js_parser: store string enum members flat so template folding cannot append to them

[robobun]

Problem

• A string enum member whose initializer is a concatenation changes value once it is interpolated into a template literal anywhere in the file: with enum A { B = "a" + "b" } and `${A.B}c`, A.B becomes "abc" in the declaration, in every inlined use, and in cross-module uses of the enum (tsc: "ab").
• Interpolating the member a second time crashes or hangs the transpiler instead: a second template literal panics with called Option::unwrap()on aNone value (Crashed while visiting), and `${A.B}${A.B}` links the string onto itself and loops forever while resolving it.
• Reached by plain bun run file.ts (the bun target enables syntax minification), bun build --minify-syntax, Bun.Transpiler with minify.syntax, and with no flags at all when the template literal is itself an enum initializer. Reproduces on 1.4.0 and main; the Zig code this was ported from had the same shape.
• Cause: enum initializers are folded eagerly, so "a" + "b" is stored as a rope (an E::String root whose next chain holds the other pieces) and ts::Data::EnumString points at that node (src/js_parser/visit/visit_stmt.rs:2223). Each inlined use copies only the root (new_expr(&*str_ptr), src/js_parser/p.rs:1564), so the declaration and every use share the chain. Template::fold (src/ast/e.rs:2231, :2275) pushes a copy of the inlined value onto the text accumulated so far and then pushes the text that follows it; that second EString::push (src/ast/e.rs:2020) walks to the last node of the shared chain and sets its next, so the member itself grows. A later interpolation walks through that node again (its end was never set: the unwrap panic) or links the chain to itself (the hang).
• fold_string_addition guarded against this per consumer (has_inlined_enum_poison + clone_rope_nodes); Template::fold did not, and the guard could not see values that wrap_inlined_enum returns unwrapped (member names containing */).

Fix

• The fix is the str_.resolve_rope_if_needed(p.arena) line in s_enum (visit_stmt.rs): the member's string is flattened before it is stored as the enum value, so it is a single node. Inlined uses still copy that node, and both folds only ever mutate their own copy, so there is nothing shared left to append to. This covers every consumer (Template::fold, fold_string_addition, the printer's cross-module substitution) and the unwrapped case, at the one place where the sharing starts. EnumString is constructed only here.
• This is the value the declaration prints anyway (A.B = "ab"); it is now resolved once, in the parser arena that owns the rest of the AST, instead of once per inlined use at print time. esbuild also stores enum string values flat.
• resolve_rope_if_needed leaves rope_len (still the string's length, which is what len() and .length folding read) and the stale end pointer behind; end is only read from a node whose next is set, and a push onto a copy of a flat node assigns both. The printer's in-place resolve leaves nodes in this exact state today.
• fold_string_addition.rs: has_inlined_enum_poison and clone_rope_nodes can no longer do anything, so they are deleted and join_strings documents the ownership rule it relies on. The existing edgecase/EnumInliningRopeStringPoison test covers the scenarios those comments described and still passes.
• p.rs: wrap_inlined_enum gets a debug_assert! that the inlined string is flat, so a future construction site that stores a rope fails in debug builds instead of producing wrong output. e.rs: EString::push documents that both chains have to be exclusively owned.
• Tests: test/bundler/transpiler/transpiler.test.js ("template literal around an inlined string enum member": member first in the literal, member after a non-constant part, member derived from another rope member, template literal inside the enum body without minification, and a bun -e run that uses one member in four template literals) and test/bundler/bundler_edgecase.test.ts (edgecase/EnumInliningRopeStringTemplateLiteral: declaration, same-file and cross-module uses, tail path). All six fail with USE_SYSTEM_BUN=1 (wrong values; the bun -e one with the panic) and pass with bun bd test.
• Also run on the debug build: test/bundler/esbuild/ts.test.ts, esbuild/dce.test.ts, esbuild/default.test.ts, bundler_minify.test.ts, bundler_regressions.test.ts, bundler_edgecase.test.ts, transpiler/transpiler.test.js, transpiler/ts-enum-redecl-panic.test.ts.

Background

• Rope: folding "a" + "b" does not copy bytes. The right operand is linked onto the left as a chain of E::String nodes (next, end, rope_len), later folds append further nodes to the chain in place, and the printer (resolve_rope_if_needed) flattens the chain into one buffer when the value is needed. Appending in place is only correct while exactly one expression owns the chain; can_be_const_value already refuses to share ropes for the same reason.
• Enum inlining: while a file is visited, A.B is replaced by an E::InlinedEnum wrapping a copy of the member's stored string, so folds such as Template::fold and fold_string_addition see a plain string literal. For an enum imported from another module, the bundler's printer substitutes the stored node itself.
• Template::fold: with syntax minification (or inside enum initializers) `x${"y"}z` is folded into "xyz" by pushing each constant piece onto the template's text with EString::push.

