panic in sync-lifted `wasi:http/handler` due to `FutureWriter::drop` outside task context

[posborne]

Attempting to write a wasi-http handler with a sync-lifted handler panics in guest code.

How to Reproduce

Using -export: to sync-lift a wasi:http/handler#handle export:

// async: ["-export:wasi:http/handler@0.3.0-rc-2026-03-15#handle"]
fn handle(_req: Request) -> Result<Response, ErrorCode> {
    let headers = Fields::new();
    let (tw, tr) = wit_future::new(|| Ok(None::<Trailers>));
    drop(tw);  // <-- panics
    let (response, _) = Response::new(headers, None, tr);
    Ok(response)
}

Results in a trap/panic in guest code.

Backtrace

  thread '<unnamed>' (1) panicked at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:193:13:
  assertion failed: !task.is_null()
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  worker error: error while executing at wasm backtrace:
     // ... panic/abort frames ....
     12:   0x4dd6 - wit_bindgen::rt::async_support::waitable::WaitableOperation<S>::register_waker::hde47cb997528dce8
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:193:13
                  - wit_bindgen::rt::async_support::waitable::WaitableOperation<S>::poll_complete_with_code::hd22a90f526e6d1de
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:338:18
     13:   0x57a5 - wit_bindgen::rt::async_support::waitable::WaitableOperation<S>::poll_complete::h605a2b23cc93fc87
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:293:14
     14:   0x5c81 - <wit_bindgen::rt::async_support::future_support::RawFutureWrite<O> as core::future::future::Future>::poll::hc72fcaed7ea6645c
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:716:14
                  - <wit_bindgen::rt::async_support::future_support::RawFutureWriter<O>::write_and_forget::DeferredWrite<O> as alloc::task::Wake>::wake::he1249cadca69f382
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:554:62
     15:   0x5a79 - wit_bindgen::rt::async_support::future_support::RawFutureWriter<O>::write_and_forget::h235167da4f91e4a1
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:506:10
                  - <wit_bindgen::rt::async_support::future_support::FutureWriter<T> as core::ops::drop::Drop>::drop::hcaaad8d86644138a
                      at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:379:17
                  - core::ptr::drop_in_place<wit_bindgen::rt::async_support::future_support::FutureWriter<core::result::Result<core::option::Option<wasi_http_concurrency_guest::wasi::http::types::Fields>,wasi_http_concurrency_guest::wasi::http::types::ErrorCode>>>::h606931453b035b43
                      at /Users/paul.osborne/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:805:1
                  - core::mem::drop::hf433209343d7ac34
                      at /Users/paul.osborne/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/mem/mod.rs:975:1
                  - <wasi_http_concurrency_guest::Component as wasi_http_concurrency_guest::exports::wasi::http::handler::Guest>::handle::h74ebd70f1b9d8869
                      at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:249:9
     16:   0x2218 - wasi_http_concurrency_guest::exports::wasi::http::handler::_export_handle_cabi::h30c4ecad9fbc835d
                      at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:90:1
     17:   0x62a2 - wasi:http/handler@0.3.0-rc-2026-03-15#handle
                      at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:90:1

Notes

• The -export: prefix generates correct ABI (fn handle, bare export symbol without [async-lift]). wasmtime 44 finds and calls the export; implicit backpressure via Task.needs_exclusive() works correctly.
• The panic path goes through FutureWriter::drop → write_and_forget → register_waker, which requires an active task context absent in a sync-lifted call. See #1456 for discussion.
• Related upstream discussion: WebAssembly/WASI #898 (instance reuse opt-out via sync-lift).

Versions

wit-bindgen 0.55.0, wasmtime 44.0.1, wasi:http 0.3.0-rc-2026-03-15