bun_ptr: RefPtr::new_cyclic; a BackRef<_, Root> can no longer be dangling - #40210
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. WalkthroughChangesThe pointer library adds validated self-referential values. Suggested reviewers: Merge Risk: 🟠 High · up to This change adds cyclic self-references used by CronJob. Retained self-pointers may outlive their owning RefPtr and dereference freed memory, while panicking initializers can leak allocations; these risks should be resolved before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Beyond the inline nits, I also checked whether new_cyclic leaks the boxed slot if init panics — it would, but every Cargo profile sets panic = "abort" (Cargo.toml:143/146/186), so no unwinding path exists. Also confirmed self_ref has no other read sites in cron.rs that relied on the Cell wrapper.
Extended reasoning...
The two inline findings are both nits with no runtime impact today (the sole new_cyclic caller only stores the back-ref; the MaybeUninit double-wrap is layout-identical). The panic-leak candidate was ruled out because Bun compiles with panic = "abort" in all profiles, so init cannot unwind past the into_raw. Deferring rather than approving because this adds new unsafe to a T0 foundation crate and the first nit is an API-design call (safe-fn-with-doc-contract vs. opaque token / unsafe fn) that a maintainer should make.
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Updated 12:05 PM PT - Sep 6th, 2026
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Both earlier notes are addressed in d789591 — init now receives an opaque SelfRoot<T> (no Deref; this_ptr requires the constructed &T as witness), and the allocation is spelled Box::<T>::new_uninit(). I re-reviewed at this commit and found no bugs. Since this adds a new unsafe-backed primitive (SelfRoot / new_cyclic) to the T0 bun_ptr crate, a human look at the API shape would still be worthwhile.
Checked: SelfRoot::this_ptr cannot be reached from inside init (no &T exists yet), so uninit memory is unreachable from safe code; the sealed DanglingOk bound correctly excludes Root; CronJob::stop's new self.self_ref.this_ptr(self) call is equivalent to the old self_ref.get().this_ptr(). Also considered leak-on-panic in init — it leaks the slot but is not UB, consistent with the crate's no-Drop RefPtr model.
Extended reasoning...
Overview
This PR tightens bun_ptr's back-reference API so a BackRef<T, Root> (which can vend a ThisPtr) can no longer be constructed dangling, and adds RefPtr::new_cyclic as the sanctioned way for a refcounted object to store a pointer to itself. Since my previous review, commit d789591 replaced the BackRef<T, Root> closure argument with a new opaque SelfRoot<T> token and fixed the Box::<MaybeUninit<T>>::new_uninit() double-wrap — both exactly as suggested. Files touched: src/ptr/lib.rs (new SelfRoot, sealed DanglingOk bound on BackRef::dangling), src/ptr/ref_count.rs (RefPtr::new_cyclic), and src/runtime/api/cron.rs (the one caller, CronJob, migrated to the new pattern).
Security risks
None user-facing. This is internal memory-safety plumbing; no untrusted input, no FFI surface change. The relevant risk class is Rust soundness (safe API over uninitialized memory), which the SelfRoot redesign now closes: the token has no Deref/get(), and this_ptr(&self, owner: &T) requires a live &T witness that cannot exist until after raw.write(value) runs. The debug-only ptr::eq assert catches passing the wrong &T.
Level of scrutiny
High. bun_ptr is a tier-0 foundation crate that ~200 other crates sit on, and this introduces a new public abstraction (SelfRoot<T>) plus a new unsafe-backed constructor (new_cyclic) with hand-written allocation, raw ptr::write, and adopt_ref. It is a small diff but an API design decision — the kind REVIEW.md flags for maintainer agreement ("new cross-cutting abstractions need maintainer agreement"). That, not correctness, is why I'm deferring rather than approving.
Other factors
- Both prior review points were addressed cleanly and the fix is the stronger of the two options I suggested (opaque token >
unsafe fn). - Verifiers examined and ruled out "new_cyclic leaks on init panic" as a bug — it does leak the
Box<MaybeUninit<T>>slot, but that is memory-safe and matches the crate's explicit no-Drop-on-RefPtrcontract. test/js/bun/cron(162 tests) exercises the one call site under debug+ASAN per the PR description.- The PR description text is now stale (still describes the pre-d7895913
BackRef<T, Root>closure signature) — cosmetic only.
