js_parser: replace skipped expressions with E::Missing and skip argument checks after a stack overflow

[robobun]

Problem

• [a.b.b..., NAME = 1] = y with 100000 levels aborts: panic: index out of bounds: the len is 5 but the index is 2000. Maximum call stack size exceeded is expected.
• On a stack overflow, visit_expr_in_out (src/js_parser/visit/visit_expr.rs:46) returns before the visit. Its caller continues, and uses the ref of an unvisited identifier as a symbol index.
• It also judges skipped arguments: require(a.b.b...) with allowUnresolved: [] logs a second error, macro m(false ? a.b.b... : 1) runs.

Fix

• The bail replaces the skipped expression with E::Missing, so no reader reaches the skipped subtree. No reader changes.
• A substitution revisit writes nothing: its node can be a shared define value. The TypeScript class lowering accepts a skipped method.
• After the overflow no macro runs, and require(), feature() and import.meta.hot.accept() check no argument.
• Verified: 15 new tests in test/bundler/transpiler/transpiler.test.js, 13 fail on main. Self-reviewed: 16 concerns raised, 15 addressed.

Background

• The visit pass gives each identifier a symbol. Its stack check (Error instead of crashing on deeply nested expressions in the transpiler #31242) logs the error once, skips each later expression, and fails the parse.
• E::Missing is the node of an array hole: readers of an expression accept it. A substitution revisit is the minifier's second visit, after it inlines a variable.
• Considered a guard in each reader (each new reader needs one) and a typed error through the visitor (3 instructions per expression).

Downsides

• bun <file> still aborts for a hoistable statement after the overflow (panic: unreachable). js_parser: fix four crashes of exports.eliminate and exports.replace #44137 has that fix.
• None found on valid input: same frame, instruction counts and depth. .text grows 1024 B.

Notes

Found by reading code. There is no issue and no user report. The first idea was to delete keep_expr_symbol_name and maybe_keep_expr_symbol_name with their ten call sites, because three of the readers are arguments of that no-op. That is not done here: #40503 gives the function a body and uses the call sites, and the other readers stay.

Repro (Bun 1.4.3-canary 367d939 and main 71d0d43)

const chain = "a" + ".b".repeat(100000);
const name = "L".repeat(2000);
new Bun.Transpiler({ loader: "js" }).transformSync(`[${chain}, ${name} = 1] = y;`);
// panic: index out of bounds: the len is 5 but the index is 2000

Input, with a member chain of 100000 levels	main 71d0d43	this PR
[chain, NAME = 1] = y, ({ a: chain, NAME = 1 } = y), for ([chain, NAME = 1] of y) {}	panic: index out of bounds	the error
x = typeof (false ? chain : NAME)	the same panic	the error
useReducer(chain, [NAME]) with bun build --react-fast-refresh	the same panic	the error
NAME().b.b... and (NAME = 1).b.b..., at the first depths that overflow	the same panic	the error
require(chain), require.resolve(chain) with allowUnresolved: []	the error, and This require() expression will not be bundled because the argument is not a string literal	the error
feature(chain) from bun:bundle	the error, and feature() argument must be a string literal	the error
a macro call m(chain)	the error, and "Cannot convert argument type to JS" error in macro	the error
a macro call m(false ? chain : 1)	the error, and the macro runs with [1]	the error, and the macro does not run
a macro with a template, m`${chain}`	the error, and template literal macro invocations are not supported after a load of the macro module	the error
import.meta.hot.accept([chain, "./dep.ts"], fn) with format: "internal_bake_dev"	the error, and Dependencies to `import.meta.hot.accept` must be statically analyzable ...	the error
require(chain) or require.resolve(y ? chain : "./d") with target: "node" and logLevel: "debug"	Parse error: the error and a debug note	the error

The name does not have to be long. It must only be at least as long as the symbol table: require(x).b.b... at the first depth that overflows ends in the len is 5 but the index is 7. With a shorter name the read is in bounds and returns another symbol.

transformSync, transform, scan, bun build, bun build --no-bundle and bun <file> all abort on main.

