as_mut.rs

use crate::utils::{
    add_where_clauses_for_new_ident, AttrParams, MultiFieldData, State,
};
use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use syn::{parse::Result, DeriveInput};

pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
    let as_mut_type = format_ident!("__AsMutT");
    let state = State::with_type_bound(
        input,
        trait_name,
        "as_mut".into(),
        AttrParams::ignore_and_forward(),
        false,
    )?;
    let MultiFieldData {
        fields,
        input_type,
        members,
        infos,
        trait_path,
        impl_generics,
        ty_generics,
        where_clause,
        ..
    } = state.enabled_fields_data();
    let sub_items: Vec<_> = infos
        .iter()
        .zip(members.iter())
        .zip(fields)
        .map(|((info, member), field)| {
            let field_type = &field.ty;
            if info.forward {
                let trait_path = quote! { #trait_path<#as_mut_type> };
                let type_where_clauses = quote! {
                    where #field_type: #trait_path
                };
                let new_generics = add_where_clauses_for_new_ident(
                    &input.generics,
                    &[field],
                    &as_mut_type,
                    type_where_clauses,
                    false,
                );
                let (impl_generics, _, where_clause) = new_generics.split_for_impl();
                let casted_trait = quote! { <#field_type as #trait_path> };
                (
                    quote! { #casted_trait::as_mut(&mut #member) },
                    quote! { #impl_generics },
                    quote! { #where_clause },
                    quote! { #trait_path },
                    quote! { #as_mut_type },
                )
            } else {
                (
                    quote! { &mut #member },
                    quote! { #impl_generics },
                    quote! { #where_clause },
                    quote! { #trait_path<#field_type> },
                    quote! { #field_type },
                )
            }
        })
        .collect();
    let bodies = sub_items.iter().map(|i| &i.0);
    let impl_genericses = sub_items.iter().map(|i| &i.1);
    let where_clauses = sub_items.iter().map(|i| &i.2);
    let trait_paths = sub_items.iter().map(|i| &i.3);
    let return_types = sub_items.iter().map(|i| &i.4);

    Ok(quote! {#(
        #[automatically_derived]
        impl #impl_genericses #trait_paths for #input_type #ty_generics #where_clauses {
            #[inline]
            fn as_mut(&mut self) -> &mut #return_types {
                #bodies
            }
        }
    )*})
}