Erased Serde

This crate provides type-erased versions of Serde's Serialize, Serializer and Deserializer traits that can be used as trait objects.

The usual Serde Serialize, Serializer and Deserializer traits cannot be used as trait objects like &dyn Serialize or boxed trait objects like Box<dyn Serialize> because of Rust's "object safety" rules. In particular, all three traits contain generic methods which cannot be made into a trait object.

This library should be considered a low-level building block for interacting with Serde APIs in an object-safe way. Most use cases will require higher level functionality such as provided by typetag which uses this crate internally.

The traits in this crate work seamlessly with any existing Serde Serialize and Deserialize type and any existing Serde Serializer and Deserializer format.

[dependencies]
serde = "1.0"
erased-serde = "0.4"

Serialization

use erased_serde::{Serialize, Serializer};
use std::collections::BTreeMap as Map;
use std::io;

fn main() {
    // Construct some serializers.
    let json = &mut serde_json::Serializer::new(io::stdout());
    let cbor = &mut serde_cbor::Serializer::new(serde_cbor::ser::IoWrite::new(io::stdout()));

    // The values in this map are boxed trait objects. Ordinarily this would not
    // be possible with serde::Serializer because of object safety, but type
    // erasure makes it possible with erased_serde::Serializer.
    let mut formats: Map<&str, Box<dyn Serializer>> = Map::new();
    formats.insert("json", Box::new(<dyn Serializer>::erase(json)));
    formats.insert("cbor", Box::new(<dyn Serializer>::erase(cbor)));

    // These are boxed trait objects as well. Same thing here - type erasure
    // makes this possible.
    let mut values: Map<&str, Box<dyn Serialize>> = Map::new();
    values.insert("vec", Box::new(vec!["a", "b"]));
    values.insert("int", Box::new(65536));

    // Pick a Serializer out of the formats map.
    let format = formats.get_mut("json").unwrap();

    // Pick a Serialize out of the values map.
    let value = values.get("vec").unwrap();

    // This line prints `["a","b"]` to stdout.
    value.erased_serialize(format).unwrap();
}

Deserialization

use erased_serde::Deserializer;
use std::collections::BTreeMap as Map;

fn main() {
    static JSON: &[u8] = br#"{"A": 65, "B": 66}"#;
    static CBOR: &[u8] = &[162, 97, 65, 24, 65, 97, 66, 24, 66];

    // Construct some deserializers.
    let json = &mut serde_json::Deserializer::from_slice(JSON);
    let cbor = &mut serde_cbor::Deserializer::from_slice(CBOR);

    // The values in this map are boxed trait objects, which is not possible
    // with the normal serde::Deserializer because of object safety.
    let mut formats: Map<&str, Box<dyn Deserializer>> = Map::new();
    formats.insert("json", Box::new(<dyn Deserializer>::erase(json)));
    formats.insert("cbor", Box::new(<dyn Deserializer>::erase(cbor)));

    // Pick a Deserializer out of the formats map.
    let format = formats.get_mut("json").unwrap();

    let data: Map<String, usize> = erased_serde::deserialize(format).unwrap();

    println!("{}", data["A"]   data["B"]);
}

How it works

This crate is based on a general technique for building trait objects of traits that have generic methods (like all of Serde's traits). This example code demonstrates the technique applied to a simplified case of a single generic method. Try it in the playground.

In erased-serde things are a bit more complicated than in the example for three reasons but the idea is the same.

We need to deal with trait methods that take self by value -- effectively by implementing the object-safe trait for Option<T> where T implements the real trait.
We need to deal with traits that have associated types like Serializer::Ok and Visitor::Value -- by carefully short-term stashing things behind a pointer.
We need to support trait methods that have a generic type in the return type but none of the argument types, like SeqAccess::next_element -- this can be flipped around into a callback style where the return value is instead passed on to a generic argument.

In the future maybe the Rust compiler will be able to apply this technique automatically to any trait that is not already object safe by the current rules.

License

^{Licensed under either of Apache License, Version
2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.}