Avoid per-tail-call owner allocations - #8472

crates/vm/src/vm/mod.rs (1)

113-117: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Use Cell instead of UnsafeCell for pending_tailcall_owner.

Cell<Option<PyObjectRef>> supports .set() and .take() directly, because Option<T> implements Default regardless of whether T does. This gives the same single-threaded interior-mutability guarantee as the current UnsafeCell, without an unsafe raw-pointer dereference in set_pending_tailcall_owner and take_pending_tailcall_owner.

The comment on line 116 justifies the choice by stating the VM is per-thread and the field is accessed only on the owning thread. That reasoning applies equally to Cell, so it does not explain why UnsafeCell is required here.

♻️ Proposed refactor to remove the unsafe raw-pointer access

-    pending_tailcall_owner: core::cell::UnsafeCell<Option<PyObjectRef>>,
+    pending_tailcall_owner: Cell<Option<PyObjectRef>>,

     pub(crate) fn set_pending_tailcall_owner(&self, owner: PyObjectRef) {
-        let slot = unsafe { &mut *self.pending_tailcall_owner.get() };
-        debug_assert!(slot.is_none(), "pending TailCall owner was not consumed");
-        *slot = Some(owner);
+        let previous = self.pending_tailcall_owner.replace(Some(owner));
+        debug_assert!(previous.is_none(), "pending TailCall owner was not consumed");
     }

     fn take_pending_tailcall_owner(&self) -> PyObjectRef {
-        unsafe { &mut *self.pending_tailcall_owner.get() }
-            .take()
-            .expect("TailCall without pending owner")
+        self.pending_tailcall_owner
+            .take()
+            .expect("TailCall without pending owner")
     }

Also applies to: 1409-1424

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/vm/src/vm/mod.rs` around lines 113 - 117, Replace the UnsafeCell-based
pending_tailcall_owner field with Cell<Option<PyObjectRef>> and update its
initialization and accessors, especially set_pending_tailcall_owner and
take_pending_tailcall_owner, to use Cell::set and Cell::take directly. Remove
the unsafe raw-pointer dereferences while preserving the existing ownership and
single-threaded behavior.