Function headers_for_prefix 

pub fn headers_for_prefix<'a>(
    header_names: impl Iterator<Item = &'a str>,
    key: &'a str,
) -> impl Iterator<Item = (&'a str, &'a str)>

Returns an iterator over pairs where the first element is the unprefixed header name that starts with the input key prefix, and the second element is the full header name.

header_names must be normalized (lowercase) names, as HTTP libraries produce them. A name matches when it starts with key lowercased, so a modeled prefix such as X-Meta- matches the wire name x-meta-foo.

This is done without allocating a lowercased copy of key, because it runs per prefix-bound member on every response. A prefix with no uppercase letters is already its own lowercase form, so a plain byte comparison is the whole answer; only an uppercase prefix pays for lowercasing each byte as it compares.