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bundler/bundler_edgecase.test.ts

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Reviewing files that changed from the base of the PR and between d745f03 and 18d3ebb.

📒 Files selected for processing (6)

• src/ast/e.rs
• src/ast/fold_string_addition.rs
• src/js_parser/p.rs
• src/js_parser/visit/visit_stmt.rs
• test/bundler/bundler_edgecase.test.ts
• test/bundler/transpiler/transpiler.test.js

Walkthrough

Changes

String rope joining no longer performs enum poison handling or deep cloning. TypeScript enum strings are flattened before metadata storage, with an assertion that inlined strings are flat. Bundler and transpiler tests cover template-literal inlining and runtime uses.

String enum rope folding

Layer / File(s)	Summary
Simplify rope string joining src/ast/e.rs, src/ast/fold_string_addition.rs	join_strings now appends shallow-cloned ropes, preserves prefer_template, and replaces the previous enum poison handling.
Flatten enum string values src/js_parser/visit/visit_stmt.rs, src/js_parser/p.rs	TypeScript enum string values are resolved before storage. wrap_inlined_enum asserts that wrapped strings have no rope continuation.
Validate enum template handling test/bundler/bundler_edgecase.test.ts, test/bundler/transpiler/transpiler.test.js	Tests cover local and imported enums, template interpolation, repeated uses, serialization, runtime propagation, and inline transpilation.

Suggested reviewers: jarred-sumner, dylan-conway

Merge Risk: 🔵 Low · up to f8123

This change fixes string-enum template folding corruption and adds broad regression coverage. However, several new subprocess tests may fail spuriously on debug or ASAN builds because they require empty stderr, so the change is mergeable with bounded test-stability follow-up.

🚥 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 and concisely describes the main change: storing string enum members as flat strings to prevent template folding from mutating shared ropes.
Description check	✅ Passed	The description provides detailed problem, cause, fix, background, and verification information. It does not use the template headings exactly, but it covers both required topics, including extensive …

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[robobun]

^{Updated 4:25 AM PT - Sep 8th, 2026}

@robobun, your commit 18d3ebb is building: #112849

[robobun]

Status: fix pushed, waiting for CI.

Reproduced on bun 1.4.0 and on main with:

enum A { B = "a" + "b" }
const t = `${A.B}c`;
console.log(t, A.B); // bun: "abc abc", tsc/node: "abc ab"

Using the member in a second template literal panics (called Option::unwrap() on a None value, "Crashed while visiting"), and `${A.B}${A.B}` hangs the transpiler.

The new tests in test/bundler/transpiler/transpiler.test.js and test/bundler/bundler_edgecase.test.ts fail with USE_SYSTEM_BUN=1 (wrong values, and the panic for the runtime one) and pass with bun bd test. Details in the PR description.

[claude]

I reviewed this PR and didn't find any bugs. The fix is at the right layer and the reasoning holds up, but since it removes the defensive clone_rope_nodes guard in favor of an invariant enforced at the enum-visitor construction site, a human look at the ownership argument would be worthwhile.

What was reviewed:

• Verified resolve_rope_if_needed mutates the arena node in place via StoreRef::DerefMut, so both the declaration expression and the stored EnumString see the flattened value.
• Confirmed ts::Data::EnumString is only constructed at this site (visit_stmt.rs:2232/2235) and copied at parse_typescript.rs:29, so the flat invariant covers all sources.
• Traced join_strings after the guard removal: with flat inputs, shallow_clone() + store_append_string produce fully independent roots and estring_push never touches the shared original — the deleted EnumInliningRopeStringPoison scenarios still hold.
• Checked the UTF-16 path: resolve_rope_if_needed early-returns on !is_utf8(), but UTF-16 strings can't be ropes (push asserts is_utf8()).

Extended reasoning...

Overview

The PR fixes a transpiler bug where TypeScript string enum members initialized with concatenation (B = "a" + "b") were stored as rope strings, and template-literal folding would append to the shared rope chain in place — corrupting the member's value everywhere, panicking on a second use, or hanging on self-reference. The fix flattens the rope at the one construction site (visit_stmt.rs s_enum handler) before storing it as ts::Data::EnumString. With that invariant established, the per-consumer has_inlined_enum_poison / clone_rope_nodes defensive clone in fold_string_addition.rs becomes dead and is removed. A debug_assert! in wrap_inlined_enum guards the invariant, and doc comments on EString::push and join_strings record the ownership contract.

