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.

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"
           '';
-        };
+        });
       });
 }