How to cast a &trait to a &ConcreteType

gistfile1.txt

// Trait for node implementations (bezier paths, circles, etc.)
pub trait NodeTrait {
    fn set_atts (&self, ...);
    fn draw (&self, ...);
}

// Node in the tree of SVG elements
pub struct Node {
    node_type: NodeType,
    parent:    Option<Weak<Node>>,
    children:  RefCell<Vec<Rc<Node>>>,
    state:     *mut RsvgState,
    node_impl: Box<NodeTrait>
}

// Wrapper for node types that are implemented in C; c_node_impl is the C-side struct
struct CNode {
    c_node_impl: *const libc::c_void,
    ... pointers to C functions ...
}

impl NodeTrait for CNode {
    ... call the corresponding C functions from our NodeTrait methods ...
}

// This is the exported function that the C code calls to implement a node type
#[no_mangle]
pub extern fn rsvg_rust_cnode_new (...,
                                   c_node_impl: *const libc::c_void,
                                   ...) -> *const Rc<Node> {
    ...
}

// The C code sometimes needs to get the c_node_impl *outside* of a NodeTrait method call
#[no_mangle]
pub extern fn rsvg_rust_cnode_get_impl (raw_node: *const Rc<Node>) -> *const libc::c_void {
    assert! (!raw_node.is_null ());
    let node: &Rc<Node> = unsafe { & *raw_node };
    
    (&*node as &CNode).c_node_impl   // <---- can't cast to the specific type like that!
}