swc-project/swc · error

free list points to occupied slot

Error message

free list points to occupied slot

What it means

An internal consistency panic in `insert` of the slot-arena (swc_arena lib.rs:217). When reusing a freed slot from the internal free list, the code expects the slot's entry to still be Vacant; if it finds the slot occupied (entry no longer Vacant), the free list has been corrupted — typically by use-after-free of an Id, double-free, or unsafe aliasing of arena mutations. The corrupted free-list state, not user payload data, is what trips it.

Source

Thrown at crates/swc_arena/src/lib.rs:217

    /// Inserts a value and returns its id.
    ///
    /// Reuses freed slots through an internal free list.
    ///
    /// # Panics
    ///
    /// Panics if the arena has reached its maximum number of slots.
    #[inline]
    pub fn insert(&mut self, value: T) -> Id<T> {
        if self.free_head != INVALID_INDEX {
            let index = self.free_head as usize;
            let slot = self
                .slots
                .get_mut(index)
                .expect("free list index should be valid");

            let next = match &slot.entry {
                SlotEntry::Vacant { next } => *next,
                SlotEntry::Occupied(_) => unreachable!("free list points to occupied slot"),
            };

            self.free_head = next;
            self.len += 1;
            slot.entry = SlotEntry::Occupied(value);

            return Id::from_parts(index as u32, slot.generation);
        }

        assert!(
            self.slots.len() < MAX_SLOTS,
            "arena reached maximum slot count"
        );

        let index = self.slots.len() as u32;
        self.slots.push(Slot {
            generation: INITIAL_GENERATION,
            entry: SlotEntry::Occupied(value),

View on GitHub (pinned to 5176682b65)

Solutions

1. Audit for use-after-free: ensure Ids are not used to mutate the arena after the slot was freed
2. Check for double-frees of the same Id
3. Add a debug assertion when freeing that the slot is still Vacant before linking it into the free list, to catch corruption earlier

When it happens

Trigger: Thrown at crates/swc_arena/src/lib.rs:217 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/b10ef428bc81d7c1. Report an issue: GitHub.