Function ambient 

pub fn ambient(
    vars: CypherKeyVars,
    lookup: impl Fn(&str) -> Option<String>,
) -> Result<Option<CypherKeyring>, KeyringError>

The keys vars names, or None when neither variable is set to anything. The file wins over the inline document, so a host can override a baked-in environment by pointing at a file.

lookup is the environment, passed in rather than read here so a test can describe one without touching the process it runs in.