The readers (each reads p.symbols[ref.inner_index()] for an identifier that the visit skipped)

• e_array and e_object default values, visit_expr.rs:1629, :1804
• the assignment arm of visit_right_and_finish, visit_binary.rs:719
• the direct eval check of e_call, visit_expr.rs:1925
• typeof, visit_expr.rs:1180
• the import check of e_index (only with bundle), visit_expr.rs:1123
• the hook name check of e_call, visit_expr.rs:2395, and the hash of the hook arguments, src/ast/expr.rs:2200
• promise_all_import_items, p.rs:1390
• KnownGlobal::minify_global_constructor, src/ast/known_global.rs:94

Each of them tests the tag of the expression after the visit, so the placeholder makes it fall through. #40503 and #44180 add more reads of the same kind, and they need no change either. #32417 changes five of these reads to load_name_from_ref, which also accepts an unvisited ref. The two changes do not depend on each other.

What reads the placeholder

• lower_class (p.rs:7524) matched the value of a method against E::Function, with unreachable!() for the rest. It runs for each class statement of a TypeScript file. class A { a = chain; m() {} } gives the error on main, so the store alone would have added an abort here. The new arm accepts E::Missing only when the overflow flag is set.
• emit_decorator_metadata_for_prop has three .e_function().expect(..). It now returns for a skipped method, with the same flag test.
• insert_after_super (lower/lower_decorators.rs:113) has the same unreachable!() for the constructor. It is not reachable: the visit also skips the key constructor, so the member is not a constructor any more. The class test has this input (class A { @dec a = chain; constructor() {} }).
• Each skipped expression gets the placeholder, a literal too. A form that kept literals in place was tried: it made the arm of insert_after_super reachable, so it was dropped.
• _parse returns after the visit pass when the log has an error (parse/parse_entry.rs:1386). The printer and the linker never see the placeholder.

The checks that do not run after the overflow

Each one tests reported_stack_overflow after the condition that already makes it rare, so a call that is none of these runs no new instruction. push_scope_for_visit_pass (p.rs:3890) already tests the flag in the same way.

Site	The flag test comes after	On main, after the overflow
check_dynamic_specifier (require(), require.resolve(), import())	options.bundle and allow_unresolved	... is not a string literal
the notes of require() (e_call) and require.resolve() (transpose_require_resolve)	warn_about_unbundled_modules	a debug note, and with it Parse error in place of the error
maybe_replace_bundler_feature_call	the test for the feature ref	feature() argument must be a string literal
the macro call in e_call	is_macro_ref	the macro runs, or "Cannot convert argument type to JS" error in macro
the macro template in e_template	the lookup in macro_.refs	a load of the macro module, and template literal macro invocations are not supported
handle_import_meta_hot_accept_call	the argument count	Dependencies to `import.meta.hot.accept` ...

Some messages still come after the overflow, as on main. Each is about a part that the visit did visit: "a" cannot be bound multiple times in the same parameter list, the private name messages of e_index, and Cannot assign to property on import.

The note of import() (p.rs:1594) needs no flag test. e_import calls the transposer only when expr_can_be_removed_if_unused is true, and that function returns false when the flag is set (p.rs:6382).

