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.

PROTOCOL.md

@@ -265,6 +265,11 @@ The reply is `{"type": "built", "wasm": "/actor/cargo/public/builds/<n>/<name>.w
 or `{"type": "build_failed", "errors": "...", "tests": {...}}`. Pass
 `wasm`, `source`, and `built_by` on to `spawn` or `upgrade`.
 
+Build scripts can compile C for `wasm32-wasip2`: the host sets
+`CC_wasm32_wasip2` and `AR_wasm32_wasip2` to a WASI clang with wasi-libc, so
+the `cc` crate (for example `rusqlite` with `bundled`) works. Its headers are
+WASI 0.1: C code that needs sockets does not build.
+
 A Rust actor depends on the SDK with `sausage-actor = "0.1"`. `cargo` points
 that dependency at the SDK of the host, which it publishes at
 `/actor/cargo/public/sdk/`.

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");

flake.nix

@@ -26,6 +26,14 @@
         # PATH. Keep this same runtime path in the packaged host and exercise
         # the installed binary below with PATH deliberately empty.
         runtimePath = pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ];
+        # A WASI clang with wasi-libc, so that build scripts (the `cc` crate)
+        # can compile C for wasm32-wasip2. Its headers are WASI 0.1: C code
+        # that needs sockets does not build.
+        wasiCc = pkgs.pkgsCross.wasi32.stdenv.cc;
+        wasiCcEnv = {
+          CC_wasm32_wasip2 = "${wasiCc}/bin/${wasiCc.targetPrefix}cc";
+          AR_wasm32_wasip2 = "${wasiCc}/bin/${wasiCc.targetPrefix}ar";
+        };
         craneLib = (crane.mkLib pkgs).overrideToolchain rust;
         common = {
           pname = "sausage";
@@ -48,9 +56,12 @@
           cargoExtraArgs = "--locked -p sausage";
           doCheck = false;
           nativeBuildInputs = [ pkgs.makeWrapper ];
-          # The `cargo` actor runs cargo at runtime. Build scripts need a C linker.
+          # The `cargo` actor runs cargo at runtime. Build scripts need a C linker,
+          # and C code for WASI needs the WASI clang.
           postInstall = ''
-            wrapProgram $out/bin/sausage --prefix PATH : ${runtimePath}
+            wrapProgram $out/bin/sausage --prefix PATH : ${runtimePath} \
+              --set-default CC_wasm32_wasip2 ${wasiCcEnv.CC_wasm32_wasip2} \
+              --set-default AR_wasm32_wasip2 ${wasiCcEnv.AR_wasm32_wasip2}
           '';
           meta.mainProgram = "sausage";
         });
@@ -84,7 +95,7 @@
         # The binary the host loop runs (`nix run`), built from the same
         # cached dependencies as the tests.
         checks.package = sausage;
-        checks.tests = craneLib.cargoTest (common // {
+        checks.tests = craneLib.cargoTest (common // wasiCcEnv // {
           inherit cargoArtifacts;
           cargoExtraArgs = "--locked ${testPackages}";
           # Nothing builds on the test run's target folder: don't store it.
@@ -114,7 +125,35 @@
           version = "0.1.0"
           edition = "2024"
           TOML
-          printf 'fn main() {}\n' > "$source/src/main.rs"
+          # C code for WASI, compiled with the packaged WASI clang. The native
+          # test build has no C, so it needs no C compiler for the host.
+          printf 'int probe_add(int a, int b) { return a + b; }\n' > "$source/src/add.c"
+          cat > "$source/build.rs" <<'RUST'
+          use std::process::Command;
+          fn main() {
+              if std::env::var("TARGET").unwrap() != "wasm32-wasip2" {
+                  return;
+              }
+              let out = std::env::var("OUT_DIR").unwrap();
+              let cc = std::env::var("CC_wasm32_wasip2").expect("CC_wasm32_wasip2");
+              let ar = std::env::var("AR_wasm32_wasip2").expect("AR_wasm32_wasip2");
+              let object = format!("{out}/add.o");
+              assert!(Command::new(cc).args(["-c", "src/add.c", "-o", &object]).status().unwrap().success());
+              assert!(Command::new(ar).args(["crs", &format!("{out}/libadd.a"), &object]).status().unwrap().success());
+              println!("cargo:rustc-link-search=native={out}");
+              println!("cargo:rustc-link-lib=static=add");
+          }
+          RUST
+          cat > "$source/src/main.rs" <<'RUST'
+          #[cfg(target_os = "wasi")]
+          unsafe extern "C" {
+              fn probe_add(a: i32, b: i32) -> i32;
+          }
+          fn main() {
+              #[cfg(target_os = "wasi")]
+              assert_eq!(unsafe { probe_add(2, 3) }, 5);
+          }
+          RUST
           cat > "$source/tests/probe.rs" <<'RUST'
           #[test]
           fn native_test_harness_runs() {
@@ -161,11 +200,11 @@
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
         apps.loop = { type = "app"; program = "${loop}/bin/sausage-loop"; };
 
-        devShells.default = pkgs.mkShell {
+        devShells.default = pkgs.mkShell (wasiCcEnv // {
           packages = [ rust pkgs.wasmtime pkgs.wasm-tools ];
           shellHook = ''
             export CARGO_HOME="$PWD/.cargo-home"
           '';
-        };
+        });
       });
 }