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Beyond the inline nit on SelfRoot::this_ptr, I also checked whether new_cyclic leaks the Box allocation if init panics — it does leak the uninitialized slot, but nothing droppable is in it and the crate builds with panic = abort, so not a concern here.
Extended reasoning...
The author addressed both nits from my previous pass (opaque SelfRoot<T> token instead of BackRef<T, Root>; Box::<T>::new_uninit()). This run's inline nit is the residual gap on the &T witness in SelfRoot::this_ptr — release builds don't tie owner to self.0. It's a soundness observation, not a runtime bug (the sole caller is correct), and it matches existing crate precedent (BackRef::<_, Shared>::dangling() has the same shape). Separately, a finder raised panic-in-init leaking the Box::new_uninit allocation; verifiers ruled it out — the slot is uninitialized so only the allocation leaks, and the workspace is panic = abort. This is unsafe foundational pointer code in a T0 crate, so I'm not auto-approving.
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b0077fa (release assert! on the ptr::eq witness) closes the gap I raised — a stack-tmp owner inside init now panics in release rather than yielding a ThisPtr to the uninit slot. Beyond the inline nit, this run also checked whether new_cyclic leaks the Box::new_uninit slot if init panics — ruled out as acceptable (matches RefPtr::new, and bun_core::handle_oom/abort-on-panic policy applies).
Extended reasoning...
The author pushed b0077fa after my previous inline comment about SelfRoot::this_ptr using debug_assert!; it now uses a release-build assert!, which makes the SAFETY comment accurate (the owner: &T witness is proven to be at self.0 on every build config before ThisPtr::new is called). That was the last soundness-shaped concern from prior rounds; the remaining inline finding is a testing-hygiene nit (no Miri-checked unit test for the new handoff shape in ref_count.rs::tests). One additional candidate was examined and ruled out: init panicking would leak the uninitialized Box slot, but that is no different from RefPtr::new (which also leaks on panic-during-construction) and the crate operates under abort-on-panic. Not auto-approving because this adds a new unsafe-heavy primitive to the T0 bun_ptr foundation crate.
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All four rounds of prior feedback are addressed — 0f7dae1 adds the Miri-covered new_cyclic_self_root_round_trip test (construction, witness assert, ThisPtr → ref_guard through the stored root, last-release destruction). No further findings this run. Deferring rather than approving only because this introduces new public unsafe-primitive API surface (SelfRoot, new_cyclic, DanglingOk) in the T0 bun_ptr crate; a human sign-off on the API shape is worth having.
Checked: new_cyclic allocation/write/adopt handoff and its Miri test; SelfRoot::this_ptr release-assert! witness (b0077fa) closes the stack-T gap and keeps allocation-root provenance for the last release; DanglingOk sealed bound leaves the two bundler BackRef::dangling() sites (both Shared) unaffected; CronJob migration drops the two-phase Cell<BackRef<_, Root>> init cleanly. Panic-in-init leaks the uninit slot — examined and treated as acceptable (internal primitive, no drop of uninit T).
Extended reasoning...
Overview
Adds RefPtr::new_cyclic (à la Arc::new_cyclic) and an opaque SelfRoot<T> token so a ref-counted value can store its own allocation-root pointer without a two-phase dangling()-then-patch init. Restricts BackRef::dangling() to Shared/Mut via a sealed DanglingOk trait so a BackRef<_, Root> (which can hand out ThisPtr) is only ever minted from a real root. Migrates the one existing pattern user, CronJob::self_ref, and adds a Miri-checked round-trip test in ref_count.rs::tests.
Prior review history
I raised four items across three prior runs; all are now addressed:
new_cyclichandedinitaBackRef<T, Root>with safeDerefto uninit memory → replaced with opaqueSelfRoot<T>(noDeref, accessors need&Twitness).Box::<MaybeUninit<T>>::new_uninit()double-wrap → fixed toBox::<T>::new_uninit().SelfRoot::this_ptrwitness onlydebug_assert!-checked → now releaseassert!; author kept the pointer sourced fromself.0(notowner) for allocation-root provenance on the last release, which is the correct call.- No Miri test for the new raw-pointer handoff shape →
new_cyclic_self_root_round_tripadded in 0f7dae1, sitting alongsideref_ptr_round_tripper the module's Tree Borrows convention.
Security risks
None user-facing. This is an internal memory-safety primitive; the risk class is UB from unsound unsafe, which the release-assert! witness and the Miri test now guard.
