Function register_edge_handler 

pub fn register_edge_handler(
    pin: Pin,
    edges: Edges,
    handler: extern "C" fn(u8, *mut ()),
    ctx: *mut (),
    nvic_priority: u8,
) -> Result<(), PosixError>

Install an edge handler for pin and unmask the line. EBUSY if the line is already claimed — call unregister_edge_handler first. The caller registers dispatch at the matching irq_slot_for_line (once per slot).