Stream: general

Topic: Async component model - string encoding support?

BigOrangeQWQ (Sep 07 2025 at 12:54):

What is the support scope of Wasmtime's async component for different string encodings?

I have a WIT file as follows:

package my:test;

interface i {
  ping: async func(x: future<string>, y: string) -> future<string>;
}

world test {
  export i;
}

world runner {
  import i;
}

When using wasm-tools component embed, if the string-encoding parameters are different, it seems to trigger an invalid error during task.return. Specifically, because the parameter y is of type string, the embed operation encodes the async lift. However, if the string-encoding of test and runner differs, it causes an issue at this point (as seen in this code line).

Alex Crichton (Sep 08 2025 at 04:07):

non-utf8 encodings aren't super well tested right now, so while wasmtime should have support for everything this is likely a guest toolchain issue unable to produce the correct component. Could you file an issue with a reproduction? That'll help narrow down where the bug lies and we can transfer it to the right repo once the source is found

BigOrangeQWQ (Sep 08 2025 at 16:10):

Yes, I agree. This issue cannot be reproduced with the wat files generated by wasm-tools embed --dummy-names legacy --async-callback (I've tried many methods but all failed). However, the wat code generated by wit-bindgen is too lengthy. I'm currently thinking about how to minimize the error case before submitting an issue.

Alex Crichton (Sep 08 2025 at 18:30):

it's ok if it's not the most minimal thing in the world, we can help minimize too once we can repro

Last updated: Dec 06 2025 at 05:03 UTC