export_replacement_value (src/runtime/api/JSTranspiler.rs:919) makes the string node with Expr::init, in the store that the next parse of the thread resets.

What replace_reexport does

Entry	Exported name	Result
replaces, or injects	default	export default <value>, visited with no entries
replaces	other	declare_symbol for the name, then export var name = <value> through inject_replacement_export
injects	other	inject_replacement_export, which declares the injected name as before
eliminates		nothing

For export * as name, the second row is what main does. The first row is new for it, and so is the fact that the last two rows do not declare name. On main, const ns = 1; export * as ns from "x" with eliminate: ["ns"] throws "ns" has already been declared, and export * as default from "x" with replace: { default: 1 } prints export var default = 1.

Why the default export is visited, and with no entries

The lowering of a top-level using wraps the module body in try. s_export_default then prints var x = <value> and export { x as default }, and the wrap pass drops each export default statement (src/js_parser/p.rs:10078). A statement that does not go through the visitor has no such pair, and the module loses the export. #38287 moves that split and asserts that no export default statement reaches the wrap pass. The visited statement is correct before and after #38287.

The visitor looks default up in the entries. With replace: { a: ["default", 1], default: 2 } it changed the value to 2. So replace_reexport takes the entries away (core::mem::take) while the visitor runs, and puts them back. The first version was a flag on the parser that the visitor reads. With it, s_export_default for ts kept the parser in a stack slot, grew 412 B, and ran 12 more instructions for each export default with no exports option.

An injected export var still goes through s_local, which looks its name up, as on main. #44138 asks which rule all forms get.

The visit cannot fail for a value of exports.replace, which is a string, a number, a boolean, null or undefined. The expect is the one that inject_replacement_export has on its own visit. With ? in its place, s_export_from can return an error, and it runs 1 more instruction for each statement with no exports option (measured: 37,130 to 37,328 for 200 statements).

declare_symbol and new_symbol

The first version made the symbol with new_symbol, as s_export_from does for the items it keeps. That symbol is not a member of the module scope, so nothing sees a collision, and the module prints two declarations of the name. declare_symbol reports the collision.

With the ts loader, declare_symbol lets a declaration take the place of an import of its name (can_merge_symbol_kinds, src/ast/scope.rs:459). So import { a } from "x"; export { a, b } from "x"; export const t = a with replace: { a: 7 } printed a module with no import, and t was 7. replace_reexport looks for an import of the name first and reports "a" has already been declared, which is what the js loader does.

An entry that injects from export default

The visitor of export default marks the value as dead code when an entry takes the export away, and it returned before it injected the export of the entry. With dead code elimination off it injected it. On main this input ends in panic: unreachable, so the missing export was not visible. The three exits for a dead value now inject, and so does the exit for a default export that is a type.

A member of a namespace

s_local selects visit_decls::<true> for a member of a TypeScript namespace too, because its gate tests is_export and the size of the map only. s_function and s_class visited such a member as dead code and then kept the statement, so its body was empty. A member is a property of the namespace object. visit_decls does the test once, and only in the instantiation for true. A test in s_local cost 3 instructions for each statement with no exports option, so it moved.

Input	Option	main	this PR
namespace NS { export let A = 1 }	eliminate: ["A"]	the namespace with no member	NS.A = 1
namespace NS { export let A = 1 }	replace: { A: 2 }	NS.A = 2	NS.A = 1
namespace NS { export function A() { return f() } }	eliminate: ["A"]	function A() {}	the function with its body

The tests

• 4 child processes in place of 1 for each input: the rows of the re-exports, the rows with no initializer, the hoistable rows, and the test of the string values from Bun.Transpiler: allocate exports.replace string values in the instance arena #38541.
• The key is the exported name: replace: { bb: 1 } matches export { a as bb } from "x", and replace: { a: 1 } does not. main looks the alias up too.
• Each list keeps one name, so no row pins the text of an empty list. js_printer: print esbuild's pretty-mode spaces in bare import, for(;;), and empty export-from #40808 changes that text.
• No row pins which of replace and eliminate wins for one name.
• The rows with using import the printed module and compare its exports, because the lowering prints helper names.
• The it.failing test expects a module that parses and exports the name, so it changes with the decision in Bun.Transpiler: exports.replace prints a module that does not parse when the module cannot declare the export name #44138. It is red when the child process did not print its line.

