Use wasm throw instruction on wasm32

.github/workflows/ci.yml

@@ -120,9 +120,13 @@ jobs:
       - name: Install rust-src
         run: rustup component add rust-src
       - name: Test with panic backend (debug)
-        run: ./cargo-wasi test --target wasm32-wasip1
+        run: LITHIUM_BACKEND=panic ./cargo-wasi test --target wasm32-wasip1
+      - name: Test with Itanium backend (debug)
+        run: LITHIUM_BACKEND=itanium ./cargo-wasi test --target wasm32-wasip1
       - name: Test with panic backend (release)
-        run: ./cargo-wasi test --target wasm32-wasip1 --release
+        run: LITHIUM_BACKEND=panic ./cargo-wasi test --target wasm32-wasip1 --release
+      - name: Test with Itanium backend (release)
+        run: LITHIUM_BACKEND=itanium ./cargo-wasi test --target wasm32-wasip1 --release
 
   darwin:
     runs-on: ${{ matrix.os }}

build.rs

@@ -23,7 +23,9 @@ fn main() {
     } else if cfg("target_os") == "emscripten" {
         println!("cargo::rustc-cfg=backend=\"emscripten\"");
     } else if version_meta().unwrap().channel == Channel::Nightly
-        && (has_cfg("unix") || (has_cfg("windows") && cfg("target_env") == "gnu"))
+        && (has_cfg("unix")
+            || (has_cfg("windows") && cfg("target_env") == "gnu")
+            || cfg("target_arch") == "wasm32")
     {
         println!("cargo::rustc-cfg=backend=\"itanium\"");
     } else if version_meta().unwrap().channel == Channel::Nightly

src/backend/itanium.rs

@@ -32,8 +32,7 @@ unsafe impl ThrowByPointer for ActiveBackend {
     unsafe fn throw(ex: *mut Header) -> ! {
         // SAFETY: We provide a valid exception header.
         unsafe {
-            #[expect(clippy::used_underscore_items, reason = "External API")]
-            _Unwind_RaiseException(ex.cast());
+            raise(ex.cast());
         }
     }
 
@@ -73,8 +72,7 @@ unsafe impl ThrowByPointer for ActiveBackend {
             //   If project-ffi-unwind changes the rustc behavior, we might have to update this
             //   code.
             unsafe {
-                #[expect(clippy::used_underscore_items, reason = "External API")]
-                _Unwind_RaiseException(ex);
+                raise(ex);
             }
         }
 
@@ -141,7 +139,8 @@ const fn get_unwinder_private_word_count() -> usize {
         target_arch = "sparc64",
         target_arch = "riscv64",
         target_arch = "riscv32",
-        target_arch = "loongarch64"
+        target_arch = "loongarch64",
+        target_arch = "wasm32"
     )) {
         2
     } else {
@@ -166,6 +165,27 @@ unsafe extern "C" fn cleanup(_code: i32, _ex: *mut Header) {
     core::intrinsics::abort();
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 extern "C-unwind" {
     fn _Unwind_RaiseException(ex: *mut u8) -> !;
 }
+
+/// Raise an Itanium EH ABI-compatible exception.
+///
+/// # Safety
+///
+/// `ex` must point at a valid instance of `_Unwind_Exception`.
+unsafe fn raise(ex: *mut u8) -> ! {
+    #[cfg(not(target_arch = "wasm32"))]
+    #[expect(clippy::used_underscore_items, reason = "External API")]
+    // SAFETY: Passthrough.
+    unsafe {
+        _Unwind_RaiseException(ex);
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    // SAFETY: Passthrough.
+    unsafe {
+        core::arch::wasm32::throw::<0>(ex);
+    }
+}

src/lib.rs

@@ -91,6 +91,10 @@
     feature(core_intrinsics)
 )]
 #![cfg_attr(backend = "seh", feature(fn_ptr_trait, std_internals))]
+#![cfg_attr(
+    all(backend = "itanium", target_arch = "wasm32"),
+    feature(wasm_exception_handling_intrinsics)
+)]
 #![deny(unsafe_op_in_unsafe_fn)]
 #![warn(
     clippy::cargo,