Stream: general

Topic: Traits for adding host function interfaces

Lachlan Gunn (Feb 12 2026 at 15:57):

I'm involved in a project where we are building a bunch of different host interfaces that will ultimately all need to be linked into to a single Wasmtime runtime. An idea that I've been toying with is to come up with a single trait that each crate will implement for its host interfaces, so that we can compose them all together without having to manually define views into the Store<T>'s T.

I've been experimenting with a macro that lets you encapsulate all the additions to the runtime in a common trait implementation on the host interface's state to go in the Store:

#[derive(Default)]
struct CounterState(i64);
impl HostInterface for CounterState {
    fn link<T: HostInterfaceStateView<Self>>(linker: &mut wasmtime::component::Linker<T>) {
        // Add host functions to the linker
    }
}

#[derive(Default)]
struct StaticState(i64);
impl HostInterface for StaticState {
    fn link<T: HostInterfaceStateView<Self>>(linker: &mut wasmtime::component::Linker<T>) {
        // Add host functions to the linker
    }
}

Then you can compose them together:

wasmtime_extensions! {
    struct MiniState (
        CounterState
    );
}

wasmtime_extensions! {
    struct State (
        MiniState,
        StaticState
    );
}

but also add them (edit: all at once) to a Linker with a single static method:

let state: State = Default::default();
let mut store = wasmtime::Store::new(&engine, state);
let mut linker = wasmtime::component::Linker::new(&engine);

State::link(&mut linker);

But before I go down the rabbit-hole of solving some of its limitations (mainly dealing with configuration) and learning to tidy it up into a proc macro that can be #[derive]ed, is there something out there like this already that I've somehow missed, that would allow these host interfaces to be encapsulated and then aggregated together in a more-or-less tidy way in the same way that wasmtime-wasi does?

Lachlan Gunn (Feb 12 2026 at 17:01):

Or is this something that might be better done using bindgen! directly?

Alex Crichton (Feb 12 2026 at 19:35):

One piece of inspiration I might recommend here is Spin Factors which while I haven't heavily used them myself I feel have done a really good job of enabling separate crates to build various implementations to get relatively easily woven in at the end

Lachlan Gunn (Feb 13 2026 at 06:16):

Thanks! I'll take a look

Lachlan Gunn (Feb 13 2026 at 08:28):

This looks great. Sadly it comes with a little bit of baggage from Spin that prevents it from being used as-is, but as you say, it looks like great inspiration, and I'm glad to have some confirmation that it's not a completely terrible idea. I had that doubt in my mind of _this seems obvious enough that if it were a good idea, then it would probably have been built into Wasmtime or some associated crate already_.

Last updated: Feb 24 2026 at 04:36 UTC