What stays wrong

Input	Option	Result
export { a as class, b } from "x"	replace: { class: 1 }	prints export var class = 1
let bb = 2; export { a as bb } from "x"	replace: { bb: 1 }	throws "bb" has already been declared
export function g() {}	replace: { g: ["default", 1] }	prints export var default = 1, as on main
export const x = 5	replace: { x: ["default", 1] }	prints export const default = 1, as on main
export default 42	replace: { default: ["default", 1] }	throws "default" has already been declared
export default class A {}	replace: { default: 1 }	prints the class, as on main
import { a } from "y"; export function f() {} export const t = a	replace: { f: ["a", 7] }, ts loader	prints a module with no import, as on main
let a = 1; export { a as bb }	replace: { bb: 1 }	the key does not match, as on main. s_export_clause looks the local name up
export { a, b } from "x"	eliminate: ["a", "b"]	prints export { } from "x", as on main. export * as prints nothing
using r = f(); export default 5	replace: { default: 1 }	prints var input_default = 5, as on main. #38287 changes the order that causes it

For the first two rows, main aborts when the imported name is as long as the symbol table, which has 6 symbols in these modules. For a shorter name it binds an unrelated symbol, and prints export var a = 1 for the one-letter name a. The first 4 rows are the it.failing test. #44138 has the rows and the questions.

#33378 has the swap of the arm in visit_decls among other changes, and conflicts with main since August. That hunk becomes empty when it rebases over this PR.

Measurements (release builds, main a4f1429 against 603fb7f, linux x64)

What	Result
.text	58,143,477 B to 58,147,061 B, +3584 B (llvm-size -A). size: text 80,660,492 to 80,664,076
replace_reexport (new, #[cold])	1410 B (ts), 603 B (js) (nm -S)
s_export_default	9107 to 10,209 B (ts), 8557 to 9554 B (js)
s_export_from, s_export_star	2521 to 2537 and 1106 to 905 B (ts), 2253 to 2307 and 1031 to 833 B (js)
s_local, s_function, s_class, append_part	+45, +28, +10 and +15 B, both instantiations
_parse	31,187 to 31,125 B (ts), 26,320 to 26,174 B (js)
export_replacement_value	527 to 563 B
Instructions with no exports option, in a module of 1300 statements	s_export_from: 37,130 to 36,736 (ts), 32,922 to 32,718 (js), for 200 statements. s_export_default: 121 on both (ts), 109 to 110 (js), for 1 statement. append_part: 28,132 to 28,202 for 14 parts (js). s_export_star, s_export_clause, s_function, s_class, s_local: 0 change (gdb step count, JIT off)
visit_binding, visit_decl, record_declared_symbol, replace_decl_and_possibly_remove, inject_replacement_export	0 of 10 function bodies differ (normalized objdump, sha256)

valgrind, strace and perf are not on the machine that ran this, so the instruction counts come from gdb.

Outputs that change (102 inputs, ts loader, one process for each input, node --check on each printed module)

	main	this PR
aborts	25	0
throws	5	10
prints a module that node --check rejects	15	8
prints a module that parses	57	84

73 inputs change. The 10 that throw: 9 bind or import the name of a replaced re-export, or have the name eval, and 1 is replace: { default: ["default", 1] }. The 8 rejected modules: 5 have a reserved word as the name, and 3 come from producers that this PR does not change (the export clause forms, and export const default = 1).

---

[human-review] gate passed · iteration 3 · 7 files touched

fails on main (without fix)

ASAN without fix: 10 failed, 22 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/transpiler/transpiler.test.js
bun test v1.4.3 (367d939d9)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [6.89ms]
(pass) Bun.Transpiler > normalizes \r\n [8.55ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [6.07ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [9.87ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [3.51ms]
(pass) Bun.Transpiler > property access inlining > works [3.88ms]
(pass) Bun.Transpiler > property access inlining > works nested [3.79ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [92.23ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [35.03ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [742.10ms]
(pass) Bun.Transpiler > property access inlining > bails out on optional-
... (truncated)

release without fix: 8 failed, 22 skipped
bun test v1.4.3-canary.1 (e1527b332)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [1.59ms]
(pass) Bun.Transpiler > normalizes \r\n [0.31ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [0.13ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [0.17ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [0.03ms]
(pass) Bun.Transpiler > property access inlining > works [0.03ms]
(pass) Bun.Transpiler > property access inlining > works nested [0.03ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [0.72ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [0.42ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [28.22ms]
(pass) Bun.Transpiler > property access inlining > bails out on optional-chain index into enum [0.84ms]
(pass) Bun.Transpiler > property access inlining > template literal around an inlined string enum member > member as the first part of the
... (truncated)

passes on PR (with fix)

