Serde Rust Serialization Framework

Serde is a powerful framework that enables serialization libraries to generically serialize Rust data structures without the overhead of runtime type information. In many situations, the handshake protocol between serializers and serializees can be completely optimized away, leaving Serde to perform roughly the same speed as a hand written serializer for a specific type.

Documentation

Simple Serde Example

Here is a simple example that uses serde_json, which uses Serde under the covers, to generate and parse JSON. First, lets start off with the Cargo.toml file:

[package]
name = "serde_example"
version = "0.1.0"
authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>"]

[dependencies]
serde_json = "0.8"

Next, the src/main.rs file itself:

extern crate serde_json;

use std::collections::HashMap;
use serde_json::Value;
use serde_json::builder::{ArrayBuilder, ObjectBuilder};

fn main() {
    // Serde has support for many of the builtin Rust types, like arrays..:
    let v = vec![1, 2];
    let serialized = serde_json::to_string(&v).unwrap();
    println!("serialized vec: {:?}", serialized);

    let deserialized: Vec<u32> = serde_json::from_str(&serialized).unwrap();
    println!("deserialized vec: {:?}", deserialized);

    // ... and maps:
    let mut map = HashMap::new();
    map.insert("x".to_string(), 1);
    map.insert("y".to_string(), 2);

    let serialized = serde_json::to_string(&map).unwrap();
    println!("serialized map: {:?}", serialized);

    let deserialized: HashMap<String, u32> = serde_json::from_str(&serialized).unwrap();
    println!("deserialized map: {:?}", deserialized);

    // It also can handle complex objects:
    let value = ObjectBuilder::new()
        .insert("int", 1)
        .insert("string", "a string")
        .insert("array", ArrayBuilder::new()
                .push(1)
                .push(2)
                .unwrap())
        .unwrap();

    let serialized = serde_json::to_string(&value).unwrap();
    println!("serialized value: {:?}", serialized);

    let deserialized: serde_json::Value = serde_json::from_str(&serialized).unwrap();
    println!("deserialized value: {:?}", deserialized);
}

This produces the following output when run:

% cargo run
serialized vec: "[1,2]"
deserialized vec: [1, 2]
serialized map: "{\"y\":2,\"x\":1}"
deserialized map: {"y": 2, "x": 1}
serialized value: "{\"array\":[1,2],\"int\":1,\"string\":\"a string\"}"
deserialized value: {"array":[1,2],"int":1,"string":"a string"}

Using Serde with Stable Rust and serde_codegen

The example before used serde_json::Value as the in-memory representation of the JSON value, but it's also possible for Serde to serialize to and from regular Rust types. However, the code to do this can be a bit complicated to write. So instead, Serde also has some powerful code generation libraries that work with Stable and Nightly Rust that eliminate much of the complexity of hand rolling serialization and deserialization for a given type.

First lets see how we would use Stable Rust, which is currently a tad more complicated than Nightly Rust due to having to work around compiler plugins being unstable. We will use serde_codegen which is based on the code generation library syntex. First we need to setup the Cargo.toml that builds the project:

[package]
name = "serde_example"
version = "0.1.0"
authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>"]
build = "build.rs"

[build-dependencies]
serde_codegen = "0.8"

[dependencies]
serde = "0.8"
serde_json = "0.8"

Next, we define our source file, src/main.rs.in. Note this is a different extension than usual because we need to do code generation:

#[derive(Serialize, Deserialize, Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let point = Point { x: 1, y: 2 };

    let serialized = serde_json::to_string(&point).unwrap();
    println!("{}", serialized);

    let deserialized: Point = serde_json::from_str(&serialized).unwrap();
    println!("{:?}", deserialized);
}

To finish up the main source code, we define a very simple src/main.rs that uses the generated code.

src/main.rs:

extern crate serde;
extern crate serde_json;

include!(concat!(env!("OUT_DIR"), "/main.rs"));

The last step is to actually drive the code generation, with the build.rs script:

extern crate serde_codegen;

use std::env;
use std::path::Path;

pub fn main() {
    let out_dir = env::var_os("OUT_DIR").unwrap();

    let src = Path::new("src/main.rs.in");
    let dst = Path::new(&out_dir).join("main.rs");

    serde_codegen::expand(&src, &dst).unwrap();
}

All this produces this when run:

% cargo run
{"x":1,"y":2}
Point { x: 1, y: 2 }

While this works well with Stable Rust, be aware that the error locations currently are reported in the generated file instead of in the source file.

