问对于已实现特征的现有类型枚举，如何在作用域中实现特征？

EN

extern crate pnet;

use pnet::packet::ipv4::Ipv4Packet;
use pnet::packet::ipv6::Ipv6Packet;

enum EthernetType {
    IPv4,
    ARP,
    VLAN,
    IPv6,
    Unknown(u16),
}

enum IPPacket<'a> {
    IPv4(Ipv4Packet<'a>),
    IPv6(Ipv6Packet<'a>),
}

fn ip_decode(pkt: &[u8]) -> IPPacket {
    let version = (pkt[0] & 0xf0) >> 4;
    if version == 4 {
        IPPacket::IPv4(Ipv4Packet::new(&pkt).unwrap())
    } else {
        IPPacket::IPv6(Ipv6Packet::new(&pkt).unwrap())
    }
}

fn main() {
    // Parse ethernet packet here...
    // ...
    let ip_packet = ip_decode(b"deadbeef");
    println!("{:?}", ip_packet.payload());
}

error[E0599]: no method named `payload` found for type `IPPacket<'_>` in the current scope
  --> src/main.rs:32:32
   |
14 | enum IPPacket<'a> {
   | ----------------- method `payload` not found for this
...
32 |     println!("{:?}", ip_packet.payload());
   |                                ^^^^^^^
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `payload`, perhaps you need to implement it:
           candidate #1: `pnet::packet::Packet`