plctag-derive 0.3.3

macros for `plctag`
Documentation 
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// plctag-rs
//
// a rust wrapper of libplctag, with rust style APIs and useful extensions.
// Copyright: 2022, Joylei <leingliu@gmail.com>
// License: MIT

use crate::shared::{get_crate, get_fields, Context};
use proc_macro2::TokenStream;
use proc_quote::quote;
use syn::{DeriveInput, Index};

pub fn expand_tag_derive(input: DeriveInput) -> syn::Result<TokenStream> {
    let ctx = Context { is_encode: false };
    let plctag = get_crate()?;
    let items = get_fields(input.data, &ctx)?;

    let sets = items
        .iter()
        .map(|(field_name, ty, attr)| {
            let ts = match attr.decode_fn {
                Some(ref f) => quote! {
                    res.#field_name =  #f(tag, offset)?;
                },
                None => {
                    let index = Index::from(attr.offset.unwrap() as usize);
                    quote! {
                        res.#field_name = #ty::decode(tag, offset + #index)?;
                    }
                }
            };
            Ok(ts)
        })
        .collect::<syn::Result<TokenStream>>()?;

    let st_name = input.ident;

    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
    Ok(quote! {
        impl  #impl_generics #plctag::Decode for #st_name #ty_generics #where_clause
         {
            fn decode(tag: &#plctag::RawTag, offset: u32) -> #plctag::Result<Self>{
                use #plctag::Decode;

                let mut res = <Self as Default>::default();
                #sets
                Ok(res)
            }
        }
    })
}