ASAN with fix: 22 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/transpiler/transpiler.test.js
bun test v1.4.3 (367d939d9)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [5.32ms]
(pass) Bun.Transpiler > normalizes \r\n [7.75ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [4.62ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [8.03ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [2.80ms]
(pass) Bun.Transpiler > property access inlining > works [3.06ms]
(pass) Bun.Transpiler > property access inlining > works nested [2.96ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [72.15ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [38.01ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [1428.82ms]
(pass) Bun.Transpiler > property access inlining > bails out on optional
... (truncated)

release with fix: 22 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 2867ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/213] gen generated_host_exports.rs
generated_host_exports.rs: 121 exports (host=5, lazy=10, generic=106, rust=0); 248 extern-C blocks audited
[2/213] gen ErrorCode+*.h
[3/213] gen NodeModuleModule.lut.h
Generating /workspace/bun/build/release/codegen/NodeModuleModule.lut.h from /workspace/bun/src/jsc/modules/NodeModuleModule.cpp
[4/213] gen cpp.rs (cppbind)
[5/213] gen JS modules (bundle-modules)
Preprocess modules (14231ms)
Bundle modules (459ms)
Postprocesss modules (253ms)
Bundle Functions (1419ms)
Generate Code (93ms)

[16.48s] Bundled "src/js" for production
  2612 kb
  198 internal modules
  13 native modules
  50 internal functions across 16 files
[6/178] build.rs build_script_build
[7/178] cxx obj/unified/UnifiedSource-src_jsc_bindings-4.cpp.o
[8/178] cxx obj/unified/UnifiedSource-src_jsc_bindings-0.cpp.o
[9/178] cxx obj/unified/UnifiedSource-src_jsc_bindings_node-0.cpp.o
[10/178] cxx obj/unified/UnifiedSource-src_jsc_bindings-3.cpp.o
[11/178] cxx obj/src/jsc/bindings/webcore/streams/JSReadable
... (truncated)

diff hotspot

src/ast/expr.rs                            |   2 +-
 src/js_parser/p.rs                         |   8 +-
 src/js_parser/parse/parse_entry.rs         |  12 +-
 src/js_parser/visit/mod.rs                 |  28 +-
 src/js_parser/visit/visit_stmt.rs          | 101 ++++++-
 src/runtime/api/JSTranspiler.rs            |   6 +-
 test/bundler/transpiler/transpiler.test.js | 447 +++++++++++++++++++++++++++++
 7 files changed, 569 insertions(+), 35 deletions(-)

gate history · 1 passed · 0 rejected · iteration 3

evidence per changed file

file                                        reads  edits  tests
src/ast/expr.rs                                 0      0    152
src/js_parser/p.rs                              9      8    155
src/js_parser/parse/parse_entry.rs              1      2    153
src/js_parser/visit/mod.rs                      4      1    154
src/js_parser/visit/visit_stmt.rs               8      2    156
src/runtime/api/JSTranspiler.rs                 1      0    156
test/bundler/transpiler/transpiler.test.js      5      6    152

[robobun]

Status: open at 603fb7f. CI is green (Buildkite #121525), and all review threads are resolved. Waits for a maintainer review. This is the one PR for the crashes of the exports option. #33376, #38541 and #44179 are closed, and their changes are here.

Reproduced on main (a4f1429) and on Bun 1.4.2:

const print = (exports, source) => new Bun.Transpiler({ loader: "ts", exports }).transformSync(source);
print({ replace: { ReadStream: "x" } }, 'export { ReadStream, WriteStream } from "node:fs"');
// panic: index out of bounds: the len is 6 but the index is 10
print({ eliminate: ["A"] }, "export let A");
// panic: called `Option::unwrap()` on a `None` value
print({ eliminate: ["A"] }, "export class A {}\nexport const keep = 1;\n");
// panic: unreachable

Verification with bun bd test test/bundler/transpiler/transpiler.test.js -t "exports.replace|exports.eliminate":

src/ from main:     2 pass, 10 fail   (the 10 new tests. The child processes abort with the panics above)
src/ from this PR:  12 pass, 0 fail   (full file: 247 pass, 0 fail)

What stays wrong, and the decision that it needs, is in #44138.

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📒 Files selected for processing (7)

• src/ast/expr.rs
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• src/js_parser/visit/mod.rs
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Walkthrough

Export replacement handling now covers named and namespace re-exports, default exports, and declarations without initializers. Parser part creation reports whether it appended a part. String replacement expressions use the supplied arena. Regression tests cover transformed output, exports, and runtime results.

