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: ast_discoverer.rs
    path of file: ./repos/autocxx/engine/src
    
    the code of the file until where you have to start completion: // Copyright 2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use indexmap::set::IndexSet as HashSet;

use autocxx_parser::{
    directive_names::{EXTERN_RUST_FUN, EXTERN_RUST_TYPE},
    RustFun, RustPath,
};
use itertools::Itertools;
use proc_macro2::Ident;
use syn::visit_mut::{visit_type_mut, VisitMut};
use syn::{
    parse_quote, punctuated::Punctuated, AssocConst, AssocType, Attribute, Expr, 
    
fn add_receiver(sig: &Signature, receiver: &Ident) -> Result<Signature, DiscoveryErr> {
    let mut sig = sig.clone();
    match sig.inputs.iter_mut().next() {
        Some(first_arg) => match first_arg {
            syn::FnArg::Receiver(rec_arg) => {
                let mut substituted_type = rec_arg.ty.clone();
                visit_type_mut(&mut SelfSubstituter::new(receiver), &mut substituted_type);
                *first_arg = parse_quote! {
                        qelf: #substituted_type
                };
                if let syn::FnArg::Typed(ref mut pat_type) = *first_arg {
                    if let syn::Pat::Ident(ref mut pat_ident) = *pat_type.pat {
                        assert_eq!(pat_ident.ident.to_string(), "qelf");
                        pat_ident.ident = Ident::new("self", pat_ident.ident.span());
                    }
                }
            }
            syn::FnArg::Typed(_) => {}
        },
        None => return Err(DiscoveryErr::NoParameterOnMethod),
    }
    Ok(sig)
}