perf(vm): lazily clone parent upvalues on Closure 🪺

[timfennis]

Context

A code-review agent flagged OpCode::Closure at ndc_vm/src/vm.rs:514 as wasteful: every closure creation pre-cloned the entire parent frame's upvalues Vec, even when the new closure didn't reference any of them. The Vec was only there to outlive the frame borrow so that capture_upvalue (which needs &mut self) could be called.

Change

Walk CaptureSource entries directly and resolve each on demand:

• Local(slot) → self.capture_upvalue(...) as before
• Upvalue(slot) → briefly re-borrow the frame and clone just that one parent upvalue Rc

The two borrows never overlap in time, so the borrow checker is happy without the intermediate Vec. Closures that only capture locals now do zero parent-upvalue work.

Performance

Bench	Baseline	New	Speedup
Targeted (parent has 16 upvalues, 200k closures)	215.0 ± 12.3 ms	200.4 ± 8.8 ms	1.07×
closures.ndc (existing, parent has 0 upvalues)	72.0 ± 3.4 ms	70.2 ± 3.2 ms	1.03× (noise)
fibonacci.ndc (regression check, no closures)	70.0 ± 2.5 ms	70.5 ± 3.0 ms	no change

The existing closures.ndc bench doesn't exercise the case this fix targets — the captured closures sit directly under the script root, so the parent's upvalues was empty and the old code's pre-clone was Vec::new(). The targeted ad-hoc bench (16 parent upvalues × 200k creations) shows the real ~7% win.

A nested_closures.ndc bench could be added to track this path going forward — happy to do that as a follow-up if useful.

🤖 Generated with Claude Code