Changes

Export replacement behavior

Layer / File(s)	Summary
Handle re-export replacements src/js_parser/visit/visit_stmt.rs, test/bundler/transpiler/transpiler.test.js	A shared helper handles matched named and namespace-alias re-export entries. Tests check transformed output, scanned export names, runtime exports, duplicate-binding errors, and names that cannot be bound.
Handle declarations and dead default exports src/js_parser/visit/mod.rs, src/js_parser/visit/visit_stmt.rs, test/bundler/transpiler/transpiler.test.js	Module replacement lookup excludes namespace members. Declarations without initializers are retained when no replacement exists. Dead default-export paths can inject eligible replacement entries. Tests cover declaration and namespace cases.
Track whether parser parts are appended src/js_parser/p.rs, src/js_parser/parse/parse_entry.rs, test/bundler/transpiler/transpiler.test.js	P::append_part returns whether it appended a part. Class and default-export statements move to before only when a part was appended. Tests check hoistable exports and statement order.
Allocate replacement expressions in the supplied arena src/ast/expr.rs, src/runtime/api/JSTranspiler.rs, test/bundler/transpiler/transpiler.test.js	String replacement expressions use the supplied arena. Tests cover replacements across module loads and synchronous and asynchronous transforms.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to 36bb2

A valid re-export can fail to transform when its replaced alias matches a local binding. The affected case is narrow and has a naming workaround; merge with explicit acceptance of this limitation or fix it first.

🚥 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 parser area and the primary change: fixing crashes in exports.eliminate and exports.replace.
Description check	✅ Passed	The description explains the problem, fix, scope, known limitations, and verification results. It does not use the exact template headings, but it provides the required information, including detailed…

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Inline comments:
Review comments at @src/js_parser/visit/visit_stmt.rs:
- Line 293: Update the synthetic default-export visit at
`p.visit_and_append_stmt` so it does not apply `exports.replace` a second time.
Preserve the replacement used to create the synthetic export, but visit that
export without allowing the `default` entry to replace its value or inject an
invalid declaration.
- Around line 297-300: In the re-export handling path around `declare_symbol`
and `inject_replacement_export`, stop using the exported alias as the local
symbol name. Declare a unique valid local binding, then export that binding
under the requested alias so re-exports do not collide with existing bindings or
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[robobun]

Both findings of the review are correct. One is fixed, and one is not part of this PR.

1. A second entry applied to the export default that an entry made. Fixed in 1de33bf. The visitor of export default skips the lookup in exports.replace while replace_reexport visits its statement. Before the fix, on export { a, b } from "x":

replace	a3d8904	1de33bf
{ a: ["default", 1], default: 2 }	export default 2	export default 1
{ a: ["default", 1], default: ["N", 2] }	no export for a (dead code elimination on)	export default 1
{ a: ["default", 1], default: ["default", 2] }	no export for a, or "default" has already been declared	export default 1

The first two rows are in the new test. An injected export var still goes through s_local, which looks its name up, as on main. #44138 asks which rule all forms get.

2. The exported alias as the local binding. Not changed here. A unique local binding with an export by name (var tmp = 1; export { tmp as bb }) is one of three possible behaviors, and #44138 asks a maintainer to choose. Until then a collision throws "bb" has already been declared, which is what export * as does on main, and a reserved word prints export var class = 1. The it.failing test records both. The thread of this finding is marked as addressed by 1de33bf, but that commit does not change it.

[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 both .expect("unreachable") calls in replace_reexport (src/js_parser/visit/visit_stmt.rs:295-305) for a user-reachable panic: declare_symbol only returns Err through mark_strict_mode_feature, which logs the reserved-word error and returns Ok(()), and with Loc::EMPTY (start == -1) range_of_identifier returns Range::NONE without indexing the source; the synthesized export default visit only visits a literal from exports.replace, which cannot fail. I also checked the is_visiting_replaced_export flag: it is set and cleared around a single infallible call with no early return in between, so it cannot leak into a later statement.

Extended reasoning...