Measurements (release builds, linux x64, main 71d0d43 against the same commit with this change)

What	main	this PR
visit_expr_in_out, frame	sub $0x248, %rsp (js), sub $0x258, %rsp (ts)	the same
visit_expr_in_out, size	17,694 B (js), 18,259 B (ts)	17,778 B, 18,353 B
.text	65,782,357 B	65,783,381 B (+1,024)
stripped binary	89,269,832 B	89,269,832 B
Largest depth of - x D + 1 that transpiles	main thread 11946/11948/11951, transform() 5831/5831/5831	the same
Largest depth of a + .b x D	main thread 12840/12842/12846, transform() 6266/6266/6266	12840/12843/12846, 6266/6266/6266
Instructions in visit_expr_in_out for one transformSync of prettier plugins/acorn.js (155,719 B)	3,027,759	3,027,759
The same for 20000 lines of f(); x = y;	15,620,000	15,620,000
The same for 1000 TypeScript classes	405,000	405,000
The same for 1000 TypeScript classes with legacy decorators and emitDecoratorMetadata	702,000	702,000
Instructions in lower_class for the two class inputs	279,000 and 1,153,000	278,000 and 1,145,000
Instructions in emit_decorator_metadata_for_prop, 2000 calls	547,000	534,000

• The sizes and frames are from llvm-nm -S, llvm-objdump and size -A. The depths are min/median/max of 20 processes for each build, interleaved.
• 12 of 88,146 functions have another size: visit_expr_in_out +84 and +94 B, lower_class +349 and +68 B, emit_decorator_metadata_for_prop -83 and -80 B, maybe_transpose_if_require_resolve +18 and +22 B, e_template +2 B twice, and one more copy each of ts::Metadata::clone (+392 B) and alloc_slice_fill_with (+16 B).
• The bail branch has 3 instructions more for each skipped expression: a test of is_revisit_for_substitution and a store of 1 byte.
• Each check has 1 compare and 1 branch more, only for its own form: a macro call, a macro template, a live feature() call, an import.meta.hot.accept() call with arguments, a require() or require.resolve() of something that is not a string with warn_about_unbundled_modules, and a specifier that is not a string with a list in allowUnresolved.

The instruction counts are from qemu-x86_64 -one-insn-per-tb -d exec,nochain -dfilter <function range>, as (2 calls - 1 call). perf, valgrind and bloaty are not on the machine that ran this.

Two other forms were built and dropped. A #[cold] helper for the branch changed what LLVM inlines into visit_expr_in_out, and its frame grew from 0x248 to 0x288. An early return in lower_class cost 6 instructions for each TypeScript class.

Probes

• The final form against main 71d0d43, release builds: 26,326 transformSync results (588 inputs with the chain in an earlier child, four loaders, 31 sets of options). 558 are the panic on main and the error here. No result crashes only here. 36 abort in both (panic: unreachable, the hoistable statement below). 112 have one message less: a note above, or a second error for a duplicate __proto__ key that the visit skipped.
• The final form, the same builds: 8,429 cases through Bun.Transpiler, Bun.build, bun build and bun <file>, compared message by message. None is worse. 86 crash only on main. 12 crash in both (reactCompiler).
• An earlier form (fd3a632), the same main: 10,524 sweeps around the first depth that overflows for 764 inputs, 17,840 builds, 1,101 files through bun <file>, 41,500 random programs, and 108,480 variants of Node.js test files with one string or identifier replaced by the chain. No crash that main does not have, and no message that main does not have, except two debug notes of require.resolve(). The notes have their flag test now.
• The same form: one Bun.Transpiler with a define object of 150 levels, 119,175 transforms around the first depth that overflows. The printed define value did not change.
• The first form (c37b8e2), a debug build against Bun 1.4.3-canary 367d939: 1,056 build, run and transpile cells and 48 dev server pages. 39 crashes became the error. One input was worse, in 2 cells and 1 page: a second import.meta.hot.accept error for a string item. The flag test removes it.

