[littledivy]

The generic form of jsg_traced! gave X<T> a no-op trace and HoldsNoGcReferences for any T. IoOwned<T> uses it, and IoContext::add_object takes any Send value. So an IoOwned, or any jsg_traced! wrapper, that holds a TracedGlobal or jsg::Rc was never visited. The handle stayed strong, and a cycle through it was never collected. A resource that holds a Slot<TracedGlobal> pointing back at its own wrapper is never dropped, while the same cycle through a traced field is.

In this patch:

• The generic form bounds every parameter by HoldsNoGcReferences, so X<T> claims no GC references only where T holds none. Weak<R> holds nothing to trace whatever R is, so it implements both traits by hand.
• add_object requires T: HoldsNoGcReferences. The values the request owns today (the span wrapper and the memory cache's fallback permit) are marked with jsg_traced!.
• New compile tests check both bounds. Without the fix they fail because the code compiles. The change is compile-time only.

[clanky-the-clanker]

✅ Approved — A compile-time-only fix that tightens the GC-reference bounds correctly. Send is still implied through Traced, and no issues were found.

_{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

The change only tightens compile-time bounds and weakens no boundary.

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📖 AGENTS.md: ✅ LGTM

The src/rust/AGENTS.md edit is accurate.

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📋 Code Quality: ✅ LGTM

The macro bounds, the Weak<R> impls, the add_object callers and the new compile tests are all consistent. Send is still enforced, because HoldsNoGcReferences is a subtrait of Traced, which requires Send.

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⚡ Performance: ✅ LGTM

The change is compile-time only and has no runtime cost.

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📝 Documentation: ✅ LGTM

The doc comments and the src/rust/AGENTS.md update match the new bounds.

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_{Reviewed commit: d3ac2b18}