Code-generate inbound (server→client) RPC dispatch for the Rust SDK

[SteveSandersonMS]

Background

The SDK protocol has two directions of RPC:

• Outbound (client→server): the SDK calls into the runtime.
• Inbound (server→client): the runtime calls back into the SDK — e.g. clientSession.* (canvas, sessionFs) and the new clientGlobal.* (llmInference) callbacks added in HTTP request callback support #1689.

For inbound dispatch, the four SDKs split into two camps:

• C#, Go, Python, TypeScript code-generate the inbound handler scaffolding (handler interfaces + the dispatch/registration glue) from api.schema.json. Adding the clientGlobal bucket in HTTP request callback support #1689 required teaching all four scripts/codegen/*.ts generators about it.

• Rust generates only the inbound wire types (generated/api_types.rs) and method-name constants (rpc_methods::* in generated/rpc.rs). The per-group dispatch + handler trait is hand-written, consistently, across all three inbound groups:

  • canvas.* → rust/src/canvas_dispatch.rs
  • sessionFs.* → rust/src/session_fs_dispatch.rs
  • llmInference.* → rust/src/copilot_request_handler.rs (dispatch)

  Each follows the identical shape: prefix-check → match rpc_methods::* constant → deserialize generated params → call the hand-written handler trait → serialize the result.

• Java has no codegen pipeline at all (entire RPC surface hand-authored), so it's out of scope here.

Proposal

Bring Rust to parity with the other generated SDKs by code-generating the inbound handler trait + dispatch helper per group into rust/src/generated/, so future inbound callbacks (new clientSession / clientGlobal groups) come for free, as they already do in C#/Go/Python/TS.

Why this is its own PR, not part of #1689

This was investigated during #1689 and deliberately deferred:

1. Do all three groups, not a one-off. Rust's hand-written-dispatch convention is currently uniform across canvas, sessionFs, and llmInference. Generating only llmInference (the group HTTP request callback support #1689 touches) would trade Rust's internal consistency for cross-SDK symmetry and leave a confusing one-off. The clean change converts all three groups together — a cross-cutting refactor of shipped, well-tested dispatchers and their public trait surfaces (CanvasHandler, SessionFsProvider, CopilotRequestHandler), out of scope for a callbacks PR.

2. Generate only the low-level wire trait; keep ergonomic traits + streaming bridges hand-written. The public CopilotRequestHandler (send_request + open_websocket) does not mirror the wire methods (httpRequestStart + httpRequestChunk); ~1200 lines bridge frame-oriented streaming ↔ that 2-method seam with stateful exchange correlation. Codegen should emit only the thin wire-level trait + dispatch (mirroring what TS generates), with the ergonomic traits and the llmInference bridge staying hand-written. Net hand-written savings are small (≈ the dispatch match + method constants); the value is future-proofing and symmetry, not LOC.

3. Validation. The streaming handler is heavily covered by Rust e2e (cancel / error / session-id) record-replay tests that should gate the refactor in CI.

Suggested scope of work

• Add inbound handler-trait + dispatch emission to scripts/codegen/rust.ts (read clientSession + clientGlobal), emitting into rust/src/generated/.
• Migrate canvas_dispatch.rs, session_fs_dispatch.rs, and copilot_request_handler.rs to implement/use the generated trait + dispatch instead of their hand-rolled match + local method-name constants.
• Keep ergonomic public traits (CanvasHandler, SessionFsProvider, CopilotRequestHandler) and the llmInference streaming bridge hand-written.
• Verify cargo build + clippy + the full Rust e2e suite stay green.

References

• PR HTTP request callback support #1689 (LLM inference callbacks) — where the clientGlobal bucket and the C#/Go/Python/TS generator changes landed.
• Generators with inbound support: scripts/codegen/{csharp,go,python,typescript}.ts (emitClientGlobalApiRegistration / emitClientSessionApiRegistration).
• Rust generator (outbound only today): scripts/codegen/rust.ts.