The tests

• Fixed input through transformSync, transform and scan, with the js and ts loaders. The member chain is twice as long as the first power of two that overflows in the test process.
• A search for the first depth that overflows, for the two readers that only see a skipped identifier there (e_call, the assignment arm). The right side of an assignment is deeper than its left side, so that test also parses the next 4 depths. On a release build of main both abort in 30 of 30 runs.
• A define value with 200 levels, and a statement that the minifier visits again. This test passes on main, and fails when the store has no condition.
• A TypeScript class with a method after the overflow. This test passes on main, and fails without the change in lower_class or in emit_decorator_metadata_for_prop.
• One test row for each check in the table above, through Bun.build or Bun.Transpiler. These rows are concurrent. The tests with a child process run one after the other.
• bun build --react-fast-refresh and bun <file>, with exactly one error: line.
• On the debug build the slowest test takes about 1.4 s (default timeout 5 s). CI passes a timeout of 90 s.
• The whole file passes on the debug build (252 pass, 0 fail). So do bundler_allow_unresolved, bundler_feature_flag, bundler_decorator_metadata, transpiler/decorator-metadata, transpiler/decorators, transpiler/es-decorators, transpiler/macro-test, transpiler/jsx-deep-nesting-stack-overflow and bake/dev/hot.

Self-review

Two passes, 16 concerns, 15 addressed.

• In the source: the two class guards accepted a missing method for any cause, and now test the flag. The comment of the bail said a thing that is not true of a define value. The macro template path had no flag test. require.resolve() logged a debug note for each skipped branch. import.meta.hot.accept() logged a second error for a skipped string. Three comments had other wording, and one line that the change does not need had a new period. A skipped literal that stays in place aborts in insert_after_super, so that idea is not in the change.
• In the tests: the assignment test passed without the change in 9 of 48 runs, and now fails on main in 30 of 30. The define test had a margin of 8 levels where x64 has 20, and now needs 3. firstOverflow did not probe its lower bound. Two tests did not count the errors. Three spawn helpers are one. The slowest test on the debug build took 3.9 s. The tests with a child process now run one after the other, and the slowest takes about 1.4 s.
• In this text: import() needs no flag test of its own, because e_import does not transpose after the overflow. Two tests pass on main and guard one clause each.
• Kept: the probe for the length of the chain runs when the test file loads. It takes under 0.1 s on a release build and 0.2 to 0.6 s on a debug build.

Left for later

• exports.eliminate on export let foo (panic: called Option::unwrap() on a None value). The cause is the inverted arm in visit_decls, and js_parser: fix four crashes of exports.eliminate and exports.replace #44137 has the fix.
• parts.pop() after a skipped hoistable statement, in the runtime loader (parse/parse_entry.rs:1338, :1350): export default function f() { return a.b.b...; } and then class B {} ends in panic: unreachable. js_parser: fix four crashes of exports.eliminate and exports.replace #44137 makes _parse move only a part that append_part appended. A todo test here has this input. Merge order does not matter.
• rewrite_private_accesses_in_expr (lower/lower_decorators.rs) recurses with no stack check. class A { @dec #a = a + a + ...; @dec #m() {} } with 20000 terms ends in Segmentation fault on main and here, with no overflow in the visit pass. No pull request has this yet.
• The lowering of the React Compiler has the same kind of recursion (react-compiler: check the stack in every pass that recurses as deep as the source nests #43230).
• visit_binding has the same bail and writes no placeholder. A deep binding pattern overflows the parser first, so the visit does not get there.

A question for a maintainer

REVIEW.md says "typed errors, never magic sentinels". Here the error still goes through the log, and _parse stops after the visit pass. E::Missing only makes the skipped subtree unreachable. The other design is a Result through visit_expr_in_out and its visitors: about 345 edits, and about 3 instructions and a branch more for each visited expression. With it, the flag tests above are not needed. Say so if you want that one.