Level of scrutiny
High — new public unsafe primitive in a T0 foundation crate that ~200 crates depend on. The implementation has been through four review iterations and is now Miri-covered, but adding SelfRoot/new_cyclic/DanglingOk to bun_ptr's public surface is an API-design decision a maintainer should confirm.
Other factors
The one candidate the verifier ruled out this run — init panicking leaks the uninit Box slot — is real but acceptable: no T is dropped (slot is uninit), the leak is bounded to one allocation, and panicking inside a struct-literal init closure is a programmer error in an internal primitive. Not a blocker.
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Inline comments:
In `@src/ptr/lib.rs`:
- Around line 176-190: Make SelfRoot::this_ptr unsafe and require callers to
uphold that the owner remains live for the returned ThisPtr’s entire use, or
change it to return an owning RefPtr<T>. Update SelfRoot::backref consistently
so it cannot provide the same unguarded escape path, preserving the
ownership/lifetime guarantee for subsequent get() calls.
In `@src/ptr/ref_count.rs`:
- Around line 616-620: Move the cyclic-construction documentation currently
attached to new_cyclic so it ends the init_ref documentation block, then place
the new_cyclic-specific contract immediately before new_cyclic. Ensure init_ref
retains its SelfRoot/token safety contract and new_cyclic no longer inherits
unrelated documentation.
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Beyond the inline findings, I also checked whether new_cyclic leaks the Box::<T>::new_uninit() slot if init panics — it does, but with no drop obligation on the uninit slot and panics in this codebase reserved for invariant violations rather than recoverable paths, this matches RefPtr::new's stance and was not flagged. The earlier review threads (opaque token without Deref, assert! vs debug_assert! on the owner witness, Miri round-trip test) all look addressed in the current diff.
Extended reasoning...
This run's new ground beyond the two posted findings was the panic-safety of new_cyclic: bun_core::heap::into_raw_nn(Box::<T>::new_uninit()) strips the Box's Drop before calling the user closure, so a panic inside init leaks the heap slot. This was examined and ruled out as acceptable — the slot holds no initialized T (no drop to run), Bun treats panics as unrecoverable invariant failures rather than error paths, and the sole intended caller (CronJob) builds a plain struct literal. The four prior-round concerns (safe Deref on an uninit BackRef, double-MaybeUninit wrap, release-only debug_assert! on the &T witness, missing Miri test in mod tests) are each visibly addressed by the current SelfRoot shape, Box::<T>::new_uninit(), assert!, and new_cyclic_self_root_round_trip.
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Both remaining issues from the last pass are addressed — cron.rs is now migrated (the two other BackRef::dangling() sites in the bundler are Mut, so the DanglingOk bound compiles everywhere), and init_ref has its doc/# Safety block back. No further findings; given this adds a new unsafe primitive to bun_ptr, a human sign-off is still worthwhile.
What was reviewed:
SelfRoot::this_ptrnow uses a releaseassert!on the owner address, closing the "stackTwith a stolen token" gap raised earlier.new_cyclic:Box::<T>::new_uninit()→into_raw_nn→write→ adopt matchesRefPtr::new's shape; the newtests::new_cyclic_self_root_round_tripcovers the Miri path the module header asks for.- Checked for a leak if
initpanics — the uninitBoxis forgotten before the closure runs with no drop guard; ruled out as a nit (init closures here are struct literals, and abort-on-panic makes it moot in practice).
Extended reasoning...
Overview
This PR adds RefPtr::new_cyclic and the SelfRoot<T> token to bun_ptr, seals BackRef::dangling() away from the Root provenance marker via a DanglingOk trait, and migrates CronJob::self_ref as the first consumer. Three files touched: src/ptr/lib.rs (+41 lines: SelfRoot, DanglingOk, sealed module, bound on dangling()), src/ptr/ref_count.rs (+68 lines: new_cyclic and a Miri round-trip test), and src/runtime/api/cron.rs (net −2: field type change, new_cyclic construction, stop call site).
