rust - 如何使调用者看到方法的特征实现？

Question

带有代码:

trait Msg {
    fn apply_to_state(&self, state: &mut State);
}

trait ApplicableToStateOne: Msg {
    fn apply_to_state(&self, state: &mut State) {
        match state {
            State::StateOne(s) => {
                self.apply_to_state_one(s) 
            }
            _ => {
                //TODO: return an error
            }
        }
    }
    fn apply_to_state_one(&self, state_one: &mut StateOne);
}

#[derive(Debug, Clone)]
pub struct MsgA {
    pub field_a: u8,
}

impl Msg for MsgA {}
impl ApplicableToStateOne for MsgA {
    fn apply_to_state_one(&self, state_one: &mut StateOne) {
        state_one.one_special += 31; // just a mutability test
    }
}

// this is a stub for receiving different kinds of messages from the network
fn recv() -> Box<dyn Msg> {
    Box::new(MsgA { field_a: 42 })
}

fn main() {
    let mut state = State::StateOne(StateOne { common: 0, one_special: 1 });
    for _ in 0..100 { // this would be loop, but that makes the playground timeout
        let incoming = recv(); // this would block
        incoming.apply_to_state(&mut state)
    }
}

(游乐场: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7c89a2bbc765380fc002864e2be80e55)
编译器提示:

error[E0046]: not all trait items implemented, missing: `apply_to_state`
  --> src/bin/sandbox6.rs:83:1
   |
2  |     fn apply_to_state(&self, state: &mut State);
   |     -------------------------------------------- `apply_to_state` from trait
...
83 | impl Msg for MsgA {}
   | ^^^^^^^^^^^^^^^^^ missing `apply_to_state` in implementation

在我的(显然是不足的)理解中，我期望会调用 trait ApplicableToStateOne的 apply_to_state实现。
我怎样才能做到这一点？

更新:
更抽象地讲，这个问题是关于:

从网络接收一个盒装的超特质对象，然后

找出它具有哪个子特性，最后

调用适合于子特征的方法(也许以某种方式通过 super 特征)。

可以使用枚举而不是特征来完成所有操作，而只是冗长而已，但这会增加枚举的层次结构。
具有枚举的层次结构是不好的，因为:

它将不必要的字节添加到序列化中，并且因为

只能有一个这样的层次结构，这会阻止以任何其他方式对消息进行分类。

Answer 1

您可以使用泛型来为实现Msg的所有内容实现ApplicableToStateOne:

struct State {}

trait Msg {
    fn apply_to_state(&self, state: &mut State);
}

trait ApplicableToStateOne: Msg {
    fn apply_to_state_one(&self, state: &mut State) {
        todo!();
    }
}

impl<T: ApplicableToStateOne> Msg for T {
    fn apply_to_state(&self, state: &mut State) {
        self.apply_to_state_one (state);
    }
}

#[derive(Debug, Clone)]
pub struct MsgA {
    pub field_a: u8,
}

impl ApplicableToStateOne for MsgA {}
// No need to implement Msg explicitly for MsgA

Playground