[robobun]

Replaces #426. That PR copied the name after code generation. The review there asked for it to be done while parsing.

Problem

• A stack frame of an async function or generator whose name was inferred from its binding has no name. Bun prints at async <anonymous> for const f = async () => { await g(); }, Node prints at async f, and f.name is "f".
• Error.captureStackTrace(err, f) in Bun looks for the frame of f by name. It finds none, so err.stack is Error: boom alone.
• The cause is in generateUnlinkedFunctionCodeBlock (bytecode/UnlinkedFunctionExecutable.cpp). It reparses the function with executable->name(), the declared name. An inferred name is only in ecmaName() and never reaches the parser.

Fix

• generateUnlinkedFunctionCodeBlock passes the inferred name of a function without a declared name to the parser (Parser::setInferredFunctionName).
• parseInner names the body function after the declared name, or else after the inferred one. The inferred name is not declared as a callee in the function's scope.
• A private name is not used (export default async function () {} is *default*), as for a declared name.
• Verified: JSTests/stress/generator-and-async-body-function-name.js in default, no-jit, dfg-eager and ftl-eager runs. It fails without the change. The Bun tests are in the Bun PR that pins this build.

Background

• A generator, an async generator and an async function that awaits are compiled as a wrapper function. Each call makes a body function that runs the code.
• The body function is the callee of the frames that run that code and of the at async frames. Its name is what stack traces, the sampling profiler and the inspector show. It is never visible to JavaScript.
• ecmaName is the name a function has for Function.prototype.name. The parser infers it from const f = ..., { f: ... }, a class field or an assignment.

Downsides

• A name that exists only at run time stays unnamed: a computed key, or a name redefined with Object.defineProperty.
• Error.captureStackTrace(err, fn) in Bun can now cut at the frame of another async function with the inferred name of fn. It already does that for declared names.
• Bytecode cache size and parse cost: measured numbers are in the Bun PR.

[github-actions]

Preview build of 1dce549: autobuild-preview-pr-820-1dce5490

[claude]

Beyond the inline findings, I also checked the lifetime of the raw inferredName pointer handed to the parser: it points at the UnlinkedFunctionExecutable's m_ecmaName (materialized first if deferred), and the executable is held by the caller for the whole reparse, so the pointer stays valid; isSymbol() also correctly keeps *default* out of the body name.

Extended reasoning...

The change is confined to USE(BUN_JSC_ADDITIONS) blocks in the parser and UnlinkedFunctionExecutable reparse path plus one new stress test, and touches no security-sensitive surface. The two confirmed inline findings (an existing stress test asserting the old empty frame name, and the new test's loop count versus the 200ms budget) are what keep this from an approve.

Findings marked 🟡 are optional suggestions and need no follow-up push.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between b1e30d0 and af807fa.

📒 Files selected for processing: 4

• JSTests/stress/generator-and-async-body-function-name.js
• Source/JavaScriptCore/bytecode/UnlinkedFunctionExecutable.cpp
• Source/JavaScriptCore/parser/Parser.cpp
• Source/JavaScriptCore/parser/Parser.h

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Walkthrough

The change adds Bun-conditional inferred-name propagation for async-function and generator wrapper parsing. A stress test captures stack traces and checks inferred names across several function forms.

Changes

Async and generator stack-frame names

Layer / File(s)	Summary
Propagate inferred names into wrapper parsing Source/JavaScriptCore/parser/Parser.h, Source/JavaScriptCore/bytecode/UnlinkedFunctionExecutable.cpp, Source/JavaScriptCore/parser/Parser.cpp	Under USE(BUN_JSC_ADDITIONS), parsing accepts an optional inferred name. Unnamed async-function, generator, and async-generator wrappers use that name when available.
Check inferred names in stack traces JSTests/stress/generator-and-async-body-function-name.js	The stress test checks captured stack-frame names across declarations, expressions, properties, class fields, assignments, nested functions, and await contexts. It repeats the checks and drains microtasks.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to af807

The change gives generator and async function bodies their inferred names in stack frames, and it only takes effect in Bun builds. I found no concrete merge-blocking risk. A non-Bun test run could in principle fail the new stress test, but nothing supplied suggests one exists.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: passing inferred names to generator and async function bodies during parsing.
Description check		The description is detailed and directly explains the problem, fix, behavior, testing, and trade-offs. It does not include the required Bugzilla link, review status line, or changed-file list from the…
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.

Full details: Description check

Explanation

The description is detailed and directly explains the problem, fix, behavior, testing, and trade-offs. It does not include the required Bugzilla link, review status line, or changed-file list from the repository template, but it is otherwise mostly complete.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.