Method promote_survivors

crates/vm/src/gc_state.rs:745–783

Promote surviving objects to the next generation. `survivors` must be strong references (`PyObjectRef`) to keep objects alive, since the generation read locks are released before this is called. Holds both source and destination list locks simultaneously to prevent a race where concurrent `untrack_object` reads a stale `gc_generation` and operates on the wrong list.

(&self, from_gen: usize, survivors: &[PyObjectRef])

    /// a race where concurrent `untrack_object` reads a stale `gc_generation`
    /// and operates on the wrong list.
    fn promote_survivors(&self, from_gen: usize, survivors: &[PyObjectRef]) {
        if from_gen >= 2 {
            return; // Already in oldest generation
        }

        let next_gen = from_gen + 1;

        for obj_ref in survivors {
            let obj = obj_ref.as_ref();
            let ptr = NonNull::from(obj);
            let obj_gen = obj.gc_generation();
            if obj_gen as usize <= from_gen && obj_gen <= 2 {
                let src_gen = obj_gen as usize;

                // Lock both source and destination lists simultaneously.
                // Always ascending order (src_gen < next_gen) → no deadlock.
                let mut src = self.generation_lists[src_gen].write();
                let mut dst = self.generation_lists[next_gen].write();

                // Re-check under locks: object might have been untracked concurrently
                if obj.gc_generation() != obj_gen || !obj.is_gc_tracked() {
                    continue;
                }

                if unsafe { src.remove(ptr) }.is_some() {
                    self.generations[src_gen]
                        .count
                        .fetch_sub(1, Ordering::SeqCst);

                    dst.push_front(ptr);
                    self.generations[next_gen]
                        .count
                        .fetch_add(1, Ordering::SeqCst);

                    obj.set_gc_generation(next_gen as u8);
                }
            }
        }
    }

    /// Get count of frozen objects
    pub fn get_freeze_count(&self) -> usize {