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: tokens.rs
    path of file: ./repos/rust-peg/peg-macros
    
    the code of the file until where you have to start completion: use peg::{Parse, ParseElem, ParseLiteral, ParseSlice, RuleResult};
use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};

#[derive(Debug, Clone)]
pub struct FlatTokenStream {
    tokens: Vec<Token>,
}

#[derive(Debug, Clone)]
pub enum Token {
    Ident(Ident),
    Literal(Literal),
    Punct(Punct),
    Begin(Group, usize),
    End(Delimiter, Span),
}

impl Token {
    fn span(&self) -> Span {
        match self {
            Token:
    
    pub fn new(stream: TokenStream) -> FlatTokenStream {
        let mut tokens = vec![];

        fn flatten(tokens: &mut Vec<Token>, tree: TokenTree) {
            match tree {
                TokenTree::Ident(i) => tokens.push(Token::Ident(i)),
                TokenTree::Literal(l) => tokens.push(Token::Literal(l)),
                TokenTree::Punct(p) => tokens.push(Token::Punct(p)),
                TokenTree::Group(g) => {
                    let start_pos = tokens.len();

                    tokens.push(Token::End(g.delimiter(), g.span())); // placeholder
                    for tree in g.stream() {
                        flatten(tokens, tree);
                    }
                    tokens.push(Token::End(g.delimiter(), g.span()));

                    let end_pos = tokens.len();
                    tokens[start_pos] = Token::Begin(g, end_pos);
                }
            }
        }

        for tree in stream {
            flatten(&mut tokens, tree);
        }

        FlatTokenStream { tokens }
    }