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[human-review] gate passed · iteration 0 · 3 files touched

fails on main (without fix)

ASAN without fix: 13 failed, 23 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 v1.4.4 (367d939d9)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [10.62ms]
(pass) Bun.Transpiler > normalizes \r\n [13.99ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [11.60ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [17.90ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [6.90ms]
(pass) Bun.Transpiler > property access inlining > works [6.70ms]
(pass) Bun.Transpiler > property access inlining > works nested [6.27ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [126.70ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [54.63ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [767.39ms]
(pass) Bun.Transpiler > property access inlining > bails out on opti
... (truncated)

release without fix: 27 failed, 23 skipped
bun test v1.4.3-canary.1 (367d939d9)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [0.19ms]
(pass) Bun.Transpiler > normalizes \r\n [0.35ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [0.39ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [0.28ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [0.05ms]
(pass) Bun.Transpiler > property access inlining > works [0.11ms]
(pass) Bun.Transpiler > property access inlining > works nested [0.06ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [1.65ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [0.38ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [20.03ms]
(pass) Bun.Transpiler > property access inlining > bails out on optional-chain index into enum [1.28ms]
(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: 23 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 v1.4.4 (367d939d9)

test/bundler/transpiler/transpiler.test.js:
(pass) Bun.Transpiler > handles errors when parsing macros [11.11ms]
(pass) Bun.Transpiler > normalizes \r\n [12.39ms]
1
(pass) Bun.Transpiler > doesn't hang indefinitely #2746 [9.58ms]
(pass) Bun.Transpiler > property access inlining > bails out with spread [16.49ms]
(pass) Bun.Transpiler > property access inlining > bails out with multiple items [6.53ms]
(pass) Bun.Transpiler > property access inlining > works [7.32ms]
(pass) Bun.Transpiler > property access inlining > works nested [6.50ms]
(pass) Bun.Transpiler > property access inlining > bails out when the array item is an optional chain [142.50ms]
(pass) Bun.Transpiler > property access inlining > bails out or strips `this` when the index is a call/assignment target [64.44ms]
(pass) Bun.Transpiler > property access inlining > preserves runtime semantics when inlining from a literal index [857.17ms]
(pass) Bun.Transpiler > property access inlining > bails out on optio
... (truncated)

release with fix: 23 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     8b8ae59323
  features     lto, baseline

23 deps, 136 codegen, 1176 objects in 2706ms

ninja: Entering directory `/workspace/bun/build/release'
[1/4] fetch lolhtml
[lolhtml] up to date
[2/4] fetch rust-argon2
[rust-argon2] up to date
[2/4] cargo plan → /workspace/bun/build/release/rust-target/plan.json
247 units: 175 lib, 16 proc-macro (host), 19 custom-build (host), 15 run custom-build, 17 lib (host), 4 run custom-build (host), 1 rlib
[3/4] reconfigure
[1/1502] mkdir stamps
[2/1502] mkdir codegen
[3/1502] gen bake.{client,server,error}.js
-> bake.client.js, bake.server.js, bake.error.js
[4/1502] gen string-map defines_table
[5/1502] gen ErrorCode+*.h
[6/1502] install /workspace/bun
bun install v1.4.3-canary.1 (367d939d9)

Checked 26 installs across 65 packages (no changes) [1.68s]
[7/1502] install /workspace/bun/packages/bun-error
bun install v1.4.3-canary.1 (367d939d9)

Checked 1 install across 2 packages (no changes) [120.00ms]
[8/1502] rustc unicode_xid 
[9/1502] rustc unicode_ide
... (truncated)

diff hotspot

src/js_parser/p.rs                         |  17 ++-
 src/js_parser/visit/visit_expr.rs          |  21 ++-
 test/bundler/transpiler/transpiler.test.js | 236 +++++++++++++++++++++++++++++
 3 files changed, 268 insertions(+), 6 deletions(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                                        reads  edits  tests
src/js_parser/p.rs                              7      4     80
src/js_parser/visit/visit_expr.rs              21     10     82
test/bundler/transpiler/transpiler.test.js      4      3     79

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Walkthrough

Expression visiting and parser processing now account for reported stack overflow when later work depends on incomplete expressions. Regression tests cover transpiler, build, and runtime overflow cases.

