Keep stack frames and positions when the runtime transpiler minifies syntax

[robobun]

Fixes #24789

Problem

• bun run enables minify_syntax itself (src/bundler/options.rs:1923). Two rewrites put a call in return position: new Error() to Error() (src/js_parser/visit/visit_expr.rs:2490) and const r = g(); return r; to return g() (src/js_parser/p.rs:2771). JSC makes that a proper tail call and drops the frame, so function makeError() { return new Error("x") } has no makeError frame. Node has it. Stack traces missing when creating errors #24789 is the let e = err1(); return e form.
• An inlined const literal kept the declaration's position (src/js_parser/p.rs:2178), so an error at its use site reported the declaration line.

Fix

• Gate both rewrites on p.options.bundle, like the function and class name drops next to them. bun build --minify keeps its output. The runtime, bun build --no-bundle and Bun.Transpiler keep new and the binding.
• The inliner refuses a return only when the binding is in tail position and the initializer ends in a call. return x.y() is still inlined: it was already a tail call.
• The inlined literal takes the position of the identifier it replaces, as in esbuild.
• Verified: test/js/bun/transpiler/runtime-stack-frames.test.ts and the runtime cases from transpiler: stop minifying new Error() (and other known-global constructs) into calls that JSC tail-calls #37388 (stack.test.ts, runtime-transpiler.test.ts). All fail on 1.4.3. More in the notes.

Background

• A proper tail call replaces the caller's frame with the callee's. JSC does it in strict mode only, so CommonJS is not affected.
• The runtime transpiler cache stores transpiled output, so EXPECTED_VERSION moves to 30.
• With new kept, the error printer reports the new keyword, as V8 does. The column snapshots here move by the length of new .
• Open question: transpiler: stop minifying new Error() (and other known-global constructs) into calls that JSC tail-calls #37388 (closed for this PR) kept new in bun build --minify output too, since a minified bundle loses the same frame and esbuild keeps new. This PR keeps the rewrite there (4 bytes per site). For that policy instead, drop the bundle condition on the e_new hunk.

Notes

• Repro on 1.4.3 (ESM file, bun file.mjs):

  function g() { throw new Error("boom"); }
  function f() { const r = g(); return r; }
  try { f(); } catch (e) { console.log(e.stack); }   // no `f` frame
  function k() { return new Error("x"); }
  console.log(k().stack);                            // no `k` frame, top frame is the caller
  function h() { const obj = null; console.log("pad"); return obj.foo; }
  try { h(); } catch (e) { console.log(e.stack); }   // points at the `const obj` line

  The same file with a // @bun pragma on line 1 (no transpile) shows all frames.

• Stack traces missing when creating errors #24789's own repro (err1 creates the error, err2 and err3 each do let e = f(); return e) shows all three frames with this branch. 1.4.3 shows only the top-level frame. transpiler: stop minifying new Error() (and other known-global constructs) into calls that JSC tail-calls #37388 alone restores err1 but not err2 or err3, because it does not touch the binding inliner.

• minify_global_constructor (src/ast/known_global.rs) is the byte-saving rewrite from feat(minify): optimize Error constructors by removing 'new' keyword #22493. With the bundle gate the runtime no longer rewrites any known-global construct into a call, so two more cases from transpiler: stop minifying new Error() (and other known-global constructs) into calls that JSC tail-calls #37388 are fixed and now tested here: the RangeError from return new Array(...lengths) names the function, and return new Function(body) takes its source origin (what import() in the body resolves against) from the returning module, as Node does. In a minified bundle both still take the call form, which is part of the open question above.

• The new Array(5, ...rest) to [5, ...rest] fold that transpiler: stop minifying new Error() (and other known-global constructs) into calls that JSC tail-calls #37388 also fixed is a separate miscompile (wrong length when rest is empty). It landed as minify: do not fold new Array(x, ...spread) into an array literal #41828, which took cache version 29, so this branch merges main and takes 30.

• Tail position follows JSC's bytecode generator: the return value itself, the last operand of a comma, the right side of &&, ||, ??, and both branches of ?:. A tagged template is a call.

• error.stack (the string) still reports the column of the constructor name for new X() frames while the printer and CallSite report new. That is the pre-existing difference error.stack: report frames at new X(...) at the new keyword #37396 addresses. One assertion in inspect-error.test.js that compared the two now compares lines for the printer-computed frames.

• Self-reviewed: 4 concerns raised, 3 addressed (the first draft added a preserve_stack_frames feature flag, now replaced by the existing bundle gate, and two walkers collapsed into one). Not taken: splitting the three hunks into three PRs. They fix one reported symptom and share one test file.

• minify: port esbuild's statement-level syntax passes #41159 also bumps the cache version. Whichever lands second bumps again.

• Suites run against the debug build: bundler transpiler and minify, test/js/bun/{transpiler,sourcemap,test,util}, test/cli/hot, test/cli/run/transpiler-cache, test/js/web/console, test/js/node/v8.

• test/cli/test/parallel.test.ts "unique JEST_WORKER_ID" and pretty-format-overflow.test.ts fail on main in this debug container too (timing).

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no test proof · iteration 4 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/util/inspect.test.js, test/js/bun/test/test-test.test.ts, test/js/bun/http/serve.test.ts, test/cli/hot/hot.test.ts

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Walkthrough

Changes

The parser now preserves source locations during constant inlining and blocks unsafe tail-call rewrites in non-bundled return statements. Constructor minification is limited to bundled builds. Runtime transpiler tests and stack-trace expectations now reflect corrected frames and columns.

