Optionally get environment variable

var_opt.rs

fn var_opt<K>(key: K) -> Result<Option<String>>
where
    K: AsRef<OsStr>,
{
    use std::env::{var, VarError};

    match var(key) {
        Ok(value) => Ok(Some(value)),
        Err(VarError::NotPresent) => Ok(None),
        _ => bail!("environment variable not found"),
    }
}