build: set RUST_CXX_NO_EXCEPTIONS automatically for wasm targets

Otherwise the cxx crate cannot build for a wasm platform.

build.rs

@@ -9,13 +9,24 @@ fn main() {
     let manifest_dir_opt = env::var_os("CARGO_MANIFEST_DIR").map(PathBuf::from);
     let manifest_dir = manifest_dir_opt.as_deref().unwrap_or(Path::new(""));
 
-    cc::Build::new()
+    let mut builder = cc::Build::new();
+    builder
         .file(manifest_dir.join("src/cxx.cc"))
         .cpp(true)
         .cpp_link_stdlib(None) // linked via link-cplusplus crate
         .std(cxxbridge_flags::STD)
-        .warnings_into_errors(cfg!(deny_warnings))
-        .compile("cxxbridge1");
+        .warnings_into_errors(cfg!(deny_warnings));
+
+    // If we are building for WASM then ensure that RUST_CXX_NO_EXCEPTIONS is defined
+    // otherwise the crate can fail to build for WASM targets
+    //
+    // Note that this could also be done via `target_family = "wasm"`
+    // but instead we are copying how the `cc` crate detects WASM
+    // https://github.com/rust-lang/cc-rs/blob/a0441c3bca087c6457824d05857e00d8e8b06753/src/lib.rs#L2016
+    #[cfg(any(target_arch = "wasm32", target_arch = "wasm64"))]
+    builder.define("RUST_CXX_NO_EXCEPTIONS", None);
+
+    builder.compile("cxxbridge1");
 
     println!("cargo:rerun-if-changed=src/cxx.cc");
     println!("cargo:rerun-if-changed=include/cxx.h");