Function arc_shape_params 

pub fn arc_shape_params(
    local_rect: Rect,
    brush: &Brush,
    center: Point,
    radius: f32,
    start_angle: f32,
    sweep_angle: f32,
    stroke: Option<Stroke>,
    inner_radius: f32,
    layer_bounds: Rect,
    layer: &GraphicsLayer,
    clip: Option<Rect>,
    blend_mode: BlendMode,
    motion_context_animated: bool,
) -> Option<ShapeDrawParams>

The DrawPrimitive::Arc arm of emit_draw_primitive; see rect_shape_params.