vendor/derive_more/src/index_mut.rs

use crate::utils::{add_where_clauses_for_new_ident, SingleFieldData, State};
use proc_macro2::{Span, TokenStream};
use quote::quote;
use syn::{parse::Result, DeriveInput, Ident};

/// Provides the hook to expand `#[derive(IndexMut)]` into an implementation of `IndexMut`
pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
    let index_type = &Ident::new("__IdxT", Span::call_site());
    let mut state = State::with_field_ignore(
        input,
        trait_name,
        quote!(::core::ops),
        String::from("index_mut"),
    )?;
    state.add_trait_path_type_param(quote!(#index_type));
    let SingleFieldData {
        field,
        field_type,
        input_type,
        trait_path_with_params,
        casted_trait,
        member,
        ..
    } = state.assert_single_enabled_field();

    let type_where_clauses = quote! {
        where #field_type: #trait_path_with_params
    };

    let new_generics = add_where_clauses_for_new_ident(
        &input.generics,
        &[field],
        index_type,
        type_where_clauses,
        true,
    );

    let (impl_generics, _, where_clause) = new_generics.split_for_impl();
    let (_, ty_generics, _) = input.generics.split_for_impl();
    Ok(quote! {
        impl#impl_generics #trait_path_with_params for #input_type#ty_generics #where_clause
        {
            #[inline]
            fn index_mut(&mut self, idx: #index_type) -> &mut Self::Output {
                #casted_trait::index_mut(&mut #member, idx)
            }
        }
    })
}