Function rect_shape_params 

pub fn rect_shape_params(
    local_rect: Rect,
    brush: &Brush,
    stroke: Option<Stroke>,
    layer_bounds: Rect,
    layer: &GraphicsLayer,
    clip: Option<Rect>,
    blend_mode: BlendMode,
    motion_context_animated: bool,
) -> Option<ShapeDrawParams>

The DrawPrimitive::Rect arm of emit_draw_primitive as a pure builder over borrowed fields. The parallel shape-run collect calls these directly from worker threads — a &DrawPrimitive cannot cross (the text variant carries Rc), but the shape variants’ fields can.