p3 bindgen forces `async: all` — no per-function sync lift, blocking call-return access to a single export

[avrabe]

Friction (from relay's P3 MC/DC work)

rust_wasm_component_bindgen(wasi_version = "p3") generates the guest wrapper with async: all — every export is lifted async, including functions whose WIT signature is plain sync (no stream/future). The generated trait method becomes async fn and the core export becomes [async-lift]...#<fn> / [task-return]<fn>.

Why it's a problem

A call-return consumer can't invoke an async-lift export. Concretely, an MC/DC harness (witness) couldn't drive relay's composed cascade for coverage, because adding a sync step(...) inside the p3 component still produced an async-lift export. We had no way to say "this one function is synchronous."

error: method should be `async` or return a future, but it is synchronous

(generated wrapper header: // async: all)

Ask

Expose a per-function async override on rust_wasm_component_bindgen (wit-bindgen supports async: { only: [...] } / an exclude list), so a component can keep its stream exports async while lifting a specific function synchronously.

Workaround (relay v1.80)

We built a separate p2 (sync) sibling component exporting the call-return function over the same shared engine library — works, but it's a whole extra component + WIT + fuse target just to expose one sync entry point. A per-function knob would remove that.

Refs: pulseengine/relay#202, pulseengine/witness#107.

[avrabe]

Fixed in #527 (open, CI running).

Added an async_interfaces parameter to rust_wasm_component_bindgen that exposes wit-bindgen's --async filters on the guest bindings — defaults unchanged (p3 → ["all"], p2 → []), so existing components are unaffected.

For your case — keep stream exports async, lift the sync step(...) to call-return — you have two options:

• async_interfaces = [] — drop the blanket entirely; wit-bindgen then follows each export's WIT signature (async-typed stays async, plain-sync stays sync). Usually the cleanest once the WIT distinguishes them.
• async_interfaces = ["-export:relay:cascade/iface@x.y.z#step"] — force just step sync, leave the rest at their WIT default.

Two gotchas I hit and documented on the rule (wit-bindgen 0.54):

1. Filter names must include the package version (pkg:iface/i@0.1.0#fn), else wit-bindgen reports unused async option.
2. Combining all with a per-export -export: exclude is rejected (also unused async option) — use the [] / allowlist forms above, not ["all", "-export:…"].

Verified with a new //test/p3:hello_p3_sync test: a p3 component with a plain sync fn step-style export that builds via the override and fails (your exact method should be async… error) without it. This removes the need for the separate p2 sibling component.