Implement RawValue type (alternative)

[dtolnay]

This PR builds on #480 but eliminates the Cow in favor of distinct &RawValue and Box<RawValue> options.

Documentation of RawValue:

---

Reference to a range of bytes encompassing a single valid JSON value in the input data.

A RawValue can be used to defer parsing parts of a payload until later, or to avoid parsing it at all in the case that part of the payload just needs to be transferred verbatim into a different output object.

When serializing, a value of this type will retain its original formatting and will not be minified or pretty-printed.

Example

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

use serde_json::{Result, value::RawValue};

#[derive(Deserialize)]
struct Input<'a> {
    code: u32,
    #[serde(borrow)]
    payload: &'a RawValue,
}

#[derive(Serialize)]
struct Output<'a> {
    info: (u32, &'a RawValue),
}

// Efficiently rearrange JSON input containing separate "code" and "payload"
// keys into a single "info" key holding an array of code and payload.
//
// This could be done equivalently using serde_json::Value as the type for
// payload, but &RawValue will perform netter because it does not require
// memory allocation. The correct range of bytes is borrowed from the input
// data and pasted verbatim into the output.
fn rearrange(input: &str) -> Result<String> {
    let input: Input = serde_json::from_str(input)?;

    let output = Output {
        info: (input.code, input.payload),
    };

    serde_json::to_string(&output)
}

fn main() -> Result<()> {
    let out = rearrange(r#" {"code": 200, "payload": {}} "#)?;

    assert_eq!(out, r#"{"info":[200,{}]}"#);

    Ok(())
}

Ownership

The typical usage of RawValue will be in the borrowed form:

#[derive(Deserialize)]
struct SomeStruct<'a> {
    #[serde(borrow)]
    raw_value: &'a RawValue,
}

The borrowed form is suitable when deserializing through serde_json::from_str and serde_json::from_slice which support borrowing from the input data without memory allocation.

When deserializing through serde_json::from_reader you will need to use the boxed form of RawValue instead. This is almost as efficient but involves buffering the raw value from the I/O stream into memory.

#[derive(Deserialize)]
struct SomeStruct {
    raw_value: Box<RawValue>,
}

[srijs]

I like where this is going! Anything I can pick up or help with?