Provide a way for non-wasm Rust code to find the WASI libc

Cargo.lock

@@ -3428,6 +3428,7 @@ dependencies = [
  "serde_json",
  "target-lexicon",
  "tempfile",
+ "wasi_libc_sys",
 ]
 
 [[package]]
@@ -4893,6 +4894,10 @@ version = "0.10.2+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fd6fbd9a79829dd1ad0cc20627bf1ed606756a7f77edff7b66b7064f9cb327c6"
 
+[[package]]
+name = "wasi_libc_sys"
+version = "0.1.0"
+
 [[package]]
 name = "wasm-bindgen"
 version = "0.2.79"

compiler/build/Cargo.toml

@@ -31,6 +31,7 @@ libloading = "0.7.1"
 tempfile = "3.2.0"
 inkwell = { path = "../../vendor/inkwell", optional = true }
 target-lexicon = "0.12.3"
+wasi_libc_sys = { path = "../../wasi-libc-sys", optional = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 serde_json = "1.0.69"
@@ -40,7 +41,7 @@ target-arm = []
 target-aarch64 = ["roc_gen_dev/target-aarch64"]
 target-x86 = []
 target-x86_64 = ["roc_gen_dev/target-x86_64"]
-target-wasm32 = ["roc_gen_wasm"]
+target-wasm32 = ["roc_gen_wasm", "wasi_libc_sys"]
 
 # This is a separate feature because when we generate docs on Netlify,
 # it doesn't have LLVM installed. (Also, it doesn't need to do code gen.)

compiler/build/src/link.rs

@@ -72,12 +72,12 @@ fn find_zig_str_path() -> PathBuf {
 }
 
 fn find_wasi_libc_path() -> PathBuf {
+    use wasi_libc_sys::WASI_LIBC_PATH;
+
     // Environment variable defined in wasi-libc-sys/build.rs
-    if let Ok(wasi_libc_path) = std::env::var("WASI_LIBC_SYS_PATH") {
-        let wasi_libc_pathbuf = PathBuf::from(&wasi_libc_path);
-        if std::path::Path::exists(&wasi_libc_pathbuf) {
-            return wasi_libc_pathbuf;
-        }
+    let wasi_libc_pathbuf = PathBuf::from(WASI_LIBC_PATH);
+    if std::path::Path::exists(&wasi_libc_pathbuf) {
+        return wasi_libc_pathbuf;
     }
 
     panic!("cannot find `wasi-libc.a`")

wasi-libc-sys/.gitignore

@@ -0,0 +1 @@
+src/generated.rs

wasi-libc-sys/build.rs

@@ -1,8 +1,8 @@
 use std::env;
+use std::ffi::OsStr;
 use std::fs;
-
-// Environment variable that can be used from other build scripts
-const WASI_LIBC_SYS_PATH: &str = "WASI_LIBC_SYS_PATH";
+use std::path::Path;
+use std::process::Command;
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
@@ -11,31 +11,36 @@ fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
     let zig_cache_dir = format!("{}/zig-cache", out_dir);
     let out_file = format!("{}/wasi-libc.a", out_dir);
-    println!("cargo:rustc-env={}={}", WASI_LIBC_SYS_PATH, &out_file);
-
-    // Compile a dummy C program with Zig, putting libc into our own private cache directory
-    let args = [
-        "build-exe",
-        "-target",
-        "wasm32-wasi",
-        "-lc",
-        "-O",
-        "ReleaseSmall",
-        "--global-cache-dir",
-        &zig_cache_dir,
-        "src/dummy.c",
-        &format!("-femit-bin={}/dummy.wasm", out_dir),
-    ];
 
+    // Compile a dummy C program with Zig, with our own private cache directory
     let zig = zig_executable();
+    run_command(
+        Path::new("."),
+        &zig,
+        [
+            "build-exe",
+            "-target",
+            "wasm32-wasi",
+            "-lc",
+            "-O",
+            "ReleaseSmall",
+            "--global-cache-dir",
+            &zig_cache_dir,
+            "src/dummy.c",
+            &format!("-femit-bin={}/dummy.wasm", out_dir),
+        ],
+    );
 
-    // println!("{} {}", zig, args.join(" "));
+    // Find the libc.a file that Zig wrote (as a side-effect of compiling the dummy program)
+    let find_cmd_output = run_command(Path::new("."), "find", [&zig_cache_dir, "-name", "libc.a"]);
+    let zig_libc_path = find_cmd_output.trim(); // get rid of a newline
 
-    run_command(Path::new("."), &zig, args);
-    let zig_libc_path = run_command(Path::new("."), "find", [&zig_cache_dir, "-name", "libc.a"]);
+    // Copy libc to where Cargo expects it
+    fs::copy(&zig_libc_path, &out_file).unwrap();
 
-    // Copy libc out of Zig's cache, to where Cargo expects it
-    fs::copy(&zig_libc_path, &out_file);
+    // Generate some Rust code to indicate where the file is
+    let generated_rust = format!("pub const WASI_LIBC_PATH: &str = \"{}\";\n", out_file);
+    fs::write("src/generated.rs", generated_rust).unwrap();
 }
 
 fn zig_executable() -> String {

wasi-libc-sys/src/lib.rs

@@ -1,7 +1,8 @@
-use core::ffi::c_void;
-
 // Rust's libc crate doesn't support Wasm, so we provide an implementation from Zig
 // We define Rust signatures here as we need them, rather than trying to cover all of libc
+#[cfg(target_family = "wasm")]
+use core::ffi::c_void;
+#[cfg(target_family = "wasm")]
 extern "C" {
     pub fn malloc(size: usize) -> *mut c_void;
     pub fn free(p: *mut c_void);
@@ -9,3 +10,10 @@ extern "C" {
     pub fn memcpy(dst: *mut c_void, src: *mut c_void, n: usize) -> *mut c_void;
     pub fn memset(dst: *mut c_void, ch: i32, n: usize) -> *mut c_void;
 }
+
+// If a non-Wasm target is using this crate, we assume it is a build script that wants to emit Wasm
+// Tell it where to find the Wasm .a file
+#[cfg(not(target_family = "wasm"))]
+mod generated;
+#[cfg(not(target_family = "wasm"))]
+pub use generated::WASI_LIBC_PATH;