Borrowed request views: ServiceRequest and StreamMessage on buffa 0.7.0

[iainmcgin]

Summary

Reworks the server-side request surface around buffa 0.7.0, which removes the OwnedView Deref impl (which let safe code hold view fields past the backing buffer's lifetime) and introduces HasMessageView plus generated FooOwnedView accessor wrappers. Rather than just absorbing the ergonomic loss, handlers move to a new request/stream-item API:

• ServiceRequest<'a, Req> — unary and server-streaming handlers now take a borrowed request (req: ServiceRequest<'_, GreetRequest>). It Derefs to the request view for field access, and offers to_owned_message() (zero-copy via the retained body bytes), bytes(), and view(). Borrowing across .await works; moving into tokio::spawn requires to_owned_message() (enforced by the compiler).
• StreamMessage<M> — client-streaming and bidi inbound items. Derefs to the buffa-generated FooOwnedView wrapper for per-field accessors, and re-encodes from the retained wire bytes when forwarded.
• Client: UnaryResponse::view() returns the reborrowed view; raw_bytes() is renamed to bytes().
• Codegen: generated service traits/dispatchers use the new signatures; the view-family impls come from buffa 0.7.0's generated code rather than connect-codegen.
• Workspace now requires buffa/buffa-types/buffa-codegen 0.7; checked-in generated code is regenerated with the 0.7.0 toolchain.

extern_path types use the same wrappers (second commit). The earlier borrowed-view fallback for inputs mapped via extern_path is gone; instead the requirement is documented (README, plugin option docs, Options rustdoc, generated trait docs): every extern_path target must be buffa ≥ 0.7.0 generated code with views enabled, the same way the stubs already rely on the JSON impls. buffa-types 0.7+ satisfies this for well-known types; a non-conforming crate fails to compile on the missing HasMessageView impl, and anthropics/buffa#161 adds a #[diagnostic::on_unimplemented] note to that trait so the error explains the fix. The output side is unchanged: Encodable view-body impls are still per-type and skipped for extern outputs (a blanket impl keyed through the HasMessageView GAT was tried and is not coherent next to the M: Message + Serialize blanket).

Status — not yet complete

• ✅ connectrpc, connectrpc-codegen, connectrpc-build, and the tests/streaming e2e suite build and pass against published buffa 0.7.0; clippy and rustfmt clean.
• ❌ Not yet ported: conformance, examples/* (eliza, multiservice, middleware, mtls-identity, streaming-tour), and benches/rpc still implement the old OwnedView<...> handler signatures, so workspace-wide clippy/test CI will be red until they are updated.
• ❌ Planned: a Heartbeat(google.protobuf.Empty) → Timestamp-style RPC in the multiservice WKT example, so the extern-input/output paths are compiled in CI rather than only asserted in codegen unit tests.
• ❌ docs/guide.md and the README handler examples still show the old signatures.
• ❌ No CHANGELOG entry yet (will cover the handler API, the buffa 0.7 floor, and the extern_path requirement).

Opening as a draft so the API surface can be reviewed holistically ahead of a connect-rust 0.7.0 release alongside buffa 0.7.0.

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