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: registry.rs
    path of file: ./repos/cargo-edit/src
    
    the code of the file until where you have to start completion: use super::errors::*;
use std::collections::HashMap;
use std::path::Path;
use url::Url;

const CRATES_IO_INDEX: &str = "https://github.com/rust-lang/crates.io-index";
const CRATES_IO_REGISTRY: &str = "crates-io";

/// Find the URL of a registry
pub fn registry_url(manifest_path: &Path, registry: Option<&str>) -> CargoResult<Url> {
    // TODO support local registry sources, directory sources, git sources: https://doc.rust-lang.org/cargo/reference/source-replacement.html?highlight=replace-with#source-replacement
    fn read_config(
        registries: &mut HashMap<String, Source>,
        path: impl AsRef<Path>,
    ) -> CargoResult<()> {
        // TODO unit test for source replacement
        let content = std::fs::read_to_string(path)?;
        let config = toml::from_str::<CargoConfig>(&content).map_err(|_| invalid_cargo_config())?;
        for (key, value) in config.registries {
            reg
    
pub fn registry_url(manifest_path: &Path, registry: Option<&str>) -> CargoResult<Url> {
    // TODO support local registry sources, directory sources, git sources: https://doc.rust-lang.org/cargo/reference/source-replacement.html?highlight=replace-with#source-replacement
    fn read_config(
        registries: &mut HashMap<String, Source>,
        path: impl AsRef<Path>,
    ) -> CargoResult<()> {
        // TODO unit test for source replacement
        let content = std::fs::read_to_string(path)?;
        let config = toml::from_str::<CargoConfig>(&content).map_err(|_| invalid_cargo_config())?;
        for (key, value) in config.registries {
            registries.entry(key).or_insert(Source {
                registry: value.index,
                replace_with: None,
            });
        }
        for (key, value) in config.source {
            registries.entry(key).or_insert(value);
        }
        Ok(())
    }
    // registry might be replaced with another source
    // it's looks like a singly linked list
    // put relations in this map.
    let mut registries: HashMap<String, Source> = HashMap::new();
    // ref: https://doc.rust-lang.org/cargo/reference/config.html#hierarchical-structure
    for work_dir in manifest_path
        .parent()
        .expect("there must be a parent directory")
        .ancestors()
    {
        let work_cargo_dir = work_dir.join(".cargo");
        let config_path = work_cargo_dir.join("config");
        if config_path.is_file() {
            read_config(&mut registries, config_path)?;
        } else {
            let config_path = work_cargo_dir.join("config.toml");
            if config_path.is_file() {
                read_config(&mut registries, config_path)?;
            }
        }
    }

    let default_cargo_home = home::cargo_home()?;
    let default_config_path = default_cargo_home.join("config");
    if default_config_path.is_file() {
        read_config(&mut registries, default_config_path)?;
    } else {
        let default_config_path = default_cargo_home.join("config.toml");
        if default_config_path.is_file() {
            read_config(&mut registries, default_config_path)?;
        }
    }

    // find head of the relevant linked list
    let mut source = match registry {
        Some(CRATES_IO_INDEX) | None => {
            let mut source = registries.remove(CRATES_IO_REGISTRY).unwrap_or_default();
            source
                .registry
                .get_or_insert_with(|| CRATES_IO_INDEX.to_string());
            source
        }
        Some(r) => registries
            .remove(r)
            .with_context(|| anyhow::format_err!("The registry '{}' could not be found", r))?,
    };

    // search this linked list and find the tail
    while let Some(replace_with) = &source.replace_with {
        let is_crates_io = replace_with == CRATES_IO_INDEX;
        source = registries.remove(replace_with).with_context(|| {
            anyhow::format_err!("The source '{}' could not be found", replace_with)
        })?;
        if is_crates_io {
            source
                .registry
                .get_or_insert_with(|| CRATES_IO_INDEX.to_string());
        }
    }

    let registry_url = source
        .registry
        .and_then(|x| Url::parse(&x).ok())
        .with_context(invalid_cargo_config)?;

    Ok(registry_url)
}