Changes

Stack overflow recovery

Layer / File(s)	Summary
Expression visit recovery src/js_parser/visit/visit_expr.rs	Expression visits replace unsafe or previously overflowed expressions with missing expressions. Macro dispatch, a warning, and feature validation are skipped after overflow.
Parser follow-on processing src/js_parser/p.rs	Parser paths skip checks and processing that depend on incomplete expressions, including class methods, decorator metadata, and import-meta hot-accept arguments.
Transpiler overflow regression tests test/bundler/transpiler/transpiler.test.js	Tests cover overflow diagnostics, depth boundaries, continued transpiler use, minifier revisits, React Fast Refresh and other builds, runtime cases, and non-crashing exits.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 8b8ae

Overflow recovery avoids checking incomplete dynamic-specifier arguments. The narrow panic after a skipped hoistable statement is unchanged from the PR base; no new material production risk is established.

Pre-merge checks | 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main parser change: replacing skipped expressions with E::Missing and skipping argument checks after a stack overflow.
Description check		The description explains the problem, implementation, scope, limitations, and verification results. It does not use the exact template headings, but it provides the required change summary and verific…

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Actionable comments posted: 1

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Inline comments:
Review comments at @test/bundler/transpiler/transpiler.test.js:
- Line 5439: Bound the doubling search in the firstOverflow test so it cannot
loop indefinitely when transformSync never returns the expected overflow. Set a
maximum depth and throw a clear diagnostic when the search exceeds it, while
preserving the existing outcome check for depths within the bound.

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• src/js_parser/p.rs
• src/js_parser/visit/visit_expr.rs
• test/bundler/transpiler/transpiler.test.js

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[claude]

I reviewed this PR and didn't find any bugs. Because it changes what the visit pass leaves in the AST on the overflow path of the core parser, and every post-visit reader now has to tolerate E::Missing, a human look at that design choice would still be worthwhile.

What was reviewed:

• The bail-path store in visit_expr_in_out and its is_revisit_for_substitution guard: a minifier revisit skips the store so a shared define value is not mutated; the revisit walks already-visited nodes, so the old behavior is preserved there.
• The unreachable!() in insert_after_super (lower_decorators.rs): the constructor key and its value are visited from the same frame in visit_class, and a skipped key becomes E::Missing so it no longer matches, which keeps that arm unreachable.
• The two p.rs sites (lower_class arm and the early return in emit_decorator_metadata_for_prop): both only fire after the overflow error is already logged, so the parse fails regardless.
• The new tests' cost on debug/ASAN lanes: the binary search runs at most ~25 transforms per shape on inputs well under the debug overflow depth, and the subprocess tests drain pipes with Promise.all.

Extended reasoning...

The change adds about 13 lines to src/js_parser (a placeholder store on the stack-overflow bail in visit_expr.rs, an EMissing arm in lower_class, and an early return in emit_decorator_metadata_for_prop) plus ~180 lines of subprocess tests in test/bundler/transpiler/transpiler.test.js. It touches no security-sensitive surface; the attack surface is a parser DoS/abort on adversarial input, which this PR reduces. The source diff is small and the bug hunt ran dry, but the fix is a cross-cutting invariant change on the core parser error path (any reader after a skipped visit must now accept E::Missing), chosen over per-reader guards, and the PR itself notes a remaining abort for a hoistable statement after the overflow that is handled elsewhere. That design trade-off and the parser ownership are why a human look is still worthwhile rather than an unattended approve.

[robobun]

Status: open at 8b8ae59. Waits for CI and review.

