Crate serde_default_utils

Overview

This is a simple set of functions to make your life easier while working with defaults in serde. Based on const generic parameters. Heavily inspired by discussions on issues about serde defaults, but mostly this one

Kudos

• JohnTheCoolingFan posted this solution, I just made it available as crate and a macro that helps to generate another const generic function for any const generic type.
• bytedream made a more powerful version of it, and although I still see const generic approach as more readable, I have to admit that for strings it’s superior, hence - included under the feature

Example

    use serde_default_utils::*;
    use serde::{Deserialize, Serialize};

        const JSON: &str =
            r#"{
                "yes_or_no":false,
                "max":60,
                "delta":-77,
                "delimeter":"☀",
                "motto":"I matter"
            }"#;
        const INLINE_JSON: &str =
            r#"{
                "inline_motto":"I matter even more",
                "slice":["I", "matter", "even", "more"],
                "slice_u64":[9000, 8000, 10000, 7000]
            }"#;
    const EMPTY_JSON: &str = r#"{}"#;
    const MAX: u32 = 7;

    serde_default!(motto, "You matter");

    #[derive(Serialize, Deserialize, Default)]
    struct Config {
        #[serde(default = "default_bool::<true>")]
        yes_or_no: bool,
        #[serde(default = "default_i16::<-3>")]
        delta: i16,
        // you can even use consts right here
        #[serde(default = "default_u32::<MAX>")]
        max: u32,
        #[serde(default = "default_char::<'☀'>")]
        delimeter: char,
        #[serde(default = "default_motto")]
        motto: &'static str,
    }

    #[cfg(feature = "inline")]
    #[serde_inline_default]
    #[derive(Serialize, Deserialize, Default)]
    struct InlineConfig {
        #[serde_inline_default("You matter even more".to_string())]
        inline_motto: String,
        #[serde_inline_default(vec!["You", "matter", "even", "more"])]
        slice: Vec<&'static str>,
        #[serde_inline_default(vec![98712, 12346, 129389, 102937])]
        slice_u64: Vec<u64>,
    }

    fn main() {
        // existing json fields are not changed
        let config: Config = serde_json::from_str(JSON).unwrap();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"yes_or_no":false,"delta":-77,"max":60,"delimeter":"☀","motto":"I matter"}"#, &s);
        // if the field is not present - it is substituted with defaults
        let config: Config = serde_json::from_str(EMPTY_JSON).unwrap();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"yes_or_no":true,"delta":-3,"max":7,"delimeter":"☀","motto":"You matter"}"#, &s);
        // the default impl is just calling underlying type defaults unless you have a custom impl Default
        let config = Config::default();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"yes_or_no":false,"delta":0,"max":0,"delimeter":"\u0000","motto":""}"#, &s);
         
        // Inline
        #[cfg(feature = "inline")]
        {
        // existing json fields are not changed
        let config: InlineConfig = serde_json::from_str(INLINE_JSON).unwrap();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"inline_motto":"I matter even more","slice":["I","matter","even","more"],"slice_u64":[9000,8000,10000,7000]}"#, &s);
        // if the field is not present - it is substituted with defaults
        let config: InlineConfig = serde_json::from_str(EMPTY_JSON).unwrap();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"inline_motto":"You matter even more","slice":["You","matter","even","more"],"slice_u64":[98712,12346,129389,102937]}"#, &s);
        // the default impl is just calling underlying type defaults unless you have a custom impl Default
        let config = Config::default();
        let s = serde_json::to_string(&config).unwrap();
        assert_eq!(r#"{"inline_motto":"","slice":[],"slice_u64":[]}"#, &s);
        }
    }

Macros

• serde_default
  Generates a function for a type provided or a custom default function This is not supposed to be used outside since const generic parameter approach is pretty limited at the moment

Functions

• default_bool
• default_char
• default_i8
• default_i16
• default_i32
• default_i64
• default_i128
• default_isize
• default_u8
• default_u16
• default_u32
• default_u64
• default_u128
• default_usize

Attribute Macros

• serde_inline_default
  The main macro of this crate. Use it to define default values of fields in structs you Serialize or Deserialize. You do not need to create a extra function to provide the default value, as it is the case in serdes’ implementation of default (#[serde(default = "...")]).