Function field 

pub fn field(
    cert: &Certificate,
    field: NameField,
) -> Result<Option<String>, Failure>

Returns the client name stored in the certificate field, lowercased.

None if the field is absent. IP addresses are formatted with IpAddr’s Display.

Errors

Failure::MultipleSan if there is more than one SAN entry of the kind, Failure::MalformedSanIp for an invalid iPAddress entry.