Using Serde with Nightly Rust and serde_macros

The prior example is a bit more complicated than it needs to be due to compiler plugins being unstable. However, if you are already using Nightly Rust, you can use serde_macros, which has a much simpler interface. First, here is the new Cargo.toml:

[package]
name = "serde_example_nightly"
version = "0.1.0"
authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>"]

[dependencies]
serde = "0.8"
serde_json = "0.8"
serde_macros = "0.8"

Note that it doesn't need a build script. Now the src/main.rs, which enables the plugin feature, and registers the serde_macros plugin:

#![feature(custom_derive, plugin)]
#![plugin(serde_macros)]

extern crate serde_json;

#[derive(Serialize, Deserialize, Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let point = Point { x: 1, y: 2 };

    let serialized = serde_json::to_string(&point).unwrap();
    println!("{}", serialized);

    let deserialized: Point = serde_json::from_str(&serialized).unwrap();
    println!("{:?}", deserialized);
}

This also produces the same output:

% cargo run
{"x":1,"y":2}
Point { x: 1, y: 2 }

You may find it easier to develop with Nightly Rust and serde\_macros, then deploy with Stable Rust and serde_codegen. It's possible to combine both approaches in one setup:

Cargo.toml:

[package]
name = "serde_example"
version = "0.1.0"
authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>"]
build = "build.rs"

[features]
default = ["serde_codegen"]
nightly = ["serde_macros"]

[build-dependencies]
serde_codegen = { version = "0.8", optional = true }

[dependencies]
serde = "0.8"
serde_json = "0.8"
serde_macros = { version = "0.8", optional = true }

build.rs:

#[cfg(not(feature = "serde_macros"))]
mod inner {
    extern crate serde_codegen;

    use std::env;
    use std::path::Path;

    pub fn main() {
        let out_dir = env::var_os("OUT_DIR").unwrap();

        let src = Path::new("src/main.rs.in");
        let dst = Path::new(&out_dir).join("main.rs");

        serde_codegen::expand(&src, &dst).unwrap();
    }
}

#[cfg(feature = "serde_macros")]
mod inner {
    pub fn main() {}
}

fn main() {
    inner::main();
}

src/main.rs:

#![cfg_attr(feature = "serde_macros", feature(custom_derive, plugin))]
#![cfg_attr(feature = "serde_macros", plugin(serde_macros))]

extern crate serde;
extern crate serde_json;

#[cfg(feature = "serde_macros")]
include!("main.rs.in");

#[cfg(not(feature = "serde_macros"))]
include!(concat!(env!("OUT_DIR"), "/main.rs"));

The src/main.rs.in is the same as before.

Then to run with stable:

% cargo build
...

Or with nightly:

% cargo build --features nightly --no-default-features
...

Serialization without Macros

Under the covers, Serde extensively uses the Visitor pattern to thread state between the Serializer and Serialize without the two having specific information about each other's concrete type. This has many of the same benefits as frameworks that use runtime type information without the overhead. In fact, when compiling with optimizations, Rust is able to remove most or all the visitor state, and generate code that's nearly as fast as a hand written serializer format for a specific type.

