serde-inline-default/src/lib.rs

#![doc = include_str!("../README.md")]

use proc_macro::TokenStream;
use syn::{parse_macro_input, spanned::Spanned, Error, Item};

mod expand;
mod utils;

/// 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 an extra function to provide the default value, as it is the case in
/// serdes' implementation of default (`#[serde(default = "...")]`).
///
/// Set this macro on a struct where you use [`Serialize`] or [`Deserialize`] and use
/// `#[serde_inline_default(...)]` on the field you want to have an inline default value.
/// Replace the `...` with the value you want, and it will be set as default if serde needs it.
///
/// Note that you must set this macro _before_ `#[derive(Serialize)]` / `#[derive(Deserialize)]` as
/// it won't work properly if it's set after the derive.
///
/// # Examples
///
/// ```rust
/// #[serde_inline_default]
/// #[derive(Deserialize)]
/// struct Test {
///     #[serde_inline_default(42)]
///     value: u32
/// }
/// ```
///
/// [`Serialize`]: https://docs.rs/serde/*/serde/trait.Serialize.html
/// [`Deserialize`]: https://docs.rs/serde/*/serde/trait.Deserialize.html
#[proc_macro_attribute]
pub fn serde_inline_default(attr: TokenStream, input: TokenStream) -> TokenStream {
    let item = parse_macro_input!(input as Item);

    match item {
        Item::Struct(s) => expand::expand_struct(s),
        Item::Enum(e) => expand::expand_enum(e),
        _ => Error::new(
            proc_macro2::TokenStream::from(attr).span(),
            "can only be used on structs and enums",
        )
        .to_compile_error()
        .into(),
    }
}