The change routes both re-export replacement sites (export { .. } from and export * as) through a new cold helper in src/js_parser/visit/visit_stmt.rs, adds a parser flag in src/js_parser/p.rs, and adds a transpiler test table; it touches no security-sensitive surface. Inline findings are being posted, so this note only records the panic paths and flag lifetime that were examined and found not to be an issue.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

Still open from earlier reviews (2):

• Unresolved: 2 minor or pre-existing.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

[robobun]

^{Updated 1:27 AM PT - Sep 29th, 2026}

✅ @robobun, your commit 603fb7f33238a9a412b10012cb22841080401910 passed in Build #121525! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 44137

That installs a local version of the PR into your bun-44137 executable, so you can run:

bun-44137 --bun

[robobun]

@robobun wake up!!

[Jarred-Sumner]

@robobun wake up!!

[robobun]

I am here. The state of this PR at e1527b3:

• CI is green: Buildkite #121286 passed, and so did the lint checks.
• All 5 review threads are resolved. No change is pending from my side.
• The PR merges cleanly into main (a4f1429). It waits for a maintainer review.

Two things stay open after this PR, and neither blocks it:

• Bun.Transpiler: exports.replace prints a module that does not parse when the module cannot declare the export name #44138 asks what a replaced export does when the module cannot declare its name. The by-name form is ready on a branch, and I open it as a PR when the choice is made.
• A string value of exports.replace is a freed node after another parse in the process. Bun.Transpiler: allocate exports.replace string values in the instance arena #38541 has the fix, and it conflicts with main since August. I rebase it now. The fix for export let A with eliminate is stacked on it, so that PR follows.

[claude]

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

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/js_parser/visit/visit_stmt.rs — pre-existing: users of the ts loader with deadCodeElimination: false lose the export an exports.replace [name, value] entry should inject when the default export is a type. The type-only early return at src/js_parser/visit/visit_stmt.rs:526-528 drops the statement without the injection the sibling returns now get via inject_for_dead_value!. Fix: every early return of s_export_default that discards the value must still inject an Inject entry's export, e.g. call the same macro before the return at :528, which covers this remaining site alongside the three at :503, :688, :858.

  Why this was flagged

  Input: new Bun.Transpiler({ loader: "ts", deadCodeElimination: false, exports: { replace: { default: ["N", 1] } } }).transformSync("interface T {}\nexport default T"). At src/js_parser/visit/visit_stmt.rs:437-443 the entry is found, mark_for_replace is set, and is_control_flow_dead stays false because dead_code_elimination is off. visit_expr leaves T Unbound; :501 is false; the TS branch at :516-528 sees local_type_names["T"] == true and returns at :528 with only restore_dead!() and record_on_exit!(), so no export var N = 1 is emitted. With the default deadCodeElimination the same input takes the :501 return and the new inject_for_dead_value! at :503 emits it, so the output differs only by that option. The base branch has the same omission at :526-528; this PR added the injection to the three other early returns but not this one.

  Verification: pre-existing. Trigger: ts loader, deadCodeElimination: false, exports.replace: { default: [name, value] } (an Inject entry), and a default export whose value is a bare identifier naming a module-level type (interface T {}\nexport default T). Mechanism verified in /home/claude/bun/src/js_parser/visit/visit_stmt.rs: at :437-443 the default entry sets mark_for_replace = true and…

• 🟣 src/js_parser/visit/visit_stmt.rs — pre-existing: users who replace default in a module with a lowered top-level using still get the original default export, not the replacement. The using branch at src/js_parser/visit/visit_stmt.rs:587-623 builds var <name> = *expr and export { <name> as default } from the original value, and only afterwards does the mark_for_replace block at :625-646 assign the Replace value into *expr or inject the Inject export. Fix: apply the default entry (Replace, and Inject when DCE is off) before the using-lowering branch so both forms and the class/function arms emit the replaced value, while keeping the replace_reexport synthesized export default path working.

  Why this was flagged

  Input: new Bun.Transpiler({ loader: "ts", exports: { replace: { default: 1 } } }).transformSync('using r = null; export default 5') through Bun.Transpiler.transformSync or transform. Bun.Transpiler's default target is not bun, so src/bundler/transpiler.rs:1641 sets lower_using, and parse_entry.rs:1046 sets will_wrap_module_in_try_catch_for_using. In s_export_default the entry is Replace, so is_control_flow_dead stays false (visit_stmt.rs:439); the visit reaches :587, which pushes S::Local { var main_default = *expr } with *expr still 5 (:600) and an export { main_default as default } clause (:616). Only then does :633-635 set *expr = replace_expr, mutating the SExportDefault that :952 pushes; finalize at src/js_parser/p.rs:10080-10082 drops every SExportDefault and merges the export clause at :10083-10098, so the module exports 5. With replace: { default: ["N", 1] } and deadCodeElimination: false, the same order keeps default (as 5) next to the injected export var N = 1 (:637). Without using the replacement applies. The base branch has the same order, so…

