[samuel-williams-shopify]

HTTP/1.0 now drops trailers instead of refusing messages whose bodies may have them. An HTTP/1.1 request of known length is sent with Content-Length and drops its trailers too, while responses stay chunked whenever their body may have trailers. A Trailer field is now treated only as a hint and no longer affects framing. Incoming bodies report may_have_trailers() only when they are chunked.

HTTP/1.0 has no chunked framing, so it cannot carry trailers, and HTTP semantics (RFC 9110 §6.5.1) let them be discarded. Until now, the HTTP/1.0 request writer refused any body whose may_have_trailers() was true, with "trailers require HTTP/1.1 chunked framing". Every BodyReader reports true, so a streamed request could never be sent over HTTP/1.0. A response to an HTTP/1.0 client failed when its body actually produced trailers.

Outgoing messages

• HTTP/1.0 requests: a body of known length is sent with Content-Length and its trailers are dropped, through both write_request and imperative begin_request/write_frame.
• HTTP/1.0 request bodies of unknown length: still refused, because an HTTP/1.0 request cannot be close-delimited. The error now gives that reason: "HTTP/1.0 request requires a body length".
• Responses to HTTP/1.0 clients: sent with their length when it is known, or close-delimited otherwise, and their trailers are dropped. A persistent connection stays reusable. A 205 whose body may have trailers now declares Content-Length: 0.
• HTTP/1.1 requests of known length: sent with Content-Length even when the body may have trailers, which are dropped after validation, instead of being chunked. This covers write_request with a body such as BodyReader::with_length, and begin_request with a content_length. Request trailers are rare, a client MUST NOT send Transfer-Encoding unless it knows the server handles HTTP/1.1 (RFC 9112 §6.1), and length-requiring servers answer 411. Bodyless GETs and known-length uploads therefore keep main's wire format, and an imperative request with content_length and a Trailer field, which main chunked, now completes with its trailer dropped instead of failing. HTTP/1.1 requests of unknown length are still chunked and keep their trailers.
• HTTP/1.1 responses: unchanged. A body that may have trailers is chunked even with a known length; buffered bodies and received Content-Length or close-delimited bodies report no trailers and keep Content-Length; HEAD, 204 and 304 take no framing.
• Dropped trailers are still validated, so trailer fields are validated the same way however they are framed.
• Tunnels: tunnel output still refuses trailer frames.

The shared BodyEncoder gains discard_trailers, which the request writer enables for every non-chunked request and the response writer enables for HTTP/1.0 output that isn't a tunnel. write_body now takes the encoder.

The Trailer field

Nobody knows ahead of time whether a message will have trailers. The Trailer field is only a SHOULD hint (gRPC, for example, omits it), so it no longer decides anything:

• On HTTP/1.0 it no longer causes a refusal.
• On HTTP/1.1 it no longer selects chunked framing for a body that has promised no trailers.

For responses, may_have_trailers() alone decides framing; for requests, a known length does.

Incoming bodies

On the receiving side, only chunked bodies report may_have_trailers(). That rules out HTTP/1.0 bodies, Content-Length bodies and close-delimited bodies. This already held, and new codec tests now pin it for both the server and the client, along with keeping received trailers out of the header fields. HTTP/1.0 messages with Transfer-Encoding were already refused on read, and the lane is not reused.

design/http1-trailers.md records this model, and design/fixtures.md, design/implementation.md, design/proposal.md and the base-http1 readme are updated to match.

Tests

• The fixtures' trailer suite now runs over HTTP/1.0. It checks in both directions that no trailer arrives and that the data arrives intact. Sabotaged HTTP/1.0 variants show these assertions can fail.
• Over HTTP/1.0, a client cannot tell a close-delimited response whose trailers were dropped from one where the server failed after the data. A server end-to-end test (http10_trailers_are_dropped) covers that case by checking that the connection serves a further request.
• The fixtures changes are kept small because Add spawned server contexts, split raw peers and a two-way streaming case to base-http-fixtures. #49 also edits crates/fixtures. After this lands, Add spawned server contexts, split raw peers and a two-way streaming case to base-http-fixtures. #49 can drop its private no-trailers upload wrapper.

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