serde-inline-default

A tiny crate to set default values for serde struct fields via inline attribute declaration.

Overview

This crate is an approach to do what serde-rs/serde#368 purposes: Defining default values for struct fields via inline declaration instead of creating a separate function for it.

So instead of writing something like this, which can get very verbose quickly with many fields:

#[derive(Deserialize)]
struct Test {
    #[serde(default = "value_default")]
    value: u32
}

fn value_default() -> u32 { 42 }

you can just do this:

#[serde_inline_default]
#[derive(Deserialize)]
struct Test {
    #[serde_inline_default(42)]
    value: u32
}

[!IMPORTANT] #[serde_inline_default] must be set before #[derive(Deserialize)]/#[derive(Serialize)]!

Internally, #[serde_inline_default(...)] gets expanded to a function which returns the set value and the attribute is replaced with #[serde(default = "<function name>")]. So this macro is just some syntax sugar for you, but can get quite handy if you want to keep your code clean or write declarative macros / macro_rules!.

Alternatives

This crate isn't perfect. Thus, you might be more satisfied with alternatives serde provides.

With #[serde(default)] + impl Default on a struct, serde uses the default implementation of the struct to get default values for each field (docs):

#[derive(Deserialize)]
#[serde(default)]
struct Test {
    value: u32
}

impl Default for Test {
    fn default() -> Self {
        Self {
            value: 42
        }
    }
}

If you still need/want serde-inline-default features, you also can combine them with #[serde(default)) and impl Default:

#[serde_inline_default]
#[derive(Deserialize)]
#[serde(default)]
struct Test {
    value: u32,
    #[serde_inline_default(0)]
    other_value: u32,
}

impl Default for Test {
    fn default() -> Self {
        Self {
            value: 42,
            other_value: 42
        }
    }
}

License

This project is licensed under either of the following licenses, at your option:

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)