Stream: wit-bindgen

Topic: [async] dropped from wasm names?

Scott Waye (Dec 06 2025 at 19:40):

HI, has [async] been dropped from the wasm function names, if so, does that mean a new version of wasmtime is needed?

Joel Dice (Dec 07 2025 at 01:04):

Correct; it's due to https://github.com/WebAssembly/component-model/pull/578. If you're using the latest wasm-tools and wit-bindgen, you'll need to use Wasmtime's main branch until v40.0.0 is released.

Scott Waye (Dec 08 2025 at 13:22):

Thanks! Looking at this wit https://github.com/bytecodealliance/wit-bindgen/blob/4284ea88f0b98e2e20e366298e95c9aa2ca4b488/tests/runtime-async/async/simple-import-params-results/test.wit#L18
It generates this c code

__attribute__((__export_name__("run")))
void __wasm_export_exports_runner_run(void) {
  exports_runner_run();
}

and nothing to indicate it is async. Is that right?

Scott Waye (Dec 08 2025 at 13:35):

I suppose more specifically why isn't [async-lift] required here any more?

Lann Martin (Dec 08 2025 at 13:54):

async-ness is now (Wasmtime main / the above PR) part of the function signature so the flag doesn't need to be part of the name any more.

Scott Waye (Dec 08 2025 at 13:56):

How is asyncness expressed in c in the signature? I'm wondering how that would be expressed in LLVM and c would give me a clue.

Lann Martin (Dec 08 2025 at 13:59):

I'm not sure if async support has even been added to the C bindings generator yet.

Scott Waye (Dec 08 2025 at 14:00):

ah ok, that might explain my confusion

Lann Martin (Dec 08 2025 at 14:00):

Semantically, async now means "may block before returning".

Alex Crichton (Dec 08 2025 at 14:22):

Oh the C generator does have async support yeah. That specific test is this one which specifies that the run function does a sync lift into an async type, hence the lack of [async-lift]

Joel Dice (Dec 08 2025 at 14:26):

The C bindings generator definitely supports async, and yes, the export should include [async-lift] by default for async-typed functions. This is what I see when I run wit-bindgen on main:

 $ cargo run -- c -w runner tests/runtime-async/async/simple-import-params-results/test.wit
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/wit-bindgen c -w runner tests/runtime-async/async/simple-import-params-results/test.wit`
Generating "runner.c"
Generating "runner.h"
Generating "runner_component_type.o"
 $ grep -B 5 -A 5 '\[async-lift\]run' runner.c
  return ret;
}

// Helper Functions

__attribute__((__export_name__("[callback][async-lift]run")))
uint32_t __wasm_export_exports_runner_run_callback(uint32_t event_raw, uint32_t waitable, uint32_t code) {
  runner_event_t event;
  event.event = (runner_event_code_t) event_raw;
  event.waitable = waitable;
  event.code = code;
--

runner_subtask_status_t a_b_i_two_arguments_and_result(uint32_t x, uint32_t y, uint32_t *result) {
  return __wasm_import_a_b_i_two_arguments_and_result((int32_t) (x), (int32_t) (y), (uint8_t*) result);
}

__attribute__((__export_name__("[async-lift]run")))
int32_t __wasm_export_exports_runner_run(void) {
  runner_callback_code_t ret = exports_runner_run();
  return ret;
}

Scott Waye (Dec 08 2025 at 14:27):

with main wit-bindgen ?

Joel Dice (Dec 08 2025 at 14:27):

correct

Alex Crichton (Dec 08 2025 at 14:31):

0.48/0.49 should work the same way too

Scott Waye (Dec 08 2025 at 14:32):

Interesting, when I run target\debug\wit-bindgen.exe test --languages rust,c tests/runtime-async/async --artifacts target/artifacts --rust-wit-bindgen-path ./crates/guest-rust --runner "wasmtime -W component-model-async" I don't get that c generated. Let me debug to see where I've gone wrong

Joel Dice (Dec 08 2025 at 14:34):

Ah; I'll bet it's this directive in runner.c: //@ args = '--rename a:b/i=test --async=-run'

Joel Dice (Dec 08 2025 at 14:35):

That tells the test crate to disable async lift for the run function. When I run wit-bindgen-cli, it ignores that directive, hence the difference between my results and yours.

Joel Dice (Dec 08 2025 at 14:35):

The upshot is that [async-lift] is indeed the default, but can be overridden via --async=-run

Last updated: Jan 09 2026 at 13:15 UTC