What's the best practice for str/String values in Rust enums?

I have this which is really pleasant to the eye, but I'm concerned about its implications:

#[derive(Eq, PartialEq, Debug)]
pub enum SmtpHost {     
    DOMAIN(String),
    IPV4(Ipv4Addr),
    IPV6(Ipv6Addr),
    UNKNOWN { label:String, literal:String },
}

I'm filling this up from a PEG grammar which gives me &str so all the stringy calls look like this - SmtpHost::Domain(s.to_string())

I would like these enums to be the outcome of the parser, like smtp_parser::host< 'input >(s: 'input & str) -> SmtpHost

I have also tried the ref approach, but that starts getting clumsy rather soon:

#[derive(Eq, PartialEq, Debug)]
pub enum SmtpHost<'a > {     
    DOMAIN(&'a str),
    IPV4(Ipv4Addr),
    IPV6(Ipv6Addr),
    UNKNOWN { label:&'a str, literal:&'a str },
}

So I'm like either / or ... but you know better. Tell me :o)

My study project for reference