Simple PoC with Wit and WASMA

.envrc

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+use_nix

.gitignore

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+/target

.vscode/settings.json

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+{
+    "nixEnvSelector.nixFile": "${workspaceFolder}/shell.nix"
+}

Cargo.toml

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+workspace = { members = ["witguest"] }
+[package]
+name = "wittest"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+async-trait = "0.1.80"
+tokio = { version = "1.37.0", features = ["full"] }
+wasmtime = "19.0.2"
+wasmtime-wasi = "19.0.2"

shell.nix

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+let
+  rust_overlay = import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz");
+  pkgs = import <nixpkgs> { overlays = [ rust_overlay ]; };
+  
+  rust = pkgs.rust-bin.stable.latest.default.override {
+    targets = [
+        "x86_64-unknown-linux-gnu"
+        "wasm32-unknown-unknown"
+        "wasm32-wasi"
+    ];
+    extensions = [
+      "rust-analyzer"
+      "rust-src"
+      #"miri-preview"
+    ];
+  };
+in
+pkgs.mkShell.override {
+  stdenv = pkgs.stdenvAdapters.useMoldLinker pkgs.clangStdenv;
+} rec {
+  buildInputs = [
+    rust
+  ] ++ (with pkgs; [
+    cargo-component wasmtime wasm-tools
+  ]);
+  LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath buildInputs;
+}

src/main.rs

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+use async_trait::async_trait;
+use wasmtime_wasi::WasiView;
+
+wasmtime::component::bindgen!({
+    world: "example",
+    path: "witguest/wit/world.wit",
+    //tracing: true,
+    async: true,
+});
+
+struct Imports {
+
+}
+
+impl WasiView for Imports {
+    fn table(&mut self) -> &mut wasmtime_wasi::ResourceTable {
+        todo!()
+    }
+
+    fn ctx(&mut self) -> &mut wasmtime_wasi::WasiCtx {
+        todo!()
+    }
+}
+
+#[async_trait]
+impl ExampleImports for Imports {
+    async fn host_test(&mut self,) ->  wasmtime::Result<()> {
+        println!("This is the host saying hi from the guest!");
+        Ok(())
+    }
+}
+
+#[tokio::main]
+async fn main() -> wasmtime::Result<()> {
+    // Instantiate the engine and store
+    let mut config = wasmtime::Config::default();
+    config.async_support(true);
+
+    let engine = wasmtime::Engine::new(&config)?;
+    let mut store = wasmtime::Store::new(&engine, Imports {});
+
+    // Load the component from disk
+    let bytes = std::fs::read("target/wasm32-wasi/debug/witguest.wasm")?;
+    let component = wasmtime::component::Component::new(&engine, bytes)?;
+
+    // Configure the linker
+    let mut linker = wasmtime::component::Linker::new(&engine);
+    wasmtime_wasi::bindings::Imports::add_to_linker(&mut linker, |s| s)?;
+    Example::add_to_linker(&mut linker, |s| s)?;
+
+    // Instantiate the component
+    println!("creating example");
+    let (example, _instance) = Example::instantiate_async(&mut store, &component, &linker).await?;
+    let result = example.call_hello_world(&mut store).await?;
+    println!("Guest says: {}", result);
+
+    Ok(())
+}

witguest/.vscode/settings.json

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+{
+    "rust-analyzer.check.overrideCommand": [
+        "cargo",
+        "component",
+        "check",
+        "--workspace",
+        "--all-targets",
+        "--message-format=json"
+    ],
+}

witguest/Cargo.toml

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+[package]
+name = "witguest"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+wit-bindgen = { version = "0.24.0", default-features = false, features = ["realloc"] }
+
+[lib]
+crate-type = ["cdylib"]
+
+[package.metadata.component]
+package = "component:witguest"
+
+[package.metadata.component.dependencies]

