Function hop_by_hop_header_names 

pub fn hop_by_hop_header_names(
    headers: &HeaderMap,
) -> impl Iterator<Item = HeaderName> + use<>

Available on crate features http and std only.

Return known hop-by-hop field names and valid Connection nominations.

Known names are included even when absent from the map. The iterator owns its nominations so callers may mutate the map while consuming it.