  Verification: pre-existing. Trigger: a Bun.Transpiler with a non-bun target (default is Browser, /home/claude/bun/src/runtime/api/JSTranspiler.rs:58, so /home/claude/bun/src/bundler/transpiler.rs:1641 sets lower_using) transforms a module whose top level has a using declaration and an export default <expr>, with exports.replace: { default: <value> }. Mechanism verified in… | pre-existing. Trigger: a…

• 🟣 src/js_parser/visit/visit_stmt.rs — pre-existing: users who set exports.replace { default: <value> } on a module whose default export is a class declaration still get the original class printed, not the replacement. In the SClass branch of s_export_default, src/js_parser/visit/visit_stmt.rs:875 stores the replacement in data.value, but visit_stmt.rs:924 unconditionally overwrites data.value with the lowered class statement before visit_stmt.rs:925 pushes it. Fix: when a Replace entry applies, emit export default <value> and return without lowering the class (as the SFunction branch does at visit_stmt.rs:729), so every export default form honors a Replace entry; add a test for export default class A {} with replace: { default: 1 }.

  Why this was flagged

  Input: new Bun.Transpiler({ loader: "ts", exports: { replace: { default: 1 } } }).transformSync("export default class A {}") through Bun.Transpiler.transformSync or transform. At src/js_parser/visit/visit_stmt.rs:437-443 mark_for_replace becomes true and is_control_flow_dead stays false because the entry is Replace, so visit_class runs and the branch reaches visit_stmt.rs:863-875, which sets data.value = js_ast::StmtOrExpr::Expr(replace_expr). Execution then continues to visit_stmt.rs:910 p.lower_class(js_ast::StmtOrExpr::Stmt(s2_copy)) and visit_stmt.rs:924 data.value = js_ast::StmtOrExpr::Stmt(class_stmts[class_stmt_idx]), which discards the replacement, and visit_stmt.rs:925 pushes the statement. The printed module is export default class A {} and the value 1 never appears. The Expr branch at visit_stmt.rs:635 and the SFunction branch at visit_stmt.rs:729 do apply the replacement, so only class declarations lose it. The base branch has the same overwrite; this diff edits the same branch (inject_for_dead_value! at visit_stmt.rs:858) but only for the dead-value path, and…

  Verification: pre-existing. Trigger: any Bun.Transpiler user with exports.replace: { default: <value> } (a Replace entry, not [name, value]) transforming a module whose default export is a class declaration (export default class A {}), through transformSync/transform. Mechanism verified in /home/claude/bun/src/js_parser/visit/visit_stmt.rs: at :437-442 mark_for_replace = true and… | pre-existing.…

[robobun]

The three findings are correct. One is fixed, and two stay as they are on main.

1. The exit for a default export that is a type. Fixed in the commit after 36bb20b. That exit now injects the export of the entry, as the three other exits do. interface T {} and export default T with replace: { default: ["N", 1] } prints export var N = 1; with dead code elimination on and off. On main it prints no export for both. The test of the hoistable exports has both rows.

2. replace: { default: 1 } with a lowered top-level using. Not changed. The PR text lists it under "What stays wrong". #38287 moves the split of export default behind the block that applies the entry, which gives the right order.

3. replace: { default: 1 } on export default class A {}. Not changed. The PR text lists it under "What stays wrong" too. This PR is for the inputs that end the process, and for the outputs that those fixes reach. #44138 now has rows 2 and 3.

[claude]

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

[robobun]

The string value part of this PR (59da676 and e946a11, from #38541) has a debug-only guard that stops a debug build on valid input. Two commits on top of 603fb7f remove the guard and add test cases. They cherry-pick onto 603fb7f with no conflict:

• 53dc63b removes the guard, restores src/ast/expr.rs to main, and replaces the one test of the string values with one child process for each case.
• 7c4c5c4 adds a scan() case and prints with console.log.

Branch: robobun/d01710db/replace-string-values-no-guard (compare). It has no PR of its own.

The guard

let _guard = bun_ast::expr::Disabler::scope(); is active while to_js_string_view runs. JSValue::is_string() is also true for a String object, and for that value the call runs user JS.

const value = new String("ignored");
value.toString = () => {
  require("./small.cjs");
  return "bar";
};
new Bun.Transpiler({ exports: { replace: { foo: value } } });

A debug build with the guard stops while it parses the required file: panic: [bun_ast::expr::Expr] called while disabled (did you forget to call enable?), exit 134. main and release builds accept this input. I reproduced this on a debug build of main a4f1429 with the hunk of #38541, which is the hunk that this PR has.