This is the sixth review pass. Earlier passes raised: (a) the original design handed init a BackRef<T, Root> with safe Deref to uninit memory — fixed by introducing the opaque SelfRoot token; (b) Box::<MaybeUninit<T>>::new_uninit() double-wrap — fixed to Box::<T>::new_uninit(); (c) debug_assert! on the owner-address check let release builds mint a ThisPtr to the uninit slot from a stack T — fixed to assert!; (d) no Miri test — added; (e) new_cyclic's doc block spliced into init_ref's — fixed; (f) the DanglingOk bound broke the build because cron.rs wasn't migrated — fixed in a5c70aa. All six are verified addressed in the current diff. I also confirmed the only other BackRef::dangling() call sites (src/bundler/Graph.rs:161, src/bundler/bundle_v2.rs:2895) are BackRef<ThreadPool, Mut>, which satisfies DanglingOk.
Security risks
None in the injection/auth/data-exposure sense. The risk class here is memory safety: new_cyclic hands out a raw NonNull<T> to an uninitialized allocation, and SelfRoot::this_ptr mints a ThisPtr (which carries the "may release the last ref" root provenance). The design now guards this correctly — the token has no Deref, and this_ptr(&self, owner: &T) asserts (in release) that owner lives at the stored address, so it cannot be followed before the value is written or from a different T. The raw.as_ptr().write(value) uses raw-pointer write (no drop of uninit), and Self(raw) adopts the +1 exactly as RefPtr::new does.
Level of scrutiny
High. bun_ptr is the crate that underpins refcounted lifetime management across the runtime; a soundness hole here is a UAF factory. REVIEW.md flags native memory safety as "the most-blocked category" and requires that safety be enforced by the compiler, not comments — which is precisely what this PR's iterations have been converging on (opaque token + release assert + sealed trait bound instead of a doc-string contract on a safe dangling()). The change is small and now well-tested, but new unsafe primitives in this crate warrant a maintainer's eyes before merge.
Other factors
The added new_cyclic_self_root_round_trip test exercises construction, this_ptr identity, ref bump/release through the token, and destructor-runs-once via the last ref obtained from the token — the shape the ref_count.rs::tests header says exists for bun run rust:miri -p bun_ptr. The PR description reports test/js/bun/cron 162/162 on debug+ASAN. The one candidate investigated and ruled out this run — init panicking leaks the new_uninit Box because into_raw_nn forgets it before the fallible call — is real but minor: the sole caller passes a struct literal, and Bun builds with panic=abort. No outstanding third-party CHANGES_REQUESTED reviews; the two coderabbit inline threads were resolved by a non-author.
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Inline comments:
In `@src/ptr/ref_count.rs`:
- Line 550: Update the allocation flow in new_cyclic to retain the
Box<MaybeUninit<T>> local while init and the post-initialization debug assertion
execute; use write’s returned value to convert it into Box<T>, then transfer
that owned Box to RefPtr instead of calling into_raw_nn before initialization.
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All six points from earlier rounds are addressed in the latest push — nothing further from me. Since this adds a new unsafe primitive to bun_ptr, a human sign-off is still worth having.
Checked in this revision: SelfRoot::this_ptr now uses a release assert! (not debug_assert!) so the owner-is-self.0 witness holds in release, while ThisPtr is still built from self.0 to keep allocation-root provenance; Box::<T>::new_uninit() replaces the double-MaybeUninit spelling; new_cyclic has its own doc block and init_ref's # Safety section is intact; the new_cyclic_self_root_round_trip test covers construction, the witness assert, a ref bump/release through the token, and last-release-via-root under the Miri suite; cron.rs is migrated in the same PR so the DanglingOk bound compiles. Also looked at the panic-in-init leak candidate — noted as already raised on this PR.
Extended reasoning...
Overview
This PR adds RefPtr::new_cyclic and SelfRoot<T> to bun_ptr, seals BackRef::dangling() away from the Root provenance marker via a DanglingOk trait, and migrates CronJob::self_ref from a two-phase Cell<BackRef<_, Root>> init to the new one-shot pattern. A unit test in ref_count.rs::tests exercises the full round-trip for the Miri lane.
Security risks
None user-facing. The risk surface is memory-safety soundness of a new unsafe primitive: an uninitialized Box<T> slot is exposed (as an opaque NonNull inside SelfRoot) to a caller-supplied closure before the value is written. The revised design closes the gap raised earlier — SelfRoot has no Deref/get, and this_ptr requires a live &T at the same address, enforced by a release assert! — so safe code cannot dereference the slot before construction completes.
