Cannot understand this error: type parameter `T` must be covered by another type when it appears before the first local type

Question

When I try the code bellow (this is a small example to reproduce my problem), I have this error from the compiler:

error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Message`)

use serde::{Serialize, Deserialize};

// A simple struct that can be serialized and deserialized
#[derive(Debug, Serialize, Deserialize)]
struct MyStruct(u64, String);

// A struct Message that can be converted into any deserializable object
struct Message(String);

impl<T: for<'a> Deserialize<'a>> TryFrom<Message> for T {
    type Error = serde_json::Error;
    fn try_from(value: Message) -> Result<Self, Self::Error> {
        serde_json::from_str(value.0)
    }
}

fn main() {
    let my_message = Message(
        serde_json::to_string(MyStruct(123, "Hello".to_string()))
    );
    
    let result_from_message = MyStruct::TryFrom(my_message).unwrap();
    println!("{result_from_message:?}");
}

I do not quite understand it, may I have some help?

Answer 1

Thanks for your help everyone, I think I get it now!

I finally rethink my code this way:

use serde::de::DeserializeOwned;
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct MyStruct(u64, String);

struct Message(String);

impl Message {
    fn into<T: DeserializeOwned>(self) -> Result<T, serde_json::Error> {
        serde_json::from_str(&self.0)
    }
}

fn main() {
    let message =
        Message(serde_json::to_string(&MyStruct(58, "Hello, World!".to_string())).unwrap());

    let my_struct = message.into::<MyStruct>().unwrap();
    println!("{my_struct:?}");
}