Expr::allocate never appends to the store, so the guard has nothing to catch. The test cases protect against a return of Expr::init.

What changes

File	Change
src/runtime/api/JSTranspiler.rs	-3 lines: the guard and its comment. Against main the file is then +2/-1.
src/ast/expr.rs	The same as main again.
test/bundler/transpiler/transpiler.test.js	describe("string values") with 7 cases in place of the one test.

The 7 cases

Each case is one child process. Only the first load of a module parses it and resets the store, and a build with the defect ends the process in the first case.

Case	Release build of main a4f1429	With the change
transformSync() after a require()	Segmentation fault, exit 139	right text
transform() after a require()	Segmentation fault, exit 139	right text
16 transform() and 16 transformSync() calls at the same time	Segmentation fault, exit 139	right text, 32 of 32
scan() with inline: true	imports is ["a","b"], exit 0	["a"]
A getter of a later value loads a module while the constructor runs	Segmentation fault, exit 139	right text
toString() of a String object loads a module	wrong text, exit 0	right text
A macro creates the transpiler, the outer transformSync runs 3 times	right text 1 time, wrong text 2 times, exit 0	right text 3 times

The last case needs no module load. Expr::init puts the node in the arena of the outer parse when a parse is active, and that parse frees its arena when it returns. Expr::allocate ignores the active parse, so the same statement covers it.

Verification

On the branch (603fb7f plus the two commits), linux x64:

• bun bd test test/bundler/transpiler/transpiler.test.js: 253 pass, 1 skip, 21 todo, 0 fail. The 7 cases take 0.6 s to 1.0 s each on the debug build.
• Debug build with src/ of main 9f70da0: 7 of 7 cases fail at the stdout assertion. 6 children end with an AddressSanitizer report, and the scan() case gets ["a","b"].
• Release 1.4.3-canary.1+367d939d9 (USE_SYSTEM_BUN=1): 7 of 7 fail at the stdout assertion. Bun 1.4.2 fails the same inputs. Bun 1.3.13 prints the right text for all of them.

Numbers for the body of this PR

Release builds of main a4f1429 with and without the one statement (Expr::allocate, no guard):

What	main	with the change
mimalloc allocations for each new Bun.Transpiler, 8 string values	987.081	987.0775
mimalloc allocations for each transformSync(), 8 string values	51.0184	51.0183
mimalloc pages held by a live instance with a string value	1.663	2.663
committed bytes for each live instance, 8 string values	62254	70070
instructions of export_replacement_value, first string value of an instance	30705	32929
instructions of export_replacement_value, each later string value	338	338
size: text, data, bss	80661482, 110424, 1822992	the same
syscalls of construct plus transformSync()	249 calls, 41 rows	the same
writes to the shared node in 55135 trapped accesses, 47226 of them on work-pool threads		0

The allocation and page numbers come from heapStats() in bun:jsc over 2000 live instances and 10000 calls. The instruction, syscall and write numbers come from gdb. valgrind, perf, strace and bloaty are not on the machine that ran this. The instruction count of the whole constructor is not measured.

So the cost is at construction only: one more mimalloc page (about 8 KB committed) for each live instance that has a string value. An instance with no string value does not change.

Not in these commits

• Expr::init where the Zig code used Expr.allocate or a clone into the allocator of the manager: src/runtime/cli/pack_command.rs:3413, :3437, :3480 and src/install/PackageManager/PackageJSONEditor.rs:212, :222, :280. I found these by a read of the code. I did not reproduce a failure.
• The body of this PR says "4 child processes" and "10 new tests". With these commits the string values have 7 child processes, and the file has 16 new tests.

[robobun]

Merge order with #44193

This PR and #44193 merge with no textual conflict, but they disagree about one input: a pair on the default key.

• This PR makes replace: { default: ["N", 1] } print export var N = 1;. On main the input ends the process with panic: unreachable (3 of 3 runs, release and debug builds).
• bun-types: declare the values that exports.replace of Bun.Transpiler accepts #44193 declares the values of exports.replace in bun.d.ts. Its type rejects a pair on the default key, because of that crash. Its doc page and its type fixture say the same.