Reproduced on main (71d0d43) and on Bun 1.4.3-canary (367d939):

const chain = "a" + ".b".repeat(100000);
const name = "L".repeat(2000);
new Bun.Transpiler({ loader: "js" }).transformSync(`[${chain}, ${name} = 1] = y;`);
// panic: index out of bounds: the len is 5 but the index is 2000

Verification with test/bundler/transpiler/transpiler.test.js -t "a stack overflow in the visit pass":

release build of main 71d0d43997:   2 pass, 13 fail   (the child processes abort with the panic above, or the build logs a second error)
debug build of this PR (bun bd):    15 pass, 0 fail   (full file: 252 pass, 0 fail)

[robobun]

^{Updated 12:09 PM PT - Oct 10th, 2026}

✅ @robobun, your commit 8b8ae59323833edbf3237750e153439ba0c3d37a passed in Build #124346! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 44467

That installs a local version of the PR into your bun-44467 executable, so you can run:

bun-44467 --bun

[claude]

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

Beyond the inline finding, I also checked the two sibling unreachable!() readers of a class method value that the diff does not touch: insert_after_super (src/js_parser/lower/lower_decorators.rs:113) and the constructor capture in visit_class (src/js_parser/visit/mod.rs:1151-1195). The member key is visited before the value (mod.rs:1117), so a skipped constructor also loses its "constructor" key and neither path sees an E::Missing method; the pre-visit StoreRef<E::Function> still points at the arena node, which this change does not free.

Extended reasoning...

The change makes the visit pass overwrite a skipped expression with E::Missing and gates seven post-visit argument consumers on reported_stack_overflow; it touches the parser only, no security-sensitive surface. One pre-existing nit about the debug log level survived verification and is posted inline; I separately ruled out the two untouched unreachable arms that read a class method value.

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

• 🟣 src/js_parser/p.rs — nit, pre-existing: with logLevel "debug" (target not bun), a stack overflow inside an import() or require() argument still throws an AggregateError "Parse error" instead of the single "Maximum call stack size exceeded" the PR aims for. The gate added at p.rs:1629 for require.resolve has two ungated twins: the import() debug note at p.rs:1594 and the require() note at visit_expr.rs:2210 still log after the overflow. Fix: suppress every unbundled-argument debug note once reported_stack_overflow is set, which covers the 3 sites. Same pattern at 3 sites (p.rs:1629, p.rs:1594, visit_expr.rs:2210).

  Why this was flagged

  Input: new Bun.Transpiler({ loader: "js", target: "node", logLevel: "debug" }).transformSync('x = import(' + chain + ' ? "./d" : "./e");') or transformSync('x = require(' + chain + ');'), with chain deep enough to overflow the visit pass (the PR's own test shape at transpiler.test.js:5506…

  Verification: pre-existing (nit): the base fails the same way; the PR gates only the require.resolve sibling. With target not bun and logLevel: "debug", parsing x = require(<chain deep enough to overflow>);, visit_expr.rs:2209-2217 logs the debug note with no !p.reported_stack_overflow.get() test, so the result is an AggregateError "Parse error" instead of the single "Maximum call stack size exceeded".

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

About the review note on the debug notes of require() and import(): the require() half holds, and it is the same on main. The import() half does not hold.

Release builds of main 71d0d43 and of e3e7c60, with new Bun.Transpiler({ loader: "js", target: "node", logLevel: "debug" }):

Input	main	e3e7c60
x = require(chain);	Parse error: the overflow error and the note	the same
x = import(chain);, x = import(chain ? "./d" : "./e");, x = import(y ? chain : "./d");	the overflow error	the overflow error

• 8b8ae59 adds the flag test to the note at visit_expr.rs:2210. require(chain) now gives the overflow error alone. The test at the debug log level has this input, and it fails without the flag test.
• The note at p.rs:1594 gets no flag test, because it cannot run after the overflow. e_import calls the transposer only when expr_can_be_removed_if_unused(&e_.options) is true (visit_expr.rs:1856), and that function returns false when the flag is set (p.rs:6382).

The same commit makes the tests with a child process run one after the other. On the debug build, several children that fill their stacks at the same time made single tests take up to 3.7 s. The slowest one now takes about 1.4 s.