hyper Add HTTP/2 103 Early Hints Support to Hyper

Summary

This tracking issue covers the implementation of HTTP/2 103 Early Hints support across the hyper ecosystem. The work has been completed across three repositories with a phased merge strategy.

Implementation Overview

HTTP 103 Early Hints (RFC 8297) allows servers to send resource preloading hints while preparing the final response, enabling browsers to start downloading critical resources early for improved performance.

Completed Work

Repository Changes

HTTP Repository (http crate)

Added StatusCode::EARLY_HINTS constant
Branch: http-103

H2 Repository (HTTP/2 protocol layer)

Implemented HTTP/2 informational responses (1xx) protocol support
Added comprehensive unit tests
Branch: http-103

Hyper Repository (Application layer)

Joel Dice's foundational work (PR #3815): Basic 1xx informational response support
Extended implementation: Complete HTTP/2 103 Early Hints with client callbacks and server support
Branch: http-103

Implementation Details

Server-Side API

async fn handler(mut req: Request<Body>) -> Result<Response<Body>, Error> {
    if let Some(mut sender) = req.extensions_mut().remove::<InformationalSender>() {
        let hints = Response::builder()
            .status(StatusCode::EARLY_HINTS)
            .header("link", "</css/critical.css>; rel=preload; as=style")
            .header("link", "</js/app.js>; rel=preload; as=script")
            .body(())
            .unwrap();
        
        sender.try_send(hints)?;
    }
    
    Ok(Response::new("Final response"))
}

Client-Side API

let config = InformationalConfig::new().with_callback(|response| {
    if response.status() == StatusCode::EARLY_HINTS {
        for link in response.headers().get_all("link") {
            start_preload(link);
        }
    }
});
let client = Client::builder().http2_only(true).build_with_config(config);

Benefits

Improved page load performance through parallel resource loading
RFC 8297 compliant implementation
Backward compatible with existing applications
Comprehensive test coverage

Pull Requests

Phase 1: Foundation Components (Ready Now)

http crate PR: Add StatusCode::EARLY_HINTS constant - https://github.com/hyperium/http/pull/794
h2 crate PR: HTTP/2 informational responses protocol support - https://github.com/hyperium/h2/pull/865
Joel Dice's hyper PR #3815: Foundational 1xx informational responses - https://github.com/hyperium/hyper/pull/3815

Phase 2: Complete Implementation (After Phase 1 Merge)

Apu Islam's hyper PR: Complete HTTP/2 103 Early Hints implementation - [Will be submitted after Phase 1 merges]

Merge Strategy

Phase 1: Foundation Components

http crate: StatusCode::EARLY_HINTS constant
h2 crate: HTTP/2 informational responses protocol support
hyper: Joel Dice's foundational 1xx informational responses (PR #3815)

Phase 2: Complete Implementation

hyper: Complete HTTP/2 103 Early Hints with client callbacks and production examples

Action Items

Phase 1 (Ready for Review)

[ ] Review and merge http PR hyperium/http#794
[ ] Review and merge h2 PR #865
[ ] Review and merge hyper PR #3815

Phase 2 (After Phase 1 Merge)

[ ] Submit comprehensive hyper PR
[ ] Review complete implementation
[ ] Verify production examples and documentation

References

RFC 8297: https://datatracker.ietf.org/doc/html/rfc8297
Original Request: https://github.com/hyperium/hyper/issues/2426
Joel Dice's Foundation Work: https://github.com/hyperium/hyper/pull/3815

Status

Implementation complete across all three repositories. Phase 1 PRs ready for immediate review and merge. Phase 2 will follow after foundation is established.

Nov 19 '25 19:11 apu031

Must be merged first - http PR - https://github.com/hyperium/http/pull/794
Should be merged next - h2 PR - https://github.com/hyperium/h2/pull/865
Joel's PR - https://github.com/hyperium/hyper/pull/3815 should be merged either with mine or before, let me know what you think?

Nov 19 '25 20:11 apu031

@seanmonstar @vikanezrimaya @dicej could you please review my PRs?

Nov 19 '25 20:11 apu031

@apu031 I'm sorry, but the prose on this tracking issue description and your two PRs looks like unfiltered LLM output due to oververbosity and the emoji overload, and this makes it hard to read. Your second message with the list of links was much clearer and the overall tracking issue description should've probably been based on that with a much shorter cover letter-style issue description for the work performed.

Also, from a quick look at your PR's commit messages they don't seem to follow Hyper's contribution guidelines, in particular the commit guidelines which outline the format the commit messages should have.

I shall be checking out the code shortly.

Nov 19 '25 20:11 vikanezrimaya

@apu031 I'm sorry, but the prose on this tracking issue description and your two PRs looks like unfiltered LLM output due to oververbosity and the emoji overload, and this makes it hard to read. Your second message with the list of links was much clearer and the overall tracking issue description should've probably been based on that with a much shorter cover letter-style issue description for the work performed.

Also, from a quick look at your PR's commit messages they don't seem to follow Hyper's contribution guidelines, in particular the commit guidelines which outline the format the commit messages should have.

I shall be checking out the code shortly.

Thanks @vikanezrimaya. I wanted it to correct my grammars and polish but added some emojis. I removed them for proper readability. I will double check the contribution and the commit guidelines once again.

Nov 19 '25 22:11 apu031