Runtime stack preservation

Layer / File(s)	Summary
Parser substitution and tail-position handling src/js_parser/p.rs	Constant inlining preserves use-site locations. Single-use substitution detects tail positions through conditional, comma, logical, and nullish-coalescing expressions.
Runtime transpiler and cache behavior src/js_parser/visit/visit_expr.rs, src/jsc/RuntimeTranspilerCache.rs	Constructor minification applies only to bundled builds. The runtime transpiler cache version increases to 29.
Runtime stack regression coverage test/js/bun/transpiler/runtime-stack-frames.test.ts	Tests cover preserved frames and source positions for returned bindings, constructed errors, inlined literals, and tail calls.
Stack and sourcemap expectation updates test/cli/hot/hot.test.ts, test/js/bun/http/serve.test.ts, test/js/bun/test/*, test/js/bun/util/*, test/js/web/console/console-log.test.ts	Updated snapshots and assertions record corrected stack columns, caret positions, and sourcemap locations.

Suggested reviewers: jarred-sumner

Merge Risk: ⚪ Minimal · up to 72bb7

This change preserves runtime stack frames and source positions while retaining bundled minification behavior. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue [#24789] by preventing tail-call-producing rewrites for non-bundled return bindings and preserving stack frames for intermediate functions such as err2 and err3. The added ru…
Out of Scope Changes check	✅ Passed	The cache version update, constructor rewrite gating, source-position correction, and snapshot changes support the stack-frame and position fixes described in the pull request objectives. No unrelated…
Title check	✅ Passed	The title clearly summarizes the primary change: preserving stack frames and source positions when the runtime transpiler applies syntax minification.
Description check	✅ Passed	The description explains the problem, implementation, scope, cache-version impact, verification steps, test coverage, and known limitations. It does not use the template headings exactly, but it provi…

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[robobun]

Status: reproduced on bun 1.4.3 with the snippet in the description (ESM file run with bun file.mjs; the same file with a // @bun pragma shows all frames). Fix is in src/js_parser/p.rs and src/js_parser/visit/visit_expr.rs, tests in test/js/bun/transpiler/runtime-stack-frames.test.ts plus the #37388 runtime cases in test/js/bun/test/stack.test.ts and test/bundler/transpiler/runtime-transpiler.test.ts.

CI: the diff is green. On build 112301 (branch merged with main, cache version 30) the only red job is test-crypto-dh-leak.js on x64-asan, which also fails on main. Ready for review.

[robobun]

^{Updated 8:51 PM PT - Sep 7th, 2026}

❌ @robobun, your commit e51e07e has 1 failures in Build #112301 (All Failures):

• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan

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[claude]

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

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Actionable comments posted: 1

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Inline comments:
In `@src/js_parser/p.rs`:
- Around line 2863-2881: Convert any_in_tail_position into an instance method,
add the existing stack-safety and reported-overflow guard at entry, and return
the appropriate failure result for its boolean contract while preserving
traversal behavior. Update its recursive calls and both callers to use
self.any_in_tail_position(...), ensuring deep EIf and right-nested EBinary
chains follow the controlled overflow path.

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• src/js_parser/p.rs
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• test/js/bun/test/stack.test.ts
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[claude]

I re-reviewed after 0131840 and found no bugs — all three nits from my earlier pass are addressed (describe.concurrent, the renamed tail-call test, and the column-preserving printerColumns helper in inspect-error.test.js). Because the substantive change is new tail-position analysis in the parser's single-use-symbol inliner, a human look at p.rs is still worthwhile.

What was reviewed

• any_in_tail_position walker in src/js_parser/p.rs — checked it covers ?:, ,, and &&/||/?? right-hands per JSC's tail-call rules, and that the guard is scoped to !self.options.bundle.
• The minify_global_constructor gate in visit_expr.rs — matches the neighboring name-drop gates on p.options.bundle.
• Cache version bump in RuntimeTranspilerCache.rs — 28→29.
• Nine snapshot/assertion updates — column shifts of 4 ("new ".length) and newly-present frames, consistent with the described output change.

Extended reasoning...

Overview

This PR gates two minify_syntax rewrites (new Error() → Error() and single-use-const substitution into return) on p.options.bundle so the runtime transpiler stops manufacturing tail calls that JSC then eliminates, dropping stack frames. It also fixes const-literal inlining to use the use-site loc, adds a tail-position walker (any_in_tail_position) to decide when substitution is safe to refuse, bumps the transpiler cache version, adds a dedicated test file, and updates nine existing snapshot/stack-trace assertions.

Security risks

None. This is a code-transformation-fidelity change in the parser/transpiler; no untrusted input handling, auth, crypto, or network paths are touched.

Level of scrutiny

Medium-high. The parser is a hot, widely-shared path, and any_in_tail_position is new recursive expression analysis whose correctness determines whether an optimization fires. It is conservatively gated (!self.options.bundle) so bundler output is unchanged, and the PR description maps each tail-position form to JSC's bytecode generator. Still, parser inlining logic is exactly the kind of change a maintainer should eyeball before merge.

Other factors

My previous review left three optional nits; commit 0131840 addressed all of them cleanly (concurrent test blocks, an accurate test name for the already-tail-call case, and a column-offset assertion that preserves the precision the base branch had instead of stripping columns). No third-party CHANGES_REQUESTED reviews are outstanding — the remaining timeline entries are bot comments self-resolved by the author. The new test file follows harness conventions (tempDir, bunEnv/bunExe, concurrent Promise.all on stdout/stderr/exited, stderr asserted before exitCode). Given the scope touches core parser logic rather than a mechanical change, deferring to a human reviewer rather than auto-approving.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.