feat: helpful message for bad macos wasm toolchain

zstd-safe/zstd-sys/build.rs

@@ -1,5 +1,6 @@
 use std::ffi::OsStr;
 use std::path::{Path, PathBuf};
+use std::process::Command;
 use std::{env, fmt, fs};
 
 #[cfg(feature = "bindgen")]
@@ -210,7 +211,7 @@ fn compile_zstd() {
     // Hide symbols from resulting library,
     // so we can be used with another zstd-linking lib.
     // See https://github.com/gyscos/zstd-rs/issues/58
-    if ! cfg!(target_env = "msvc") {
+    if !cfg!(target_env = "msvc") {
         config.flag("-fvisibility=hidden");
     }
     config.define("XXH_PRIVATE_API", Some(""));
@@ -263,13 +264,87 @@ fn cargo_print(content: &dyn fmt::Display) {
     }
 }
 
+fn check_clang_wasm_support() {
+    // Use cc-rs to find the exact compiler it will use for this target,
+    // so we probe the same binary that compile_zstd() will invoke.
+    let compiler = cc::Build::new().get_compiler();
+    let clang = compiler.path().to_string_lossy().to_string();
+
+    // We must attempt actual code generation (not just -fsyntax-only),
+    // because Apple clang can parse for wasm32 but cannot emit code for it.
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let test_obj = out_dir.join("__wasm_probe.o");
+
+    let output = Command::new(compiler.path())
+        .args([
+            "--target=wasm32-unknown-unknown",
+            "-x", "c",
+            "-c", // compile to object (exercises the backend)
+            "-o",
+        ])
+        .arg(&test_obj)
+        .arg("-") // read from stdin
+        .stdin(std::process::Stdio::null())
+        .output();
+
+    match output {
+        Err(e) => {
+            panic!(
+                "\n\n\
+                Could not run `{}`: {}\n\
+                \n\
+                To build for wasm32-unknown-unknown on macOS you need LLVM's clang,\n\
+                which ships with wasm32 target support. Fix:\n\
+                \n\
+                    brew install llvm\n\
+                    export PATH=\"/opt/homebrew/opt/llvm/bin:$PATH\"\n\
+                \n\
+                Then re-run your build.\n",
+                clang, e
+            );
+        }
+        Ok(out) => {
+            let stderr = String::from_utf8_lossy(&out.stderr);
+            if stderr
+                .contains("No available targets are compatible with triple")
+            {
+                panic!(
+                    "\n\n\
+                    The `clang` on your PATH ({}) does not support\n\
+                    target `wasm32-unknown-unknown`.\n\
+                    \n\
+                    This is likely the Apple/Xcode clang, which omits wasm backends.\n\
+                    Fix by installing LLVM via Homebrew and prepending it to PATH:\n\
+                    \n\
+                        brew install llvm\n\
+                        export PATH=\"/opt/homebrew/opt/llvm/bin:$PATH\"\n\
+                    \n\
+                    Then re-run your build.\n\
+                    \n\
+                    Alternatively, pin the env var for just this build:\n\
+                    \n\
+                        CC=/opt/homebrew/opt/llvm/bin/clang cargo build \
+                        --target wasm32-unknown-unknown\n",
+                    clang
+                );
+            }
+            // Probe succeeded — clean up the test object
+            let _ = fs::remove_file(&test_obj);
+        }
+    }
+}
+
 fn main() {
     cargo_print(&"rerun-if-env-changed=ZSTD_SYS_USE_PKG_CONFIG");
 
     let target_arch =
         std::env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
     let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
 
+    if target_arch == "wasm32" {
+        check_clang_wasm_support();
+    }
+
     if target_arch == "wasm32" || target_os == "hermit" {
         cargo_print(&"rustc-cfg=feature=\"std\"");
     }