Function send_early_hints 

pub async fn send_early_hints(
    req: &mut Request<impl Body>,
    headers: HeaderMap,
) -> Result<(), Error>

Sends a 103 Early Hints response for the given request.

Early hints allow the server to push Link (or other) headers to the client before the final response is ready, enabling the browser to begin preloading resources (stylesheets, scripts, fonts, etc.) while the handler is still computing the response.

Requirements

The connection must be HTTP/2 or HTTP/3, or if HTTP/1.1, the server must have been created with Http1Options::enable_early_hints set to true. When that option is enabled the handler inserts an EarlyHints token into each request’s extensions. If the token is absent (i.e. early hints were not enabled) this function returns an error.

Errors

Returns Err if:

• Early hints are not enabled on the connection ("early hints not supported").
• The connection handler has already shut down and can no longer accept interim responses.
• Writing the 103 response to the underlying I/O stream fails.

Example

use http::HeaderMap;

let mut link_headers = HeaderMap::new();
link_headers.insert(
    http::header::LINK,
    "</style.css>; rel=preload; as=style".parse().unwrap(),
);
send_early_hints(&mut request, link_headers).await?;