cargo: compile C for wasm32-wasip2 with a WASI clang

The Nix package, the dev shell, and the test check set CC_wasm32_wasip2
and AR_wasm32_wasip2 to the clang of pkgsCross.wasi32 (wasi-libc), so
build scripts with the cc crate work, for example rusqlite with bundled
SQLite. The runtime-toolchain check links C into a probe with the
packaged host and an empty PATH.

cli/src/cargo_actor.rs

@@ -8,6 +8,9 @@
 //! The broker knows nothing about Rust. This actor is one way to get a
 //! `.wasm` file. Other actors can add other languages.
 //!
+//! Build scripts compile C for WASI with the clang in `CC_wasm32_wasip2`
+//! and `AR_wasm32_wasip2`. The Nix package and the dev shell set both.
+//!
 //! Actors depend on the SDK with `sausage-actor = "0.1"`. The SDK is not
 //! published: this actor copies the SDK of this host to `public/sdk/` (so
 //! that every actor can read it), and points the dependency there in the

cli/tests/crates/cargo-c/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "cargo-c"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+sausage-actor = "0.1"
+
+[build-dependencies]
+cc = "1"
+
+[workspace]

cli/tests/crates/cargo-c/build.rs

@@ -0,0 +1,5 @@
+// Compile C with the `cc` crate: natively for the unit test, and with the
+// WASI clang (`CC_wasm32_wasip2`) for the actor.
+fn main() {
+    cc::Build::new().file("src/add.c").compile("add");
+}

cli/tests/crates/cargo-c/src/add.c

@@ -0,0 +1 @@
+int c_add(int a, int b) { return a + b; }

cli/tests/crates/cargo-c/src/main.rs

@@ -0,0 +1,25 @@
+//! Adds two numbers with a C function: `{"type": "add", "a", "b"}` -> `{"type": "sum", "value"}`.
+use sausage_actor::{json, messages, reply};
+
+unsafe extern "C" {
+    fn c_add(a: i32, b: i32) -> i32;
+}
+
+fn add(a: i32, b: i32) -> i32 {
+    unsafe { c_add(a, b) }
+}
+
+fn main() {
+    for message in messages() {
+        let (a, b) = (message.body["a"].as_i64().unwrap_or(0), message.body["b"].as_i64().unwrap_or(0));
+        let _ = reply(&message, json!({"type": "sum", "value": add(a as i32, b as i32)}));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn c_code_runs_on_the_host_target() {
+        assert_eq!(super::add(2, 3), 5);
+    }
+}

cli/tests/host.rs

@@ -769,6 +769,22 @@ async fn cargo_actor_runs_unit_tests_by_default_and_can_skip_them() {
     host.stop().await;
 }
 
+#[tokio::test]
+async fn cargo_actor_compiles_c_code_for_wasi() {
+    let world = create_world("cargo-c");
+    let (host, mut mailbox) = start(&world).await;
+    let source = cargo_source("cargo-c");
+    let id = host.broker.send("console", Some("cargo"), None, None, json!({"type": "build", "source": source.display().to_string()}));
+    let built = receive_reply(&mut mailbox, &id, Duration::from_secs(300)).await;
+    assert_eq!(built.body["type"], "built", "{built:?}");
+    assert_eq!(built.body["tests"]["passed"], 1, "{built:?}");
+    let wasm = host.world.resolve("console", built.body["wasm"].as_str().unwrap()).unwrap();
+    spawn_type(&host, &mut mailbox, "c-add", &wasm, &source).await;
+    let sum = ask_type(&host, &mut mailbox, "c-add", json!({"type": "add", "a": 2, "b": 3}), "sum").await;
+    assert_eq!(sum.body["value"], 5);
+    host.stop().await;
+}
+
 #[tokio::test]
 async fn rejects_invalid_actor_ids() {
     let world = create_world("invalid-id");