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: subpattern.rs
    path of file: ./repos/logos/logos-codegen/src/parser
    
    the code of the file until where you have to start completion: use proc_macro2::TokenStream;
use syn::Ident;

use crate::error::Errors;
use crate::mir::Mir;
use crate::parser::definition::{bytes_to_regex_string, Literal};

#[derive(Default)]
pub struct Subpatterns {
    map: Vec<(Ident, String)>,
}

impl Subpatterns {
    pub fn add(&mut self, param: Ident, pattern: TokenStream, errors: &mut Errors) {
        let lit = match syn::parse2::<Literal>(pattern) {
            Ok(lit) => lit,
            Err(e) => {
                errors.err(e.to_string(), e.span());
                return;
            }
        };

        if let Some((name, _)) = self.map.iter().find(|(name, _)| *name == param) {
            errors
                .err(format!("{} can only be assigned once", param), param.span())
                .err("Previously assigned here", name.span());
            return;
        }

        let fixed = self.fix(&lit, errors);

        // Validate the literal as proper regex. If it's not, emit an error.
        let mir = match &lit {
            Literal::Ut
    
    pub fn fix(&self, lit: &Literal, errors: &mut Errors) -> String {
        let mut i = 0;
        let mut pattern = match lit {
            Literal::Utf8(s) => s.value(),
            Literal::Bytes(b) => bytes_to_regex_string(b.value()),
        };

        while let Some(f) = pattern[i..].find("(?&") {
            i += f;
            pattern.replace_range(i..i + 3, "(?:");
            i += 3;

            let subref_end = if let Some(f) = pattern[i..].find(')') {
                i + f
            } else {
                pattern.truncate(i); // truncate so latter error doesn't suppress
                break; // regex-syntax will report the unclosed group
            };

            let name = &pattern[i..subref_end];
            let name = match syn::parse_str::<Ident>(name) {
                Ok(name) => name,
                Err(_) => {
                    errors.err(
                        format!("subpattern reference `{}` is not an identifier", name),
                        lit.span(),
                    );
                    // we emitted the error; make something up and continue
                    pattern.replace_range(i..subref_end, "_");
                    i += 2;
                    continue;
                }
            };

            match self.map.iter().find(|(def, _)| *def == name) {
                Some((_, subpattern)) => {
                    pattern.replace_range(i..subref_end, subpattern);
                    i += subpattern.len() + 1;
                }
                None => {
                    errors.err(
                        format!("subpattern reference `{}` has not been defined", name),
                        lit.span(),
                    );
                    // leaving `(?:name)` is fine
                    i = subref_end + 1;
                }
            }
        }

        pattern
    }