To see it in action, lets look at how a simple type like i32 is serialized. The Serializer is threaded through the type:

impl serde::Serialize for i32 {
    fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
        where S: serde::Serializer,
    {
        serializer.serialize_i32(*self)
    }
}

As you can see it's pretty simple. More complex types like BTreeMap need to pass a MapVisitor to the Serializer in order to walk through the type:

impl<K, V> Serialize for BTreeMap<K, V>
    where K: Serialize + Ord,
          V: Serialize,
{
    #[inline]
    fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
        where S: Serializer,
    {
        serializer.serialize_map(MapIteratorVisitor::new(self.iter(), Some(self.len())))
    }
}

pub struct MapIteratorVisitor<Iter> {
    iter: Iter,
    len: Option<usize>,
}

impl<K, V, Iter> MapIteratorVisitor<Iter>
    where Iter: Iterator<Item=(K, V)>
{
    #[inline]
    pub fn new(iter: Iter, len: Option<usize>) -> MapIteratorVisitor<Iter> {
        MapIteratorVisitor {
            iter: iter,
            len: len,
        }
    }
}

impl<K, V, I> MapVisitor for MapIteratorVisitor<I>
    where K: Serialize,
          V: Serialize,
          I: Iterator<Item=(K, V)>,
{
    #[inline]
    fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
        where S: Serializer,
    {
        match self.iter.next() {
            Some((key, value)) => {
                let value = try!(serializer.serialize_map_elt(key, value));
                Ok(Some(value))
            }
            None => Ok(None)
        }
    }

    #[inline]
    fn len(&self) -> Option<usize> {
        self.len
    }
}

Serializing structs follow this same pattern. In fact, structs are represented as a named map. Its visitor uses a simple state machine to iterate through all the fields:

extern crate serde;
extern crate serde_json;

struct Point {
    x: i32,
    y: i32,
}

impl serde::Serialize for Point {
    fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
        where S: serde::Serializer
    {
        serializer.serialize_struct("Point", PointMapVisitor {
            value: self,
            state: 0,
        })
    }
}

struct PointMapVisitor<'a> {
    value: &'a Point,
    state: u8,
}

impl<'a> serde::ser::MapVisitor for PointMapVisitor<'a> {
    fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
        where S: serde::Serializer
    {
        match self.state {
            0 => {
                self.state += 1;
                Ok(Some(try!(serializer.serialize_struct_elt("x", &self.value.x))))
            }
            1 => {
                self.state += 1;
                Ok(Some(try!(serializer.serialize_struct_elt("y", &self.value.y))))
            }
            _ => {
                Ok(None)
            }
        }
    }
}

fn main() {
    let point = Point { x: 1, y: 2 };
    let serialized = serde_json::to_string(&point).unwrap();

    println!("{}", serialized);
}

Deserialization without Macros

Deserialization is a little more complicated since there's a bit more error handling that needs to occur. Let's start with the simple i32 Deserialize implementation. It passes a Visitor to the Deserializer. The Visitor can create the i32 from a variety of different types:

impl Deserialize for i32 {
    fn deserialize<D>(deserializer: &mut D) -> Result<i32, D::Error>
        where D: serde::Deserializer,
    {
        deserializer.deserialize(I32Visitor)
    }
}

struct I32Visitor;

impl serde::de::Visitor for I32Visitor {
    type Value = i32;

    fn visit_i16<E>(&mut self, value: i16) -> Result<i32, E>
        where E: Error,
    {
        self.visit_i32(value as i32)
    }

    fn visit_i32<E>(&mut self, value: i32) -> Result<i32, E>
        where E: Error,
    {
        Ok(value)
    }

    ...

Since it's possible for this type to get passed an unexpected type, we need a way to error out. This is done by way of the Error trait, which allows a Deserialize to generate an error for a few common error conditions. Here's how it could be used:

    ...

    fn visit_string<E>(&mut self, _: String) -> Result<i32, E>
        where E: Error,
    {
        Err(serde::de::Error::custom("expect a string"))
    }

    ...

