Function http_alpn_override 

pub fn http_alpn_override(
    application_protocol: Option<&Protocol>,
    target_version: Option<Version>,
) -> Result<Option<ApplicationProtocol>, Box<dyn Error + Sync + Send>>

Available on crate feature tls only.

Resolve an explicit target HTTP version to an ALPN offer override.

Non-HTTP protocols ignore the target. An absent application protocol still allows an explicit target; an absent target leaves the existing offer alone. Pass only an explicit target, not a fallback HTTP version. The caller applies the offer and any backend-specific settings coupled to it, such as ALPS.