Deserializing a RawValue inside an untagged enum always fails

[untitaker]

cargo script compatible example:

//! ```cargo
//! [dependencies]
//! serde = "1.0.79"
//! serde_json = { version = "1.0.32", features = ["raw_value"] }
//! serde_derive = "1.0.79"
//! ```

#[macro_use] extern crate serde_derive;
extern crate serde;
extern crate serde_json;

#[derive(Debug, Deserialize, Serialize)]
struct FooNewtype<T>(T);

#[derive(Debug, Deserialize, Serialize)]
#[serde(untagged)]
enum FooEnum<T> {
    Single(T)
}

fn main() {
    // works
    let val: FooNewtype<Box<serde_json::value::RawValue>> = serde_json::from_str("42").unwrap();
    println!("{:?}", val);

    // thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("data did not match any variant of untagged enum FooEnum", line: 0, column: 0)', libcore/result.rs:945:5
    let val: FooEnum<Box<serde_json::value::RawValue>> = serde_json::from_str("42").unwrap();
    println!("{:?}", val);
}

I haven't figured out what's causing this yet.

[untitaker]

Ok, I think I get it now. RawValue assumes the Deserializer from serde-json but untagged enums use their own deserializer.

@dtolnay Do you have opinions on putting deserialize_raw onto the Deserialize trait (with a default impl that errors)? It's the first fix that comes to mind

[dtolnay]

Right, untagged enums are deserialized from a data structure that has been buffered into heap memory, not from the original JSON deserializer.

I don't think a deserialize_raw method is the right fix. The underlying limitation is the same as serde-rs/serde#1183. Both issues would be better resolved by supporting a customizable buffer type per deserializer (proof of concept in serde-rs/serde#1354).

[dtolnay]

Closing in favor of serde-rs/serde#1183 since this issue is a direct consequence.