sval

A lightweight, no-std, object-safe, serialization-only API for structured values with serde and std::fmt support.

Producers of structured values use the value module. Consumers of structured values use the stream module.

sval offers a JSON-like data model, which is more limiting than serde's, but capable enough to represent Rust data-structures in one form or another.

This library is designed to plug a no-std-object-safe sized hole in Rust's current serialization ecosystem. The driving use-case is structured logging, where individual events are typically small, and there's no complete schema that can tie values in any one event to values in another.

sval_json and sval_derive are mostly pilfered from dtolnay's excellent miniserde project.

Supported formats

• JSON, the ubiquitous JavaScript Object Notation used by many HTTP APIs.

Minimum rustc

This library requires Rust 1.31.0.

See also

• serde
• miniserde

Cargo features

sval has the following optional features that can be enabled in your Cargo.toml:

• std: assume std is available and add support for std types.
• derive: add support for #[derive(Value)].
• serde: enable integration with serde.
  • serde_std (same as serde): enable serde integration along with std. This is for maximum ecosystem compatibility.
  • serde_no_std: enable serde integration without needing std. Some implementations of sval::Value may not be representable. This is for specialized no_std use-cases.
• fmt: support converting any Value into a Debug.
• arbitrary-depth: support stateful values with any depth.
• test: add helpers for testing implementations of Value.

How to use it

Add sval to your crate dependencies:

[dependencies.sval]
version = "0.4.6"

To support my data-structures

Simple struct-like data-structures can derive sval::Value:

[dependencies.sval]
features = ["derive"]

#[macro_use]
extern crate sval;

#[derive(Value)]
struct MyData {
    id: u64,
    name: String,
}

Other data-structures can implement sval::Value manually:

use sval::value::{self, Value};

struct MyId(u64);

impl Value for MyId {
    fn stream(&self, stream: &mut value::Stream) -> value::Result {
        stream.u64(self.0)
    }
}

To format my data

The sval_json crate can format any sval::Value as JSON:

[dependencies.sval_json]
version = "0.4.6"
features = ["std"]

let my_json = sval_json::to_string(my_data)?;

To integrate with serde

sval has out-of-the-box serde integration between sval::Values and serde::Serializes. Add the serde feature to sval to enable it:

[dependencies.sval]
features = ["serde"]

Use the to_serialize function to turn any sval::Value into a serde::Serialize:

let my_serialize = sval::serde::to_serialize(my_data);

Use the to_value function to turn any serde::Serialize into a sval::Value:

let my_value = sval::serde::to_value(my_data);

When the serde feature is available, structures that already derive Serialize can also always derive Value. The Value implementation will behave the same as Serialize:

#[derive(Serialize, Value)]
#[sval(derive_from = "serde")]
struct MyData {
    id: u64,
    name: String,
    #[serde(flatten)]
    props: serde_json::Map<String, serde_json::Value>,
}

To integrate with std::fmt

sval can provide a compatible Debug implementation for any sval::Value. Add the fmt feature to sval to enable it:

[dependencies.sval]
features = ["fmt"]

Use the to_debug function to turn any sval::Value into a std::fmt::Debug:

fn with_value(value: impl Value) {
    dbg!(sval::fmt::to_debug(&value));

    ..
}