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: builtin_git.rs
    path of file: ./repos/nurl/src/fetcher
    
    the code of the file until where you have to start completion: use std::io::Write;

use anyhow::{bail, Context, Result};
use rustc_hash::FxHashMap;
use serde_json::json;

use crate::{fetcher::Fetcher, Url};

pub struct BuiltinsFetchGit;

impl<'a> Fetcher<'a> for BuiltinsFetchGit {
    fn fetch_nix(
        &self,
        out: &mut impl Write,
        url: &'a Url,
        r
    
    fn to_json(&'a self, out: &mut impl Write, url: &'a Url, rev: Option<String>) -> Result<()> {
        let rev = rev.context("builtins.fetchGit does not support feching the latest revision")?;
        let rev_type = if rev.len() == 40 { "rev" } else { "ref" };

        serde_json::to_writer(
            out,
            &json!({
                "fetcher": "builtins.fetchGit",
                "args": {
                    "url": url.to_string(),
                    rev_type: rev,
                },
            }),
        )?;

        Ok(())
    }