[bartlomieju]

Under capture_async_api_throws, C++ JSG returns a rejected promise from
a method, property or call handler whose result is a promise, instead
of throwing (LiftKj_<v8::Localv8::Promise> in jsg/util.h). Argument
conversion errors and errors from the method itself are both caught; a
termination keeps propagating. Rust JSG threw them all synchronously,
so a Rust port of such a method behaved differently from the C++ one
it replaces, for example for ctx.restore(42).catch(...).

The generated callback of a #[jsg_method], #[jsg_property] getter or
#[jsg_callable] that returns a jsg::Promise or a v8::Localv8::Promise,
alone or in a Result, now runs its body through
jsg::capture_throws_as_rejection(). If the isolate captures throws as
rejections, the body runs in Lock::try_catch(), and what it throws is
returned as a rejected promise. Isolate::captures_throws_as_rejections()
reads the flag as C++ JSG does (jsg::getCaptureThrowsAsRejections()),
and Isolate::has_pending_exception() tells the helper that the body
threw. The jsg-macros README documents the behavior.

The new promise tests cover each way a callback fails, with the flag
on and off, a method that returns no promise, and a termination.
Without the macro change, the test with the flag on fails.

Compatibility: no compatibility flag. No Rust API that returns a
promise is exposed to Workers yet; this makes those to come match the
C++ ones under the existing capture_async_api_throws flag.

[clanky-the-clanker]

Superseded by the review of 2ae33ceb.

This review, of 9c03481f

✅ Approved — No findings. With the flag on, a promise-returning Rust callback now rejects its promise with what it throws, as C++ JSG does. Terminations still propagate, and nothing changes with the flag off.

_{The decision is advisory: this review is a comment and never approves, blocks or requests changes on the pull request.}

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✅ All checks passed

🔒 Security: ✅ LGTM

No security concerns.

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📖 AGENTS.md: ✅ LGTM

No AGENTS.md concerns.

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📋 Code Quality: ✅ LGTM

No problems found.

• A termination is thrown again and keeps propagating.
• The rejected promise is created only after the TryCatch has gone out of scope.
• The generated callbacks' early returns behave the same as before.
• Tests cover the flag-on, flag-off and termination paths.

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⚡ Performance: ✅ LGTM

The added work is one flag read per call, and only on methods that return a promise. try_catch runs only when the flag is on.

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📝 Documentation: ✅ LGTM

The new README bullet correctly describes how promise-returning callbacks behave under capture_async_api_throws. It also narrows the general statement just above it that a Result error throws.

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_{Reviewed commit: 9c03481f}

[clanky-the-clanker]

✅ Approved — No findings. With the flag on, a promise-returning Rust callback now rejects its promise with what it throws, as C++ JSG does. Terminations still propagate, and nothing changes with the flag off.

_{The decision is advisory: this review is a comment and never approves, blocks or requests changes on the pull request.}

---

✅ All checks passed

🔒 Security: ✅ LGTM

No security concerns.

---

📖 AGENTS.md: ✅ LGTM

No AGENTS.md concerns.

---

📋 Code Quality: ✅ LGTM

No problems found.

• A termination is thrown again and keeps propagating.
• The rejected promise is created only after the TryCatch has gone out of scope.
• The generated callbacks' early returns behave the same as before.
• Tests cover the flag-on, flag-off and termination paths.

---

⚡ Performance: ✅ LGTM

The added work is one flag read per call, and only on methods that return a promise. try_catch runs only when the flag is on.

---

📝 Documentation: ✅ LGTM

The new README bullet correctly describes how promise-returning callbacks behave under capture_async_api_throws. It also narrows the general statement just above it that a Result error throws.

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_{Reviewed commit: 2ae33ceb — same changes as 9c03481f, so this is that commit's review (no new review was run; line numbers are as of 9c03481f)}