Level of scrutiny
High. bun_ptr is the foundational refcounting/provenance crate; new unsafe fn-backed constructors here affect every future adopter. This has been through five automated review rounds with specific soundness feedback, all of which the current diff addresses. That said, REVIEW.md's bar for unsafe and the "who frees this, when, on which paths" question mean a human maintainer should still confirm the provenance story (root NonNull stored inside the value, later used to heap::take on last release) matches the crate's Tree Borrows model.
Other factors
All six earlier inline threads map to concrete changes in the last three commits: opaque token instead of BackRef in init, release-checked witness assert, corrected Box::<T>::new_uninit(), restored init_ref doc/# Safety, added Miri-run unit test, and the cron.rs migration landing alongside the DanglingOk restriction. No outstanding third-party CHANGES_REQUESTED; the two coderabbit threads were resolved by a non-author. The one remaining candidate this run (heap-slot leak if init panics) was flagged as already raised on the PR, so not repeated here.
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LGTM — all six points from earlier rounds are addressed in the current diff.
What was reviewed: SelfRoot::this_ptr now hard-assert!s the owner address (release-checked, not debug-only); new_cyclic uses Box::<T>::new_uninit() and hands out an opaque token with no Deref; init_ref kept its own # Safety doc; the new_cyclic_self_root_round_trip test covers the alloc→write→adopt→release chain for Miri; cron.rs is migrated so the DanglingOk bound doesn't break the build; and the two remaining BackRef::dangling() calls in src/bundler/ are on Mut, which implements DanglingOk.
Extended reasoning...
Overview
This PR adds RefPtr::new_cyclic and an opaque SelfRoot<T> token to bun_ptr, restricts BackRef::dangling() to non-Root provenance markers via a sealed DanglingOk trait, and migrates CronJob::self_ref from a two-phase Cell<BackRef<_, Root>> (dangling-then-set) to a SelfRoot populated inside new_cyclic. Three files touched: src/ptr/lib.rs (~40 lines), src/ptr/ref_count.rs (~65 lines including a new Miri-targeted unit test), and src/runtime/api/cron.rs (a handful of lines swapping the field type and construction).
Security risks
None in the classic sense (no untrusted input parsing, auth, or network). The relevant risk class is memory safety: bun_ptr is unsafe refcount/provenance infrastructure, and a bug here would be a UAF or uninit-read reachable from any RefPtr client. The change is a net tightening: it removes the ability to construct a dangling Root back-reference (which could previously mint a ThisPtr to garbage) and replaces the one caller's two-phase init with an atomic construction. The SelfRoot token has no Deref and its only accessor requires a live &T witness at the stored address, checked with a release assert!, so safe code cannot follow it into the uninitialized slot during init.
Level of scrutiny
High, and it has received it — this is the fifth review pass. Across earlier rounds I raised: (1) the closure originally received a BackRef<T, Root> with safe Deref to uninit memory — fixed by the opaque SelfRoot design; (2) Box::<MaybeUninit<T>>::new_uninit() double-wrapping — now Box::<T>::new_uninit(); (3) the owner check was debug_assert!-only — now a hard assert!; (4) no Miri coverage — new_cyclic_self_root_round_trip added, exercising construction, this_ptr witness, ref bump/release through the token, and destruction via heap::take; (5) cron.rs wasn't in the diff so DanglingOk broke the build — now migrated; (6) new_cyclic's doc comment had swallowed init_ref's # Safety block — now separated. All six are visibly addressed in the current diff. I also confirmed the two remaining BackRef::dangling() calls in the bundler are on BackRef<_, Mut>, which implements DanglingOk, so no other call sites break.
Other factors
new_cyclic mirrors the existing RefPtr::new shape (into_raw_nn → debug_assert rc_has_one_ref → Self(ptr)), uses the crate-convention bun_core::heap helpers, and the CronJob migration is a mechanical field-type swap with the Cell wrapper and post-construction .set() deleted. No third-party CHANGES_REQUESTED reviews are outstanding; the CodeRabbit threads were resolved by a non-author. Exit reason was dry_streak. Given the small size, the safety-improving direction of the change, the added Miri test, and every prior concern being addressed, approving is appropriate.
…ared/Mut new_cyclic builds a refcounted value that stores its own root pointer as an opaque SelfRoot<T>; the token can only be turned into a ThisPtr through the constructed &T, so it cannot be followed early or late. A Root back-reference can hand out ThisPtrs, so BackRef::dangling() is limited to Shared/Mut.
