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: env.rs
    path of file: ./repos/linkerd2-proxy/linkerd/app/src
    
    the code of the file until where you have to start completion: use crate::{dns, gateway, identity, inbound, oc_collector, outbound, policy, spire};
use linkerd_app_core::{
    addr,
    config::*,
    control::{Config as ControlConfig, ControlAddr},
    proxy::http::{h1, h2},
    tls,
    transport::{Keepalive, ListenAddr},
    Addr, AddrMatch, Conditional, IpNet,
};
use linkerd_tonic_stream::ReceiveLimits;
use rangemap::RangeInclusiveSet;
use std::{
    collections::{HashMap, HashSet},
    fs,
    net::{IpAddr, SocketAddr},
    path::PathBuf,
    str::FromStr,
    time::Duration,
};
use thiserror::Error;
use tracing::{debug, error, info, warn};

/// The strings used to build a configuration.
pub trait Strings {
    /// Retrieves the value for the key `key`.
    ///
    /// `key` must be one of the `ENV_` values below.
    fn get(&self, key: &str) -> Result<Option<String>, EnvError>;
}

/// An implementation of `Strings` that reads the values from environment variables.
pub struct Env;

/// Errors produced when loading a `Config` struct.
#[derive(Clone, Debug, Error)]
pub enum EnvError {
    #[error("invalid environment variable")]
    Inv
    
    fn dns_suffixes() {
        fn p(s: &str) -> Result<Vec<String>, ParseError> {
            let mut sfxs = parse_dns_suffixes(s)?
                .into_iter()
                .map(|s| format!("{}", s))
                .collect::<Vec<_>>();
            sfxs.sort();
            Ok(sfxs)
        }

        assert_eq!(p(""), Ok(vec![]), "empty string");
        assert_eq!(p(",,,"), Ok(vec![]), "empty list components are ignored");
        assert_eq!(p("."), Ok(vec![".".to_owned()]), "root is valid");
        assert_eq!(
            p("a.b.c"),
            Ok(vec!["a.b.c".to_owned()]),
            "a name without trailing dot"
        );
        assert_eq!(
            p("a.b.c."),
            Ok(vec!["a.b.c.".to_owned()]),
            "a name with a trailing dot"
        );
        assert_eq!(
            p(" a.b.c. , d.e.f. "),
            Ok(vec!["a.b.c.".to_owned(), "d.e.f.".to_owned()]),
            "whitespace is ignored"
        );
        assert_eq!(
            p("a .b.c"),
            Err(ParseError::NotADomainSuffix),
            "whitespace not allowed within a name"
        );
        assert_eq!(
            p("mUlti.CasE.nAmE"),
            Ok(vec!["multi.case.name".to_owned()]),
            "names are coerced to lowercase"
        );
    }