Maps follow a similar pattern as before, and use a MapVisitor to walk through the values generated by the Deserializer.

impl<K, V> serde::Deserialize for BTreeMap<K, V>
    where K: serde::Deserialize + Eq + Ord,
          V: serde::Deserialize,
{
    fn deserialize<D>(deserializer: &mut D) -> Result<BTreeMap<K, V>, D::Error>
        where D: serde::Deserializer,
    {
        deserializer.deserialize(BTreeMapVisitor::new())
    }
}

pub struct BTreeMapVisitor<K, V> {
    marker: PhantomData<BTreeMap<K, V>>,
}

impl<K, V> BTreeMapVisitor<K, V> {
    pub fn new() -> Self {
        BTreeMapVisitor {
            marker: PhantomData,
        }
    }
}

impl<K, V> serde::de::Visitor for BTreeMapVisitor<K, V>
    where K: serde::de::Deserialize + Ord,
          V: serde::de::Deserialize
{
    type Value = BTreeMap<K, V>;

    fn visit_unit<E>(&mut self) -> Result<BTreeMap<K, V>, E>
        where E: Error,
    {
        Ok(BTreeMap::new())
    }

    fn visit_map<V_>(&mut self, mut visitor: V_) -> Result<BTreeMap<K, V>, V_::Error>
        where V_: MapVisitor,
    {
        let mut values = BTreeMap::new();

        while let Some((key, value)) = try!(visitor.visit()) {
            values.insert(key, value);
        }

        try!(visitor.end());

        Ok(values)
    }
}

Deserializing structs goes a step further in order to support not allocating a String to hold the field names. This is done by custom field enum that deserializes an enum variant from a string. So for our Point example from before, we need to generate:

extern crate serde;
extern crate serde_json;

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

enum PointField {
    X,
    Y,
}

impl serde::Deserialize for PointField {
    fn deserialize<D>(deserializer: &mut D) -> Result<PointField, D::Error>
        where D: serde::de::Deserializer
    {
        struct PointFieldVisitor;

        impl serde::de::Visitor for PointFieldVisitor {
            type Value = PointField;

            fn visit_str<E>(&mut self, value: &str) -> Result<PointField, E>
                where E: serde::de::Error
            {
                match value {
                    "x" => Ok(PointField::X),
                    "y" => Ok(PointField::Y),
                    _ => Err(serde::de::Error::custom("expected x or y")),
                }
            }
        }

        deserializer.deserialize(PointFieldVisitor)
    }
}

impl serde::Deserialize for Point {
    fn deserialize<D>(deserializer: &mut D) -> Result<Point, D::Error>
        where D: serde::de::Deserializer
    {
        static FIELDS: &'static [&'static str] = &["x", "y"];
        deserializer.deserialize_struct("Point", FIELDS, PointVisitor)
    }
}

struct PointVisitor;

impl serde::de::Visitor for PointVisitor {
    type Value = Point;

    fn visit_map<V>(&mut self, mut visitor: V) -> Result<Point, V::Error>
        where V: serde::de::MapVisitor
    {
        let mut x = None;
        let mut y = None;

        loop {
            match try!(visitor.visit_key()) {
                Some(PointField::X) => { x = Some(try!(visitor.visit_value())); }
                Some(PointField::Y) => { y = Some(try!(visitor.visit_value())); }
                None => { break; }
            }
        }

        let x = match x {
            Some(x) => x,
            None => try!(visitor.missing_field("x")),
        };

        let y = match y {
            Some(y) => y,
            None => try!(visitor.missing_field("y")),
        };

        try!(visitor.end());

        Ok(Point{ x: x, y: y })
    }
}


fn main() {
    let serialized = "{\"x\":1,\"y\":2}";

    let deserialized: Point = serde_json::from_str(&serialized).unwrap();

    println!("{:?}", deserialized);
}

Design Considerations and tradeoffs for Serializers and Deserializers

Serde serialization and deserialization implementations are written in such a way that they err on being able to represent more values, and also provide better error messages when they are passed an incorrect type to deserialize from. For example, by default, it is a syntax error to deserialize a String into an Option<String>. This is implemented such that it is possible to distinguish between the values None and Some(()), if the serialization format supports option types.

However, many formats do not have option types, and represents optional values as either a null, or some other value. Serde Serializers and Deserializers can opt-in support for this. For serialization, this is pretty easy. Simply implement these methods:

...

    fn visit_none(&mut self) -> Result<(), Self::Error> {
        self.visit_unit()
    }

    fn visit_some<T>(&mut self, value: T) -> Result<(), Self::Error> {
        value.serialize(self)
    }
...

