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: arena_clone.rs
    path of file: ./repos/gluon/codegen/src
    
    the code of the file until where you have to start completion: use proc_macro2::{Ident, Span, TokenStream};
use syn::{self, Data, DeriveInput, Fields, GenericParam, Generics};

use crate::{
    attr::{Container, CrateName},
    shared::{map_type_params, split_for_impl},
};

pub fn derive(input: TokenStream) -> TokenStream {
    l
    
pub fn derive(input: TokenStream) -> TokenStream {
    let derive_input = syn::parse2(input).expect("Input is checked by rustc");

    let container = Container::from_ast(&derive_input);

    let DeriveInput {
        ident,
        data,
        generics,
        ..
    } = derive_input;

    let tokens = match data {
        Data::Struct(_) | Data::Enum(_) => gen_impl(&container, ident, generics, &data),
        Data::Union(_) => panic!("Unions are not supported"),
    };

    tokens.into()
}