Function parse_loopback_client_id 

fn parse_loopback_client_id(
    client_id: &str,
) -> Option<Result<ClientMetadataDocument, ClientMetadataError>>

Recognises the atproto OAuth profile’s “Localhost Client Development” client_id form — exactly http://localhost, no port, path empty or / — and builds its virtual client metadata document straight from the redirect_uri (repeatable) and scope query parameters, fetching nothing.

None means client_id is not this form, including a localhost client_id that carries a port or a deeper path, which resolve_client judges like any other. Some carries the built-and-validated document, or the one way building it can fail.