…ched-in Root back-reference
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Re-reviewed after the latest push — all six items from earlier rounds are now addressed (opaque SelfRoot token, Box::<T>::new_uninit(), release-mode assert! on the owner identity check, the Miri round-trip test, the CronJob migration so DanglingOk doesn't break the build, and init_ref keeping its own doc block). I didn't find anything further; a human pass is still worthwhile given this is unsafe ref-count/provenance infrastructure.
Checked: SelfRoot::this_ptr now hard-asserts owner is at self.0 in release, closing the earlier safe-code-reaches-uninit gap; new_cyclic uses heap::into_raw_nn + ptr::write so a panicking init leaks but never drops uninit; the two remaining BackRef::dangling() callers in src/bundler/ are Mut-typed and satisfy DanglingOk; the new new_cyclic_self_root_round_trip test lives in ref_count.rs::tests where the Miri lane covers it.
Extended reasoning...
Overview
The PR adds RefPtr::new_cyclic and an opaque SelfRoot<T> token to bun_ptr so a ref-counted value can store its own allocation-root pointer without a two-phase construct-then-patch dance, gates BackRef::dangling() behind a sealed DanglingOk trait that excludes Root, and migrates the sole consumer (CronJob in src/runtime/api/cron.rs) to the new pattern. Net change is roughly +100 lines across three files, including a unit test in the Miri-covered ref_count.rs::tests module.
Security risks
No user-facing input handling, network, auth, or crypto is touched. The risk surface is memory safety: new_cyclic does a raw Box::<T>::new_uninit() → into_raw_nn → ptr::write handoff and stores a NonNull<T> inside the value for later ThisPtr minting. The earlier soundness gap (safe code inside init reaching the uninitialized slot) is now closed structurally — SelfRoot has no Deref, and this_ptr(&self, owner: &T) release-asserts ptr::eq(self.0, owner) before constructing the ThisPtr, so a mismatched witness panics rather than yielding a pointer to uninit memory. The DanglingOk sealed trait makes a dangling Root back-reference unrepresentable at the type level, matching REVIEW.md's "make invalid states unrepresentable" guidance.
Level of scrutiny
High — this is core unsafe smart-pointer infrastructure with pointer-provenance implications (the whole point of SelfRoot is preserving allocation-root provenance for the last Box::from_raw). I've reviewed it across six iterations and every concrete issue raised has been fixed in the current push, but changes to bun_ptr's ref-counting primitives warrant a maintainer's eyes before merge rather than bot-only approval.
Other factors
Verified the two remaining BackRef::dangling() call sites in src/bundler/{Graph.rs,bundle_v2.rs} are BackRef<_, Mut>, which implements DanglingOk, so the trait bound doesn't break them. No CODEOWNERS entry covers the changed paths. The added test exercises construction, the owner-identity assert, a ref bump through the token, and drop ordering via the module's existing DROPS counter, and sits where bun run rust:miri -p bun_ptr will run it under Tree Borrows. If init panics, the uninitialized box leaks (no drop of uninit T), which is acceptable and matches the leak-on-panic posture elsewhere in the crate.
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What
Follow-up to #40192 (raised in review of #40203):
BackRef::dangling()was generic over the provenance marker, so aBackRef<T, Root>— which can hand out aThisPtr— could be created dangling and "patched later" (CronJob::self_refdid exactly that:Cell::new(BackRef::dangling())thenset(BackRef::from(job.this_ptr()))afterRefPtr::new).RefPtr::new_cyclic(|this: SelfRoot<T>| -> T)(à laArc::new_cyclic): allocates the slot, handsinitan opaqueSelfRoot<T>token to store, writes the value, adopts the ref.SelfRoothas no accessors of its own —this_ptr(&self, owner: &T)/backref(&self, owner: &T)need the constructed&T, so the pointer cannot be followed insideinit.BackRef::dangling()is now only available forShared/Mut(sealedDanglingOkbound), so aRootback-reference is always minted from a real root (From<ThisPtr>,from_root, ornew_cyclic).CronJob::self_refbecomes aSelfRoot<CronJob>field initialised throughnew_cyclic(self.self_ref.this_ptr(self)); noCell, no two-phase init.Testing
test/js/bun/cron162/162 on debug+ASAN; clippy clean on bun_ptr / bun_runtime / bun_bundler (the two bundlerBackRef::dangling()placeholders areSharedand unchanged).