For deserialization, this can be implemented by way of the Deserializer::visit_option hook, which presumes that there is some ability to peek at what is the next value in the serialized token stream. This following example is from serde_tests::TokenDeserializer, where it checks to see if the next value is an Option, a (), or some other value:

...

    fn visit_option<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
        where V: de::Visitor,
    {
        match self.tokens.peek() {
            Some(&Token::Option(false)) => {
                self.tokens.next();
                visitor.visit_none()
            }
            Some(&Token::Option(true)) => {
                self.tokens.next();
                visitor.visit_some(self)
            }
            Some(&Token::Unit) => {
                self.tokens.next();
                visitor.visit_none()
            }
            Some(_) => visitor.visit_some(self),
            None => Err(Error::EndOfStreamError),
        }
    }

...

Annotations

serde_codegen and serde_macros support annotations that help to customize how types are serialized. Here are the supported annotations:

Container Annotations:

Annotation	Function
#[serde(rename="name")]	Serialize and deserialize this container with the given name
#[serde(rename(serialize="name1"))]	Serialize this container with the given name
#[serde(rename(deserialize="name1"))]	Deserialize this container with the given name
#[serde(deny_unknown_fields)]	Always error during serialization when encountering unknown fields. When absent, unknown fields are ignored for self-describing formats like JSON.
#[serde(bound="T: MyTrait")]	Where-clause for the Serialize and Deserialize impls. This replaces any bounds inferred by Serde.
#[serde(bound(serialize="T: MyTrait"))]	Where-clause for the Serialize impl.
#[serde(bound(deserialize="T: MyTrait"))]	Where-clause for the Deserialize impl.

Variant Annotations:

Annotation	Function
#[serde(rename="name")]	Serialize and deserialize this variant with the given name
#[serde(rename(serialize="name1"))]	Serialize this variant with the given name
#[serde(rename(deserialize="name1"))]	Deserialize this variant with the given name

Field Annotations:

Annotation	Function
#[serde(rename="name")]	Serialize and deserialize this field with the given name
#[serde(rename(serialize="name1"))]	Serialize this field with the given name
#[serde(rename(deserialize="name1"))]	Deserialize this field with the given name
#[serde(default)]	If the value is not specified, use the Default::default()
#[serde(default="$path")]	Call the path to a function fn() -> T to build the value
#[serde(skip_serializing)]	Do not serialize this value
#[serde(skip_deserializing)]	Always use Default::default() or #[serde(default="$path")] instead of deserializing this value
#[serde(skip_serializing_if="$path")]	Do not serialize this value if this function fn(&T) -> bool returns true
#[serde(serialize_with="$path")]	Call a function fn<S>(&T, &mut S) -> Result<(), S::Error> where S: Serializer to serialize this value of type T
#[serde(deserialize_with="$path")]	Call a function fn<D>(&mut D) -> Result<T, D::Error> where D: Deserializer to deserialize this value of type T
#[serde(bound="T: MyTrait")]	Where-clause for the Serialize and Deserialize impls. This replaces any bounds inferred by Serde for the current field.
#[serde(bound(serialize="T: MyTrait"))]	Where-clause for the Serialize impl.
#[serde(bound(deserialize="T: MyTrait"))]	Where-clause for the Deserialize impl.

Using in no_std crates

The core serde package defines a number of features to enable usage in a variety of freestanding environments. Enable any or none of the following features, and use default-features = false in your Cargo.toml:

• alloc (implies nightly)
• collections (implies alloc and nightly)
• std (default)

If you only use default-features = false, you will receive a stock no_std serde with no support for any of the collection types.

Serialization Formats Using Serde

Format	Name
Bincode	bincode
env vars	envy
Hjson	serde_hjson
JSON	serde_json
MessagePack	rmp
XML	serde_xml
YAML	serde_yaml