witguest/src/bindings.rs

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+// Generated by `wit-bindgen` 0.18.0. DO NOT EDIT!
+#[allow(unused_unsafe, clippy::all)]
+pub fn host_test(){
+  
+  #[allow(unused_imports)]
+  use wit_bindgen::rt::{alloc, vec::Vec, string::String};
+  unsafe {
+    
+    #[cfg(target_arch = "wasm32")]
+    #[link(wasm_import_module = "$root")]
+    extern "C" {
+      #[link_name = "host-test"]
+      fn wit_import();
+    }
+    
+    #[cfg(not(target_arch = "wasm32"))]
+    fn wit_import(){ unreachable!() }
+    wit_import();
+  }
+}
+const _: () = {
+  
+  #[doc(hidden)]
+  #[export_name = "hello-world"]
+  #[allow(non_snake_case)]
+  unsafe extern "C" fn __export_hello_world() -> i32 {
+    #[allow(unused_imports)]
+    use wit_bindgen::rt::{alloc, vec::Vec, string::String};
+    
+    // Before executing any other code, use this function to run all static
+    // constructors, if they have not yet been run. This is a hack required
+    // to work around wasi-libc ctors calling import functions to initialize
+    // the environment.
+    //
+    // This functionality will be removed once rust 1.69.0 is stable, at which
+    // point wasi-libc will no longer have this behavior.
+    //
+    // See
+    // https://github.com/bytecodealliance/preview2-prototyping/issues/99
+    // for more details.
+    #[cfg(target_arch="wasm32")]
+    wit_bindgen::rt::run_ctors_once();
+    
+    let result0 = <_GuestImpl as Guest>::hello_world();
+    let ptr1 = _RET_AREA.0.as_mut_ptr() as i32;
+    let vec2 = (result0.into_bytes()).into_boxed_slice();
+    let ptr2 = vec2.as_ptr() as i32;
+    let len2 = vec2.len() as i32;
+    ::core::mem::forget(vec2);
+    *((ptr1 + 4) as *mut i32) = len2;
+    *((ptr1 + 0) as *mut i32) = ptr2;
+    ptr1
+  }
+  
+  const _: () = {
+    #[doc(hidden)]
+    #[export_name = "cabi_post_hello-world"]
+    #[allow(non_snake_case)]
+    unsafe extern "C" fn __post_return_hello_world(arg0: i32,) {
+      let l0 = *((arg0 + 0) as *const i32);
+      let l1 = *((arg0 + 4) as *const i32);
+      wit_bindgen::rt::dealloc(l0, (l1) as usize, 1);
+    }
+  };
+};
+use super::Component as _GuestImpl;
+pub trait Guest {
+  fn hello_world() -> wit_bindgen::rt::string::String;
+}
+
+#[allow(unused_imports)]
+use wit_bindgen::rt::{alloc, vec::Vec, string::String};
+
+#[repr(align(4))]
+struct _RetArea([u8; 8]);
+static mut _RET_AREA: _RetArea = _RetArea([0; 8]);
+
+#[cfg(target_arch = "wasm32")]
+#[link_section = "component-type:example"]
+#[doc(hidden)]
+pub static __WIT_BINDGEN_COMPONENT_TYPE: [u8; 201] = [0, 97, 115, 109, 13, 0, 1, 0, 0, 25, 22, 119, 105, 116, 45, 99, 111, 109, 112, 111, 110, 101, 110, 116, 45, 101, 110, 99, 111, 100, 105, 110, 103, 4, 0, 7, 77, 1, 65, 2, 1, 65, 4, 1, 64, 0, 1, 0, 3, 0, 9, 104, 111, 115, 116, 45, 116, 101, 115, 116, 1, 0, 1, 64, 0, 0, 115, 4, 0, 11, 104, 101, 108, 108, 111, 45, 119, 111, 114, 108, 100, 1, 1, 4, 1, 26, 99, 111, 109, 112, 111, 110, 101, 110, 116, 58, 119, 105, 116, 103, 117, 101, 115, 116, 47, 101, 120, 97, 109, 112, 108, 101, 4, 0, 11, 13, 1, 0, 7, 101, 120, 97, 109, 112, 108, 101, 3, 0, 0, 0, 70, 9, 112, 114, 111, 100, 117, 99, 101, 114, 115, 1, 12, 112, 114, 111, 99, 101, 115, 115, 101, 100, 45, 98, 121, 2, 13, 119, 105, 116, 45, 99, 111, 109, 112, 111, 110, 101, 110, 116, 6, 48, 46, 50, 49, 46, 48, 16, 119, 105, 116, 45, 98, 105, 110, 100, 103, 101, 110, 45, 114, 117, 115, 116, 6, 48, 46, 49, 56, 46, 48];
+
+#[inline(never)]
+#[doc(hidden)]
+#[cfg(target_arch = "wasm32")]
+pub fn __link_section() {}

witguest/src/lib.rs

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+mod bindings;
+
+use bindings::Guest;
+
+struct Component;
+
+impl Guest for Component {
+    /// Say hello!
+    fn hello_world() -> String {
+        bindings::host_test();
+        "Hello, World!".to_string()
+    }
+}

witguest/wit/world.wit

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+package component:witguest;
+
+/// An example world for the component to target.
+world example {
+    import host-test: func();
+    export hello-world: func() -> string;
+}