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: lexer.rs
    path of file: ./repos/darkfi/src/zkas
    
    the code of the file until where you have to start completion: /* This file is part of DarkFi (https://dark.fi)
 *
 * Copyright (C) 2020-2024 Dyne.org foundation
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use std::{io::Result, str::Chars};

use super::error::ErrorEmitter;

const SPECIAL_CHARS: [char; 9] = ['{', '}', '(', ')', '[', ']', ',', ';', '='];

fn is_letter(ch: char) -> bool {
    ch.is_ascii_lowercase() || ch.is_ascii_uppercase() || ch == '_'
}

fn is_digit(ch: char) -> bool {
    ch.is_ascii_digit()
}

#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub enum TokenType {
    Symbol,
    String,
    Number,
    LeftBrace,
    RightBrace,
    LeftParen,
    RightParen,
    LeftBracket,
    RightBracket,
    Comma,
    Semicolon,
    Assign,
}

#[derive(Clone, Debug)]
pub struct Token {
    pub token: String,
    pub token_type: TokenType,
    pub line: usize,
    pub column: usize,
}

impl Token {
    fn new(token: &str, token_type: TokenType, line: usize, column: usize) -> Self {
        Self { token: token.to_string(), token_type, line, column }
    }
}

pub struct Lexer<'a> {
    source: Chars<'a>,
    error: ErrorEmitter,
}

impl<'a> Lexer<'a> {
    pub fn new(filename: &str, source: Chars<'a>) -> Self {
        // For nice error reporting, we'll load everything into a string
        // vector so we have references to lines.
        let lines: Vec<String> = source.as_str().lines().map(|x| x.to_string()).collect();
        let error = ErrorEmitter::new("Lexer", filename, lines);

        Self { source, error }
    }

    pub fn lex(&self) -> Result<Vec<Token>> {
        let mut tokens = vec![];
        let mut lineno = 1;
        let mut column = 0;

        // We use this as a buffer to store a single token, which is then
        // reset after a token is pushed to the returning vec.
        let mut buf = String::new();

        // We use these to keep state when iterating.
        let mut in_comment = false;
        let mut in_string = false;
        let mut in_number = false;
        let mut in_symbol = false;

        macro_rule
    
    pub fn lex(&self) -> Result<Vec<Token>> {
        let mut tokens = vec![];
        let mut lineno = 1;
        let mut column = 0;

        // We use this as a buffer to store a single token, which is then
        // reset after a token is pushed to the returning vec.
        let mut buf = String::new();

        // We use these to keep state when iterating.
        let mut in_comment = false;
        let mut in_string = false;
        let mut in_number = false;
        let mut in_symbol = false;

        macro_rules! new_symbol {
            () => {
                tokens.push(Token::new(&buf, TokenType::Symbol, lineno, column - buf.len()));
                in_symbol = false;
                buf = String::new();
            };
        }
        macro_rules! new_string {
            () => {
                tokens.push(Token::new(&buf, TokenType::String, lineno, column - buf.len()));
                in_string = false;
                buf = String::new();
            };
        }
        macro_rules! new_number {
            () => {
                tokens.push(Token::new(&buf, TokenType::Number, lineno, column - buf.len()));
                in_number = false;
                buf = String::new();
            };
        }

        #[allow(clippy::explicit_counter_loop)]
        for c in self.source.clone() {
            column += 1;

            if c == '\n' {
                if in_symbol {
                    new_symbol!();
                }

                if in_string {
                    return Err(self.error.abort("Strings can't contain newlines", lineno, column))
                }

                if in_number {
                    return Err(self.error.abort("Numbers can't contain newlines", lineno, column))
                }

                in_comment = false;
                lineno += 1;
                column = 0;
                continue
            }

            if c == '#' || in_comment {
                if in_symbol {
                    new_symbol!();
                }

                if in_number {
                    new_number!();
                }

                if in_string {
                    buf.push(c);
                    continue
                }

                in_comment = true;
                continue
            }

            if c.is_whitespace() {
                if in_symbol {
                    new_symbol!();
                }

                if in_number {
                    new_number!();
                }

                if in_string {
                    // For now we forbid whitespace in strings.
                    return Err(self.error.abort(
                        "Strings/Namespaces can't contain whitespace",
                        lineno,
                        column,
                    ))
                }

                continue
            }

            // Main cases, in_comment is already checked above.
            if !in_number && !in_symbol && !in_string && is_digit(c) {
                in_number = true;
                buf.push(c);
                continue
            }

            if in_number && !is_digit(c) {
                new_number!();
            }

            if in_number && is_digit(c) {
                buf.push(c);
                continue
            }

            if !in_number && !in_symbol && !in_string && is_letter(c) {
                in_symbol = true;
                buf.push(c);
                continue
            }

            if !in_number && !in_symbol && !in_string && c == '"' {
                // " I need to fix my Rust vis lexer
                in_string = true;
                continue
            }

            if (in_symbol || in_string) && (is_letter(c) || is_digit(c)) {
                buf.push(c);
                continue
            }

            if in_string && c == '"' {
                // " I need to fix my vis lexer
                if buf.is_empty() {
                    return Err(self.error.abort("String cannot be empty", lineno, column))
                }
                new_string!();
                continue
            }

            if SPECIAL_CHARS.contains(&c) {
                if in_symbol {
                    new_symbol!();
                }

                if in_number {
                    new_number!();
                }

                if in_string {
                    // TODO: Perhaps forbid these chars inside strings.
                }

                match c {
                    '{' => {
                        tokens.push(Token::new("{", TokenType::LeftBrace, lineno, column));
                        continue
                    }
                    '}' => {
                        tokens.push(Token::new("}", TokenType::RightBrace, lineno, column));
                        continue
                    }
                    '(' => {
                        tokens.push(Token::new("(", TokenType::LeftParen, lineno, column));
                        continue
                    }
                    ')' => {
                        tokens.push(Token::new(")", TokenType::RightParen, lineno, column));
                        continue
                    }
                    '[' => {
                        tokens.push(Token::new("[", TokenType::LeftBracket, lineno, column));
                        continue
                    }
                    ']' => {
                        tokens.push(Token::new("]", TokenType::RightBracket, lineno, column));
                        continue
                    }
                    ',' => {
                        tokens.push(Token::new(",", TokenType::Comma, lineno, column));
                        continue
                    }
                    ';' => {
                        tokens.push(Token::new(";", TokenType::Semicolon, lineno, column));
                        continue
                    }
                    '=' => {
                        tokens.push(Token::new("=", TokenType::Assign, lineno, column));
                        continue
                    }
                    _ => {
                        return Err(self.error.abort(
                            &format!("Invalid token `{}`", c),
                            lineno,
                            column - 1,
                        ))
                    }
                }
            }

            return Err(self.error.abort(&format!("Invalid token `{}`", c), lineno, column - 1))
        }

        Ok(tokens)
    }