If both PRs merge as they are, the type rejects an input that the runtime accepts. The PR that merges second owes one edit:

• packages/bun-types/bun.d.ts: remove the sentence "The default key takes a value, and not a pair." and the & { default?: ... } part of the type.
• docs/runtime/transpiler.mdx: remove the same comment line and the same part of the type.
• test/integration/bun-types/fixture/transpiler.ts: remove the expectType<ExportsReplace["default"]> row and the three @ts-expect-error comments. The three rows stay as accepted input.

If #44193 merges first, the edit goes into this PR. If this PR merges first, the edit goes into #44193. Nothing changes before one of the two merges.

Checked on the branch of #44193 at c92c229:

• With only the type changed, tsc reports 4 diagnostics in transpiler.ts: TS2554 at line 13, and TS2578 (unused @ts-expect-error) at lines 117, 121 and 126.
• With the whole edit, test/integration/bun-types/bun-types.test.ts passes (22 pass, 0 fail).

The edit, as a diff against #44193 (3 files, +3 -15)

diff --git a/docs/runtime/transpiler.mdx b/docs/runtime/transpiler.mdx
index ad86e8d7e6..55ebffcdbc 100644
--- a/docs/runtime/transpiler.mdx
+++ b/docs/runtime/transpiler.mdx
@@ -214,7 +214,6 @@ interface TranspilerOptions {
       // For a pair or a map that you keep in a variable, write as const
       // Write each key as an identifier or default, and each name as an identifier
       // The constructor throws a TypeError for a key or a name such as a-b
-      // The default key takes a value, and not a pair
       replace?: Record<
         string,
         | string
@@ -223,9 +222,7 @@ interface TranspilerOptions {
         | null
         | undefined
         | readonly [name: string, value: string | number | boolean | null | undefined]
-      > & {
-        default?: string | number | boolean | null | undefined;
-      };
+      >;
   },
 
   // Whether to remove unused imports from transpiled file
diff --git a/packages/bun-types/bun.d.ts b/packages/bun-types/bun.d.ts
index 7b8b7d66e6..fee5c4a82b 100644
--- a/packages/bun-types/bun.d.ts
+++ b/packages/bun-types/bun.d.ts
@@ -3029,8 +3029,6 @@ declare module "bun" {
        * identifier. The constructor throws a `TypeError` for a key or a
        * `name` such as `a-b`.
        *
-       * The `default` key takes a value, and not a pair.
-       *
        * @example
        * ```ts
        * const transpiler = new Bun.Transpiler({
@@ -3059,9 +3057,7 @@ declare module "bun" {
         | null
         | undefined
         | readonly [name: string, value: string | number | boolean | null | undefined]
-      > & {
-        default?: string | number | boolean | null | undefined;
-      };
+      >;
     };
     treeShaking?: boolean;
     trimUnusedImports?: boolean;
diff --git a/test/integration/bun-types/fixture/transpiler.ts b/test/integration/bun-types/fixture/transpiler.ts
index 8d3c2704cd..ca3b9c5674 100644
--- a/test/integration/bun-types/fixture/transpiler.ts
+++ b/test/integration/bun-types/fixture/transpiler.ts
@@ -10,7 +10,6 @@ expectType<ExportsReplace[string]>().is<
   | undefined
   | readonly [name: string, value: string | number | boolean | null | undefined]
 >();
-expectType<ExportsReplace["default"]>().is<string | number | boolean | null | undefined>();
 
 // The example in the JSDoc of `exports.replace`.
 {
@@ -109,21 +108,17 @@ new Bun.Transpiler({
   new Bun.Transpiler({ exports: { replace: inferredMap } });
 }
 
-// The `default` key takes a value, and not a pair.
+// The `default` key takes a value or a pair.
 {
   new Bun.Transpiler({ exports: { replace: { default: 60 } } });
   new Bun.Transpiler({ exports: { replace: { default: "bar", revalidate: 60 } } });
-
-  // @ts-expect-error - the default key takes a value, and not a pair
   new Bun.Transpiler({ exports: { replace: { default: ["__N_SSG", true] } } });
 
   const entry = ["__N_SSG", true] as const;
-  // @ts-expect-error - the default key takes a value, and not a pair
   new Bun.Transpiler({ exports: { replace: { default: entry } } });
 
   const replace: ExportsReplace = {};
   replace.default = 60;
-  // @ts-expect-error - the default key takes a value, and not a pair
   replace.default = ["__N_SSG", true];
 }
 

[Jarred-Sumner]

This is Claude (an AI assistant) commenting on Jarred's behalf.

This fix went in with #46021, which is merged. Closing.