Security risks

None. This is transpiler-internal AST manipulation with no user-controlled resource access, auth, or external data handling. The pre-fix bug was a correctness/crash/hang issue, not a security exposure.

Level of scrutiny

High. This is core js_parser code that runs on every TypeScript file, and the change shifts the safety model from defensive per-consumer cloning to an invariant enforced at construction. Rope-string in-place mutation is exactly the kind of shared-mutable-arena-state pattern the review guidelines flag ("Never let a pointer or slice outlive the memory it points into", "join_strings mutates BOTH inputs"). The reasoning is subtle enough that I traced each consumer and each construction site rather than trusting the description.

Other factors

• The PR description is unusually thorough — it names the exact mechanism, cites source lines, explains why the old guard was insufficient (Template::fold bypassed it; */ in member names bypassed EInlinedEnum detection), and states which existing tests were re-run.
• Tests cover the full variant matrix per the review guidelines: member first in template, member after non-constant part, derived rope member, template inside enum body without minification, runtime execution with multiple uses (the crash/hang repros), and cross-module inlining. All verified to fail with USE_SYSTEM_BUN=1.
• I confirmed can_be_const_value (expr.rs:2357) already rejects ropes, which is the other half of the "shared strings are always flat" invariant join_strings now relies on.
• The stale rope_len/end fields left behind by resolve_rope_if_needed are addressed in the description and I verified: end is only dereferenced from a node whose next is set, and a fresh push onto a flat copy assigns both.
• Dead code deletion follows repo policy; the existing edgecase/EnumInliningRopeStringPoison test still covers the scenarios the deleted comments described.

Deferring because the removal of a working defensive guard in favor of a construction-site invariant in memory-safety-adjacent parser code is the kind of architectural shift a maintainer should sign off on, even though I found no fault in the reasoning.

[robobun]

For whoever picks up the ownership question the review above defers: the argument is that ts::Data::EnumString has a single construction site (s_enum, which now flattens the string first; parse_typescript.rs only copies the StoreRef), so the string behind every E::InlinedEnum is one node with no chain. Both folds copy the root before they mutate anything (join_strings via shallow_clone, Template::fold via Expr::init(shallow_clone())), and the only in-place mutation either of them does is to chain nodes, of which there are none. The debug_assert! in wrap_inlined_enum checks that at every inlining site, and edgecase/EnumInliningRopeStringPoison is the existing test for the cases the removed cloning used to handle; it passes on the debug build along with the bundler and transpiler suites listed in the description.

[claude]

I reviewed this PR and didn't find bugs in the fix itself — flattening the enum string at its single construction site in s_enum correctly eliminates the shared rope chain, and the join_strings simplification follows from that invariant. Given that this removes a defensive clone path (clone_rope_nodes) based on an aliasing/ownership argument in a StoreRef-mutation-heavy area, a human look would still be worthwhile. Also noting: CI reports failures on the latest commit, and comment-cop has three unaddressed inline flags on the new doc comments.

What was reviewed:

• Confirmed ts::Data::EnumString is only constructed in s_enum (parse_typescript.rs copies the StoreRef, doesn't build a new rope), so the flatten covers every producer.
• Traced resolve_rope_if_needed — leaves rope_len/end stale but next = None; EString::push on a copy of a flat node reassigns both, so no stale-end read.
• Checked that join_strings' shallow_clone of a flat left/right yields a fresh root with no chain, so the removed clone_rope_nodes is indeed dead for enum inputs.

Extended reasoning...

Overview

Fixes rope-string aliasing when TypeScript string enum members with concatenated initializers (B = "a" + "b") are inlined into template literals. The core fix is one line in visit_stmt.rs (str_.resolve_rope_if_needed(p.arena) before storing into ts::Data::EnumString); fold_string_addition.rs deletes the now-dead has_inlined_enum_poison/clone_rope_nodes guard, p.rs adds a debug_assert! in wrap_inlined_enum, and e.rs documents EString::push's ownership contract. Tests added in transpiler.test.js and bundler_edgecase.test.ts.

Security risks

None. This is transpiler output correctness — no auth, crypto, permissions, or untrusted-input parsing surface changes.

Level of scrutiny

Medium-high. The mechanism is sound and the fix is at the right layer (the one construction site of EnumString, per REVIEW.md's "fix at the layer that owns the invariant"). However, it removes a defensive deep-clone in favor of an ownership invariant enforced by a debug_assert!, and the correctness argument depends on StoreRef::DerefMut in-place mutation semantics and the claim that shallow_clone of a flat node fully decouples it. I verified these hold, but rope aliasing via arena StoreRef is exactly the memory-safety-adjacent territory REVIEW.md flags for extra scrutiny — a human should confirm the invariant argument before the guard is deleted.

