implementing a world that imports wasi + wasi-http · wasmtime

Hi! I'm trying to implement a world that imports both wasi types and wasi-http types and running into issues with satisfying wasmtime bindgen Host trait requirements. So if I use just wasi then I can implement all my interfaces for WasiImpl<MyState> and it automatically satisfies all the wasi import traits because they are implemented for WasiImpl<T>, but then I can't add wasi-http implementations since they are only implemented for WasiHttpImpl<T> newtype, and the wasmtime bindgen generated world requires the World::add_to_linker(linker, state) to have state satisfy FnOnce(T) -> U, where U implements all my world interfaces, wasi interfaces, and wasi-http interfaces in one type, which seems impossible unless I create a new newtype and implement all wasi and wasi-http implementations for it?

Marek Vavrusa (Mar 19 2025 at 20:55):

I guess I can implement a custom add_to_linker without the Host traits as they are satisfied via newtype conversions already

Alex Crichton (Mar 19 2025 at 20:57):

Are you implementing the wasi traits by hand? Or using the impls in the wasmtime-wasi* crates? If you're only doing your own functionality by hand then you might also be interested in the *_get_host variant of the add_to_linker functions that are generated

Marek Vavrusa (Mar 19 2025 at 20:59):

Alex Crichton (Mar 19 2025 at 21:02):

ah ok, in that case what led you to work with WasiImpl? In theory that's just an implementation detail of those crates

Marek Vavrusa (Mar 19 2025 at 21:06):

The wasmtime-wasi* crates add_to_linker()are fine because they require T to be WasiView/WasiHttpView and then wrap the T in newtype and add bindings for that. The problem is bindgen generated add_to_linker for my world which generates a function that looks like this:

pub fn add_to_linker<T, U>(
            linker: &mut wasmtime::component::Linker<T>,
            get: impl Fn(&mut T) -> &mut U + Send + Sync + Copy + 'static,
        ) -> wasmtime::Result<()>
        where
            T: Send, U: host::app::init::Host +  __with_name9::network::Host  + __with_name5::wall_clock::Host ...

Marek Vavrusa (Mar 19 2025 at 21:07):

Like ideally bindgen would generate U constraints like host::app::init::Host + WasiView

Marek Vavrusa (Mar 19 2025 at 21:16):

I ignored the generated add_to_linker, and instead implemented add_to_linker manually for my concrete MyState which seems like the best solution

Alex Crichton (Mar 19 2025 at 21:28):

Makes sense, I'd recommend avoiding the generated add_to_linker for wasi, you probably want the top-level handwritten one instead which avoids dealing with WasiImpl

Marek Vavrusa (Mar 19 2025 at 21:33):

Yeah makes sense thanks! I was thinking about this for a while but only after writing this it occurred to me that I can just write my own add_to_linker :melting_face:

Andrew Gazelka (Mar 22 2025 at 00:58):

any recommended tutorials for using wasmtime with hot reloading? trying to figure it out for a game I am making

Stream: wasmtime

Topic: implementing a world that imports wasi + wasi-http

Marek Vavrusa (Mar 19 2025 at 20:45):