Pumpkin-MC/Pumpkin · error

Cannot construct CrafterCraftEvent from WASM

Error message

Cannot construct CrafterCraftEvent from WASM

What it means

`from_wasm_event` for CrafterCraftEvent unconditionally panics when it receives an Event::CrafterCraftEvent. This impl exists only to satisfy the trait; the crate deliberately forbids a plugin from *returning* a CrafterCraftEvent to the host (the event flows host->plugin only). Hitting this means a WASM plugin handed a CrafterCraftEvent back across the boundary.

Solutions

1. Fix the plugin so it never returns the CrafterCraftEvent it received; consume it and return the expected value instead.
2. Check the plugin's generated bindings for a wrong return type on the event handler method.
3. Update the plugin SDK to match the host's WIT version so the handler's return type is correct.
4. If you need the host to observe the event, use the cancellation/data-mutation API rather than returning the event.

Example fix

// plugin side, before
fn on_craft(event: CrafterCraftEvent) -> Event { Event::CrafterCraftEvent(event) }
// after
fn on_craft(event: CrafterCraftEvent) -> Event { let _ = event; /* handle, don't re-emit */ }

// plugin side: never return the event object you received
assert!(!matches!(return_value, Some(Event::CrafterCraftEvent(_))), "do not return CrafterCraftEvent to host");

// host-side narrowing before calling from_wasm_event
fn is_crafter_event(e: &Event) -> bool { matches!(e, Event::CrafterCraftEvent(_)) }

When it happens

Trigger: A plugin method whose WIT signature allows returning an Event returns Event::CrafterCraftEvent, and the host calls ToFromWasmEvent::from_wasm_event on it.

Common situations: A plugin (or generated bindings) misusing the event API by echoing/re-emitting the event object it received; a plugin language binding that constructs the wrong direction of event.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot construct EntityBlockFormEvent from WASM
• Cannot construct BlockReceiveGameEvent from WASM
• failed to add item stack resource
• Cannot construct BlockShearEntityEvent from WASM
• unexpected event type

AI-assisted analysis of Pumpkin-MC/Pumpkin@8d4639e25a (2026-09-09). Data as JSON: /api/errors/6e7e2bceb033629e. Report an issue: GitHub.

Appendix: source

Thrown at crates/pumpkin/src/plugin/loader/wasm/wasm_host/wit/v0_1/events/block.rs:1180

            .expect("failed to add item stack resource");
        Event::CrafterCraftEvent(CrafterCraftEventData {
            block_pos: to_wasm_block_position(self.block_pos),
            target_world,
            result,
            cancelled: self.cancelled,
        })
    }

    fn apply_wasm_event(&mut self, event: Event, state: &mut PluginHostState) {
        cleanup_event(&event, state);
        if let Event::CrafterCraftEvent(data) = event {
            self.cancelled = data.cancelled;
        }
    }

    fn from_wasm_event(event: Event, _state: &mut PluginHostState) -> Self {
        match event {
            Event::CrafterCraftEvent(_) => panic!("Cannot construct CrafterCraftEvent from WASM"),
            _ => panic!("unexpected event type"),
        }
    }
}

impl ToFromWasmEvent for EntityBlockFormEvent {
    fn to_wasm_event(&self, state: &mut PluginHostState) -> Event {
        let target_world = state
            .add_world(self.world.clone())
            .expect("failed to add world resource");
        Event::EntityBlockFormEvent(EntityBlockFormEventData {
            entity_id: self.entity.get_entity().entity_id,
            block_pos: to_wasm_block_position(self.block_pos),
            target_world,
            new_state_id: self.new_state_id.as_u16(),
            cancelled: self.cancelled,
        })
    }

View on GitHub (pinned to 8d4639e25a)