Function request_connect_protocol 

pub fn request_connect_protocol<Body>(
    request: &Request<Body>,
) -> Option<Protocol>

Available on crate features http and std only.

Application protocol requested by an HTTP upgrade or Extended CONNECT.

HTTP/2 and HTTP/3 use the Protocol extension on CONNECT. HTTP/1 requires both Upgrade and Connection: Upgrade; an advertisement alone returns None. This identifies intent, not peer support or a completed upgrade.