Build WASI libc using Zig, instead of using a checked-in binary

Cargo.toml

@@ -55,6 +55,7 @@ exclude = [
     "compiler/test_mono_macros",
     # `cargo build` would cause roc_std to be built with default features which errors on windows 
     "roc_std",
+    "wasi-libc-sys",
 ]
 # Needed to be able to run `cargo run -p roc_cli --no-default-features` -
 # see www/build.sh for more.

compiler/build/src/link.rs

@@ -72,16 +72,12 @@ fn find_zig_str_path() -> PathBuf {
 }
 
 fn find_wasi_libc_path() -> PathBuf {
-    let wasi_libc_path = PathBuf::from("compiler/builtins/bitcode/wasi-libc.a");
-
-    if std::path::Path::exists(&wasi_libc_path) {
-        return 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;
         }
-
-    // when running the tests, we start in the /cli directory
-    let wasi_libc_path = PathBuf::from("../compiler/builtins/bitcode/wasi-libc.a");
-    if std::path::Path::exists(&wasi_libc_path) {
-        return wasi_libc_path;
     }
 
     panic!("cannot find `wasi-libc.a`")

wasi-libc-sys/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+authors = ["The Roc Contributors"]
+description = "Rust wrapper for a WebAssembly test platform built on libc"
+edition = "2018"
+license = "UPL-1.0"
+name = "wasi_libc_sys"
+repository = "https://github.com/rtfeldman/roc"
+version = "0.1.0"

wasi-libc-sys/build.rs

@@ -0,0 +1,70 @@
+use std::env;
+use std::fs;
+
+// Environment variable that can be used from other build scripts
+const WASI_LIBC_SYS_PATH: &str = "WASI_LIBC_SYS_PATH";
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=src/dummy.c");
+
+    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),
+    ];
+
+    let zig = zig_executable();
+
+    // println!("{} {}", zig, args.join(" "));
+
+    run_command(Path::new("."), &zig, args);
+    let zig_libc_path = run_command(Path::new("."), "find", [&zig_cache_dir, "-name", "libc.a"]);
+
+    // Copy libc out of Zig's cache, to where Cargo expects it
+    fs::copy(&zig_libc_path, &out_file);
+}
+
+fn zig_executable() -> String {
+    match std::env::var("ROC_ZIG") {
+        Ok(path) => path,
+        Err(_) => "zig".into(),
+    }
+}
+
+fn run_command<S, I, P: AsRef<Path>>(path: P, command_str: &str, args: I) -> String
+where
+    I: IntoIterator<Item = S>,
+    S: AsRef<OsStr>,
+{
+    let output_result = Command::new(OsStr::new(&command_str))
+        .current_dir(path)
+        .args(args)
+        .output();
+    match output_result {
+        Ok(output) => match output.status.success() {
+            true => std::str::from_utf8(&output.stdout).unwrap().to_string(),
+            false => {
+                let error_str = match std::str::from_utf8(&output.stderr) {
+                    Ok(stderr) => stderr.to_string(),
+                    Err(_) => format!("Failed to run \"{}\"", command_str),
+                };
+                panic!("{} failed: {}", command_str, error_str);
+            }
+        },
+        Err(reason) => panic!("{} failed: {}", command_str, reason),
+    }
+}

wasi-libc-sys/src/dummy.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+int main(int argc, char **argv)
+{
+    printf("I am a dummy C program. I only exist to help the build script to find libc.a and copy it");
+}

wasi-libc-sys/src/lib.rs

@@ -0,0 +1,11 @@
+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
+extern "C" {
+    pub fn malloc(size: usize) -> *mut c_void;
+    pub fn free(p: *mut c_void);
+    pub fn realloc(p: *mut c_void, size: usize) -> *mut c_void;
+    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;
+}