auto_enums_derive 0.3.3

This library provides an attribute macro like a wrapper of `#[derive]`, implementing the supported traits and passing unsupported traits to `#[derive]`.
Documentation 
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use proc_macro2::TokenStream;
use quote::quote;

use crate::utils::*;

pub(crate) const NAME: &[&str] = &["FnOnce"];

pub(crate) fn derive(data: &Data) -> Result<TokenStream> {
    let root = std_root();
    let trait_path = quote!(#root::ops::FnOnce);
    let trait_ = quote!(#trait_path(__T) -> __U);
    let fst = data.fields().iter().next();

    let mut impls = data.impl_with_capacity(2)?;

    *impls.trait_() = Some(Trait::new(
        syn::parse2(trait_path.clone())?,
        parse_quote!(#trait_path<(__T,)>)?,
    ));
    impls.push_generic_param(param_ident("__T"));
    impls.push_generic_param(param_ident("__U"));

    impls.push_where_predicate(parse_quote!(#fst: #trait_)?);
    data.fields()
        .iter()
        .skip(1)
        .try_for_each(|f| parse_quote!(#f: #trait_).map(|f| impls.push_where_predicate(f)))?;

    impls.append_items_from_trait(parse_quote! {
        trait FnOnce {
            type Output;
            #[inline]
            extern "rust-call" fn call_once(self, args: (__T,)) -> Self::Output;
        }
    }?)?;

    Ok(impls.build())
}