Add a RawValue type

[alteous]

It would be helpful to have a type similar to Go's json.RawMessage that is not tokenized during deserialization, but rather its raw contents stored as a Vec<u8> or &'de [u8].

The following pseudo-code demonstrates the idea.

#[derive(Deserialize)]
struct Struct {
    /// Deserialized normally.
    core_data: Vec<i32>,

    /// Contents of `user_data` are copied / borrowed directly from the input
    /// with no modification.
    ///
    /// `RawValue<'static>` is akin to `Vec<u8>`.
    /// `RawValue<'a>` is akin to `&'a [u8]`.
    user_data: serde_json::RawValue<'static>,
}

fn main() {
    let json = r#"
    {
        "core_data": [1, 2, 3],
        "user_data": { "foo": {}, "bar": 123, "baz": "abc" }
    }
    "#;
    
    let s: Struct = serde_json::from_bytes(&json).unwrap();
    println!("{}", s.user_data); // "{ \"foo\": {}, \"bar\": 123, \"baz\": \"abc\" }"
}

The main advantage of this is would be to have 'lazily-deserialized' values.

[shepmaster]

@dtolnay mentions that the work happening in #416 might lay some groundwork for such a change to take place.

[alexreg]

Yeah, this sounds useful. Ideally we could get some of the building blocks for this sort of thing (custom 'built-in' types) integrated into serde itself, in time.

[srijs]

I need this for a project I'm working on, and would be happy to give it shot following the same approach as #416. Is this something that you'd accept a PR for?

[dtolnay]

Yes I would like a PR for this. When you say you need this feature, is it because deserializing to serde_json::Value is too slow in your project and you are expecting this to have better performance, or some other reason?

[srijs]

Yeah, I've got an http web server that's backed by a postgres database which stores JSONB values, and would like to shuffle data back and forth (and embed into other payloads) with as few allocations as possible.

[dtolnay]

As a first step, please put together a short benchmark that you trust as reasonably representative of the shuffling data back and forth piece of your application. Ideally it would depend on serde / serde_derive / serde_json but nothing else -- so no postgres or http stuff. Focus on the data structures so that they have the same ratio and pattern of raw vs meaningful content as your real use case. Write it using serde_json::Value for now, and then the goal will be to swap in a raw value once it's implemented and observe a speedup.

[srijs]

Wrote a small microbenchmark here: https://gist.github.com/srijs/7fe21b43c2cf5d2ceeb593ae4591c6f5

Please let me know if this is what you had in mind!

[dtolnay]

👍 looks good, let's make it work.