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: code_builder.rs
    path of file: ./repos/turbo/crates/turbopack-core/src
    
    the code of the file until where you have to start completion: use std::{
    cmp::min,
    io::{BufRead, Result as IoResult, Write},
    ops,
};

use anyhow::Result;
use turbo_tasks::Vc;
use turbo_tasks_fs::rope::{Rope, RopeBuilder};
use turbo_tasks_hash::hash_xxh3_hash64;

use crate::{
    source_map::{GenerateSourceMap, OptionSourceMap, SourceMap, SourceMapSection},
    source_pos::SourcePos,
};

/// A mapping of byte-offset in the code string to an associated source map.
pub type Mapping = (usize, Option<Vc<Box<dyn GenerateSourceMap>>>);

/// Code stores combined output code and the source map of that output code.
#[turbo_tasks::value(shared)]
#[derive(Debug, Clone)]
pub struct Code {
    code: Rope,

    mappings: Vec<Mapping>,
}

/// CodeBuilder provides a mutable container to a
    
    pub async fn generate_source_map(&self) -> Result<Vc<OptionSourceMap>> {
        let mut pos = SourcePos::new();
        let mut last_byte_pos = 0;

        let mut sections = Vec::with_capacity(self.mappings.len());
        let mut read = self.code.read();
        for (byte_pos, map) in &self.mappings {
            let mut want = byte_pos - last_byte_pos;
            while want > 0 {
                let buf = read.fill_buf()?;
                debug_assert!(!buf.is_empty());

                let end = min(want, buf.len());
                pos.update(&buf[0..end]);

                read.consume(end);
                want -= end;
            }
            last_byte_pos = *byte_pos;

            let encoded = match map {
                None => SourceMap::empty(),
                Some(map) => match *map.generate_source_map().await? {
                    None => SourceMap::empty(),
                    Some(map) => map,
                },
            };

            sections.push(SourceMapSection::new(pos, encoded))
        }

        Ok(Vc::cell(Some(SourceMap::new_sectioned(sections).cell())))
    }