What causes "Can't delete {:?} mid-drag" in a-b-street/abstreet?

Defer the delete until the drag finishes (end drag handler performs the delete).

Can't delete mid-drag

Error message

Can't delete {:?} mid-drag

What it means

World::maybe_delete panics if asked to delete the object currently being dragged. Deleting mid-drag would leave the drag state (dragging_from) pointing at a removed object, breaking internal invariants, so even the tolerant delete refuses in that case.

Solutions

Defer the delete until the drag finishes (end drag handler performs the delete).
Cancel/end the drag state before calling maybe_delete.
Check world's dragging state (if exposed) before deleting a hovered object.

Example fix

// before
world.maybe_delete(hovered_id);
// after
if !world.is_dragging() { world.maybe_delete(hovered_id); } else { pending_deletes.push(hovered_id); }

Defensive patterns

Strategy: validation

Validate before calling

// Defer deletes during active drags
if drag_in_progress && hovered == Some(id) { defer_delete(id); } else { world.maybe_delete(id); }

Prevention

Queue deletions triggered by keyboard/timers until mouse-up
Never mutate the World's object set from inside drag handlers
Document that hovered+dragging objects must not be deleted

When it happens

Trigger: Calling maybe_delete(id) while that same ID is hovered AND a drag is in progress (dragging_from is Some) — e.g. deleting an object from a keyboard/timer event during an active mouse drag.

Common situations: Key handlers or async events removing objects while the user still holds the mouse button on them; test harnesses simulating drag+delete concurrently.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/e8872361c37e46b7. Report an issue: GitHub.

Appendix: source

Thrown at widgetry/src/mapspace/world.rs:422

    pub fn delete_before_replacement(&mut self, id: ID) {
        if self.objects.remove(&id).is_some() {
            if self.quadtree.remove(id).is_none() {
                // This can happen for objects that're out-of-bounds. One example is intersections
                // in map_editor.
                warn!("{:?} wasn't in the quadtree", id);
            }
        } else {
            panic!("Can't delete {:?}; it's not in the World", id);
        }
    }

    /// Like delete, but doesn't crash if the object doesn't exist
    pub fn maybe_delete(&mut self, id: ID) {
        if self.hovering == Some(id) {
            self.hovering = None;
            self.draw_hovering = None;
            if self.dragging_from.is_some() {
                panic!("Can't delete {:?} mid-drag", id);
            }
        }

        if self.objects.remove(&id).is_some() {
            if self.quadtree.remove(id).is_none() {
                // This can happen for objects that're out-of-bounds. One example is intersections
                // in map_editor.
                warn!("{:?} wasn't in the quadtree", id);
            }
        }
    }

    /// After adding all objects to a `World`, call this to initially detect if the cursor is
    /// hovering on an object. This may also be called after adding or deleting objects to
    /// immediately recalculate hover before the mouse moves.
    // TODO Maybe we should automatically do this after mutations? Except we don't want to in the
    // middle of a bulk operation, like initial setup or a many-step mutation. So maybe the caller
    // really should handle it.

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