Other factors

• CI failures: robobun reports Build #97926 has failures on commit fedad63. Unclear whether they're related to this change or pre-existing flakes; should be resolved before merge.
• Outstanding bot comments: comment-cop flagged three multi-line comments (e.rs:2023, fold_string_addition.rs:9, visit_stmt.rs:2228) as "paragraph-long comment to justify a workaround". These are documentation of an invariant rather than a workaround, but the flags haven't been addressed or dismissed.
• Test coverage: The new tests are solid (both Template::fold paths, cross-module, no-minify enum-body case, runtime crash/hang repro). The existing EnumInliningRopeStringPoison test still covers the scenarios the removed clone_rope_nodes handled.
• No prior claude[bot] review on this PR.

[robobun]

One more shape of this bug, so that a search for the crash text finds this PR:

const enum A { B = "x" + "y" }
console.log(`${A.B}-${A.B}`);
console.log(A.B);

bun build --minify-syntax a.ts on bun 1.4.0 and on current main grows to several GB, then fails with panic: BabyVec capacity overflow or Bun has run out of memory (Crashed while visiting a.ts). The first push of the tail - writes into the shared enum rope. The second ${A.B} then links that chain back onto itself, and resolve_rope_if_needed never ends. The expected output is console.log("xy-xy");console.log("xy");.

The change on this branch covers it: the member is stored flat, so each push only writes into nodes that the template literal owns.

[robobun]

A differential fuzz run over enum and namespace lowering hit this bug again in three shapes. All three have the root cause this PR fixes (the stored member string is a rope whose tail nodes every inlined use shares), and all three pass on this branch:

• enum E { A = "v" + "w" }; console.log(`${E.A}${E.A}`): bun run and bun build --minify-syntax allocate without bound (OOM, or panic: BabyVec capacity overflow). Covered by the existing runtime test here (`${Routes.Base}${Routes.Base}`).
• `${S.A}x` and `${S.A}y` in two statements: panic: called Option::unwrap() on a None value, "Crashed while visiting". Covered by the existing runtime test here.
• const x = `${T.C}c` then T.C, T.C.length, T.C === "ab", { [T.C]: 1 }: bun prints ["abc","abc",2,false,{"abc":1}], node prints ["abc","ab",2,true,{"ab":1}].

8d2f3c5 adds tests for the third shape and for two more paths that hand out the same stored string: a member whose name contains */ (inlined without the /* name */ wrapper, so the removed has_inlined_enum_poison check never saw it, and E["a*/b"] + "x" twice crashes on 1.4.2), and const k = E.A propagation through const_values. It also pins the Bun.Transpiler inline: true output, where 1.4.2 prints T["C"] = "abc" for the enum's own assignment. Tests only, no source change. All of them fail on bun 1.4.2 and pass on this branch merged with current main (transpiler.test.js 216 pass, edgecase/EnumInlining* pass).

The earlier CI results on this PR are not about the change: the GitHub Actions jobs failed in bun install (failed to download bun-tracestrings: HTTP 5xx) and Buildkite build 98310 was canceled. This push reruns them.

[robobun]

22ceafc and f81239e only shorten the three code comments the comment lint flagged (e.rs push, join_strings, the flatten in s_enum). Each is one line now. No code change. The lint threads are answered and resolved. The CodeRabbit rate limit notices above need no action.

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📒 Files selected for processing (6)

• src/ast/e.rs
• src/ast/fold_string_addition.rs
• src/js_parser/p.rs
• src/js_parser/visit/visit_stmt.rs
• test/bundler/bundler_edgecase.test.ts
• test/bundler/transpiler/transpiler.test.js

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[robobun]

Rebased onto current main (b803ab2, no conflicts, same diff). The previous CI run compared a branch 756 commits behind main against the latest canary, which is where the binary size delta came from. On the rebased build, transpiler.test.js (216 pass), the EnumInlining bundler cases, and esbuild/ts.test.ts pass. The cert expiry failure in test-http-should-accept-custom-certs-when-provided.ts is not from this change and is reported separately. The stderr review thread above is answered and closed.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Closing in favor of #41973. It carries the two commits from this PR unchanged and adds a regression test for a third symptom of the same sharing: folding S.A + "k" + S.A + "k" + ... over an inlined string member was quadratic in memory (8 192 terms, 49 KB of source, 1.4 GB), because join_strings re-cloned the whole accumulated rope every time a member appeared on the right. Storing members flat removes that clone, so the same change fixes it. #41973 also bumps the runtime transpiler cache version, since the output of affected files changes.