Function is_loopback_literal 

pub fn is_loopback_literal(host: &str) -> bool

Whether a hostname is one of the bare loopback names the atproto profile permits a port on.

Narrower than is_loopback_host, and deliberately: agents.localhost resolves to this machine, but an atproto client reads did:web:agents.localhost%3A3000 as a handle rather than a DID, so a DID in that shape is one nothing else can resolve.