display_json

Rust's standard library offers traits your types can implement in order to serialize (std::fmt::{Display, Debug}) or deserialize (std::str::FromStr) them to and from strings. Serialization and deserialization of data types in rust is usually done by implementing the Serialize and Deserialize traits from the serde crate. display_json is a crate that allows you to easily integrate serde's functionality with these traits from std using custom derive procedural macros. These macros (de)serialize your types to and from json strings by wrapping serde_json's serialization and deserialization capabilities into the traits from std.

Table of Contents

• Serializing objects to json with rust std's fmt traits
  • DisplayAsJson
  • DebugAsJson
  • Pretty json
  • Mixing Display and Debug
• Deserializing objects from json with rust std's FromStr trait
  • FromStrAsJson

Serializing objects to json with rust std's fmt traits

display_json provides you with an easy way of combining the serialization of your objects to json and the usage of rust's built-in formatting features provided by the Display and Debug traits. For string serialization to json, display_json exposes the custom derive procedural macros DisplayAsJson, DisplayAsJsonPretty, DebugAsJson and DebugAsJsonPretty. This allows you to use std's These custom derives create a stringified json version of an object using serde_json. The four custom derive macros are basically neat wrappers that wrap the to_string and to_string_pretty functions from the serde_json crate into an implementation of Display or Debug for your type.

DisplayAsJson

Without display_json, you'd have to serialize your object to a json string like this:

use serde::Serialize;
use serde_json::{Result, to_string};

#[derive(Serialize)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

fn main() -> Result<()> {
    let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

    let s = to_string(&f)?;

    assert_eq!(s, r#"{"bar":"bar","baz":0,"bat":true}"#);

    Ok(())
} 

The same can be accomplished with less code by using the DisplayAsJson custom derive on Foo:

use serde::Serialize;
use display_json::DisplayAsJson;

#[derive(Serialize, DisplayAsJson)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

assert_eq!(f.to_string(), r#"{"bar":"bar","baz":0,"bat":true}"#);

DisplayAsJson is a wrapper around serde_json::to_string. It takes the serialized json string and provides it to the Display trait which is implemented by DisplayAsJson. This makes it more convenient to format your objects to json. For example, you can use this to create well-ingestable log messages or to serialize your data for sending it as the body of an http request:

use serde::Serialize;
use display_json::DisplayAsJson;

#[derive(Serialize, DisplayAsJson)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

// log `f` to stdout:
println!("{}", f);

// or you could construct an http request body from it

// or process your serialized object any other way you please

DebugAsJson

DebugAsJson works the same as DisplayAsJson, only instead of implementing the Display trait, it implements the Debug trait:

use serde::Serialize;
use display_json::DebugAsJson;

#[derive(Serialize, DebugAsJson)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

// note that we use the debug formatter for serializing `f` to json
let f_ser = format!("{:?}", f);

assert_eq!(f_ser, r#"{"bar":"bar","baz":0,"bat":true}"#);

Pretty json

DisplayAsJsonPretty and DebugAsJsonPretty work the same as their non-pretty counterparts, except producing a multiline, indented json string instead of a compact json string:

use serde::Serialize;
use display_json::{DisplayAsJsonPretty, DebugAsJsonPretty};

#[derive(Serialize, DisplayAsJsonPretty, DebugAsJsonPretty)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

let result = r#"{
  "bar": "bar",
  "baz": 0,
  "bat": true
}"#;

let f_ser = format!("{}", f);
let f_ser_dbg = format!("{:?}", f);

assert_eq!(f_ser, f_ser_dbg);

assert_eq!(f_ser, result);
assert_eq!(f_ser_dbg, result);

Mixing Display and Debug

As you can see in the example above, you can combine the DisplayAsJson and DebugAsJson variants however you like. For example, you could use DisplayAsJson to serialize your object for the body of an http request and DebugAsJsonPretty for creating well-readable debugging messages for you to debug your code.

Deserializing objects from json with rust std's FromStr trait

While deserialization of json strings is usually done by integrating your program directly with the serde and serde_json crates, sometimes you want to or have to use rust's standard library's trait std::str::FromStr, which is how the types in std implement deserialization from string. This could be due to interoperability constraints with other crate's that perform deserialization from strings, without serde integration. An example of such a crate would be clap's derive api.

If you find yourself confronted with the fact that your type needs to implement std::str::FromStr, even though it implements serde::Deserialize and you have no intention of thinking up a custom format with a parser just to cumbersomely write an implementation for FromStr by hand, display_json is a great choice.

FromStrAsJson

display_json exposes the FromStrAsJson custom derive procedural macro you can derive on your type. FromStrAsJson implements std::str::FromStr as a wrapper around serde_json::from_str. If you need to implement the FromStr trait for your type and all you want to do is to deserialize a json string with it, FromStrAsJson is the solution for you with the least amount of code, making sure your focus lies on what your program does, without you having to stare at boilerplate code.

FromStrAsJson is used as follows:

use serde::Deserialize;
use display_json::FromStrAsJson;

use std::str::FromStr;

#[derive(Deserialize, FromStrAsJson, PartialEq, Debug)]
struct Foo {
    bar: String,
    baz: i32,
    bat: bool,
}

let f_as_json = r#"{"bar":"bar","baz":0,"bat":true}"#;

let f = Foo { bar: "bar".to_owned(), baz: 0, bat: true };

assert_eq!(Foo::from_str(f_as_json).unwrap(), f);