You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: variant_index.rs
    path of file: ./repos/visual-assembly-canvas/machine/poom_macros/src
    
    the code of the file until where you have to start completion: extern crate proc_macro;

use proc_macro::TokenStream;
use quote::{quote};
use syn::{parse_macro_input, Data, DeriveInput};

use crate::enums::variant_arity;

pub fn insert_variant_index_method(input: TokenStream) -> TokenStream {
    let ast = parse_macro_input!(input as DeriveInput);

    // Ensure that the input is an enum
    if let Data::Enum(data_enum) = &ast.data {
        let enum_name = &ast.ident;
        let index_values = data_enum.variants.iter().enumerate().map(|(index, variant)| {
            let field_count = variant_arity(variant);

            // Extract the variant's identifier.
            let variant_ident = &variant.ident;

            // If the arity is zero, match against the variant identifier, e.g. `Foo::Bar => 0`
            if field_count == 0 {
                return quote! {
                    #enum_name::#variant_ident => #index
                };
            }

            // Otherwise, match against the fields as well, e.g. `Foo::Baz(..) => 2`
            quote! {
                #enum_name::#variant_ident(..) => #index
            }
        });

        
    
pub fn insert_variant_index_method(input: TokenStream) -> TokenStream {
    let ast = parse_macro_input!(input as DeriveInput);

    // Ensure that the input is an enum
    if let Data::Enum(data_enum) = &ast.data {
        let enum_name = &ast.ident;
        let index_values = data_enum.variants.iter().enumerate().map(|(index, variant)| {
            let field_count = variant_arity(variant);

            // Extract the variant's identifier.
            let variant_ident = &variant.ident;

            // If the arity is zero, match against the variant identifier, e.g. `Foo::Bar => 0`
            if field_count == 0 {
                return quote! {
                    #enum_name::#variant_ident => #index
                };
            }

            // Otherwise, match against the fields as well, e.g. `Foo::Baz(..) => 2`
            quote! {
                #enum_name::#variant_ident(..) => #index
            }
        });

        // Insert the variant index method into the enum.
        let expanded = quote! {
            impl #enum_name {
                pub fn index(&self) -> usize {
                    match self {
                        #(#index_values,)*
                    }
                }
            }
        };

        expanded.into()
    } else {
        panic!("Arity can only be derived for enums");
    }
}