How can I mass implement Deserialize for all types that implement a specific trait?

Question

I am deserializing a YAML config file with Serde. For most structs I deserialize into, things are quite simple — there's a one-to-one relationship between the fields of the structs and the properties in my YAML file.

In a few cases, things are a bit more complicated. For these, the properties in the YAML file are better viewed as parameters to the constructor. The actual struct will have different fields, calculated from those.

For these cases, I have written separate config structs that I deserialize into. For simplicity, consider this silly example:

struct Message {
    text: String,
}

impl Message {
    fn from_config(config: MessageConfig) -> Message {
        Message {
            text: format!("{} {}", config.first_half, config.second_half),
        }
    }
}

#[derive(Deserialize)]
struct MessageConfig {
    first_half: String,
    second_half: String,
}

To have Serde do the conversion from MessageConfig to Message for me, I implemented Deserialize for Message :

impl<'de> Deserialize<'de> for Message {
    fn deserialize<D>(deserializer: D) -> Result<Message, D::Error>
    where
        D: Deserializer<'de>,
    {
        MessageConfig::deserialize(deserializer).map(|config| Message::from_config(config))
    }
}

This works, but there would be a lot of copy pasting of the deserialization code involved if I were to do this for every struct, so I figured I should make a trait out of it:

use serde::{Deserialize, Deserializer};
use serde_json;
#[macro_use]
extern crate serde_derive;

trait Configurable {
    type Config;
    fn from_config(config: Self::Config) -> Self;
}

impl<'de, T, C> Deserialize<'de> for T
where
    T: Configurable<Config = C>,
    C: Deserialize<'de>,
{
    fn deserialize<D>(deserializer: D) -> Result<T, D::Error>
    where
        D: Deserializer<'de>,
    {
        Self::Config::deserialize(deserializer).map(|config| Self::from_config(config))
    }
}

struct Message {
    text: String,
}

impl<'de> Configurable for Message {
    type Config = MessageConfig;

    fn from_config(config: MessageConfig) -> Message {
        Message {
            text: format!("{} {}", config.first_half, config.second_half),
        }
    }
}

#[derive(Deserialize)]
struct MessageConfig {
    first_half: String,
    second_half: String,
}

However, the compiler is not happy about this:

error[E0119]: conflicting implementations of trait `_IMPL_DESERIALIZE_FOR_MessageConfig::_serde::Deserialize<'_>` for type `std::boxed::Box<_>`:
  --> src/lib.rs:11:1
   |
11 | / impl<'de, T, C> Deserialize<'de> for T
12 | | where
13 | |     T: Configurable<Config = C>,
14 | |     C: Deserialize<'de>,
...  |
21 | |     }
22 | | }
   | |_^
   |
   = note: conflicting implementation in crate `serde`:
           - impl<'de, T> _IMPL_DESERIALIZE_FOR_MessageConfig::_serde::Deserialize<'de> for std::boxed::Box<T>
             where T: _IMPL_DESERIALIZE_FOR_MessageConfig::_serde::Deserialize<'de>;
   = note: downstream crates may implement trait `Configurable` for type `std::boxed::Box<_>`

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g. `MyStruct<T>`)
  --> src/lib.rs:11:1
   |
11 | / impl<'de, T, C> Deserialize<'de> for T
12 | | where
13 | |     T: Configurable<Config = C>,
14 | |     C: Deserialize<'de>,
...  |
21 | |     }
22 | | }
   | |_^ type parameter `T` must be used as the type parameter for some local type
   |
   = note: only traits defined in the current crate can be implemented for a type parameter

The error messages make little sense to me. What does Box have to do with anything? And is it somehow possible to make this trait work?

Answer 1

I am not sure if there's a way to define such a broad trait without causing conflicting implementations. What you could do is use a macro to avoid repetition:

use serde::{Deserialize, Deserializer};
use serde_json;

use serde_json::Error;

#[macro_use]
extern crate serde_derive;

struct Message {
    text: String,
}

#[derive(Deserialize)]
struct MessageConfig {
    first_half: String,
    second_half: String,
}

impl Message {
    fn from_config(config: MessageConfig) -> Message {
        Message {
            text: format!("{} {}", config.first_half, config.second_half),
        }
    }
}

macro_rules! derive_configurable_serializer {
    ( $t:ident, $c:ident ) => {
        impl<'de> Deserialize<'de> for $t {
            fn deserialize<D>(deserializer: D) -> Result<$t, D::Error>
            where
                D: Deserializer<'de>,
            {
                $c::deserialize(deserializer).map(|config| $t::from_config(config))
            }
        }
    };
}

derive_configurable_serializer!(Message, MessageConfig);

fn main() -> Result<(), Error> {
    let data = r#"{ "first_half": "John", "second_half": "Doe" }"#;

    let p: Message = serde_json::from_str(data)?;

    println!("Hello, {}!", p.text);

    Ok(())
}