Async model

The harness is async at every layer, and the core crate is runtime-agnostic.

rusty_harness  (async fn APIs, Runtime trait — no runtime by default)
├── rt-tokio → TokioRuntime ─┬─ Rust applications
│                            └─ rusty-harness-py   (awaitables + *_sync)
└── rt-wasm  → WasmRuntime  ─── rusty-harness-wasm (Promise exports)

Rules the core follows

• Public APIs are async fn or return a plain impl Future. No runtime type appears in a public signature.
• Anything that needs a runtime — spawning, timers, I/O — goes through the Runtime trait in rusty_harness::runtime, with two feature-gated implementations:
  • rt-tokio — TokioRuntime, for native consumers and the Python bindings.
  • rt-wasm — WasmRuntime (wasm-bindgen-futures::spawn_local, gloo-timers), compiled only for wasm32.
• There is no default runtime feature. tokio is absent from the core's dependency tree unless rt-tokio is on, and CI compiles the core for wasm32-unknown-unknown on every pull request to keep it that way.
• Bounds use MaybeSend, which is Send on native targets and empty on wasm32, where JS futures are !Send and there is one thread.

Bring your own runtime

APIs that need no runtime work under any executor.

With tokio:

#[tokio::main]
async fn main() {
    println!("{}", rusty_harness::hello().await);
}

With smol:

fn main() {
    smol::block_on(async {
        println!("{}", rusty_harness::hello().await);
    });
}

APIs that do need a runtime take one explicitly:

use rusty_harness::{Runtime, runtime::TokioRuntime};

#[tokio::main]
async fn main() {
    let rt = TokioRuntime::current();
    rt.spawn(async { println!("{}", rusty_harness::hello().await) });
    rt.sleep(std::time::Duration::from_millis(10)).await;
}

In the bindings

• Python — every async API is an awaitable driven by one shared, multi-thread tokio runtime owned by the extension module (pyo3-async-runtimes); nothing creates a runtime per call. Each also has a blocking *_sync twin that releases the GIL while it waits.
• JavaScript — every async API returns a Promise, bridged by wasm-bindgen-futures.