std support for wasm32 panic=unwind

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -5,6 +5,7 @@
 };
 use crate::llvm;
 use libc::c_int;
+use rustc_codegen_ssa::base::wants_wasm_eh;
 use rustc_codegen_ssa::traits::PrintBackendInfo;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::small_c_str::SmallCStr;
@@ -98,8 +99,12 @@ fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
             }
         }
 
-        if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
-            add("-enable-emscripten-cxx-exceptions", false);
+        if sess.panic_strategy() == PanicStrategy::Unwind {
+            if sess.target.os == "emscripten" {
+                add("-enable-emscripten-cxx-exceptions", false);
+            } else if wants_wasm_eh(sess) {
+                add("-wasm-enable-eh", false);
+            }
         }
 
         // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
@@ -520,6 +525,10 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
             .map(String::from),
     );
 
+    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
+        features.push("+exception-handling".into());
+    }
+
     // -Ctarget-features
     let supported_features = sess.target.supported_target_features();
     let mut featsmap = FxHashMap::default();

library/panic_unwind/src/lib.rs

@@ -16,6 +16,10 @@
 #![doc(issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
+#![cfg_attr(
+    all(target_family = "wasm", not(target_os = "emscripten")),
+    feature(link_llvm_intrinsics)
+)]
 #![feature(panic_unwind)]
 #![feature(staged_api)]
 #![feature(std_internals)]
@@ -56,9 +60,16 @@
     ))] {
         #[path = "gcc.rs"]
         mod real_imp;
+    } else if #[cfg(all(target_family = "wasm", panic = "unwind"))] {
+        // for now, PanicStrategy::Unwind is not the default for wasm targets,
+        // so we need the panic = "unwind" in the cfg above. to use llvm.wasm.throw,
+        // we need to pass -wasm-enable-eh to LLVM, but that only happens if rustc
+        // is compiling with -C panic=unwind. So, this lets us -Zbuild-std with
+        // panic=unwind, while keeping the default panic=abort working.
+        #[path = "wasm.rs"]
+        mod real_imp;
     } else {
         // Targets that don't support unwinding.
-        // - family=wasm
         // - os=none ("bare metal" targets)
         // - os=uefi
         // - os=espidf

library/panic_unwind/src/wasm.rs

@@ -0,0 +1,32 @@
+//! Unwinding panics for wasm32.
+use alloc::boxed::Box;
+use core::any::Any;
+
+// The type of the exception payload that the wasm engine propagates
+// through unwinding for us. LLVM requires that it be a thin pointer.
+type Payload = Box<Box<dyn Any + Send>>;
+
+extern "C" {
+    /// LLVM lowers this intrinsic to the `throw` instruction.
+    #[link_name = "llvm.wasm.throw"]
+    fn wasm_throw(tag: i32, ptr: *mut u8) -> !;
+}
+
+pub unsafe fn panic(payload: Box<dyn Any + Send>) -> u32 {
+    // The payload we pass to `wasm_throw` will be exactly the argument we get
+    // in `cleanup` below. So we just box it up once, to get something pointer-sized.
+    let payload_box: Payload = Box::new(payload);
+    // The wasm `throw` instruction takes a "tag", which differentiates certain
+    // types of exceptions from others. LLVM currently just identifies these
+    // via integers, with 0 corresponding to C++ exceptions and 1 to C setjmp()/longjmp().
+    // Ideally, we'd be able to choose something unique for Rust, such that we
+    // don't try to treat a C++ exception payload as a `Box<Box<dyn Any>>`, but
+    // otherwise, pretending to be C++ works for now.
+    wasm_throw(0, Box::into_raw(payload_box) as *mut u8)
+}
+
+pub unsafe fn cleanup(payload_box: *mut u8) -> Box<dyn Any + Send> {
+    // Recover the underlying `Box`.
+    let payload_box: Payload = Box::from_raw(payload_box as *mut _);
+    *payload_box
+}

library/std/src/sys/personality/mod.rs

@@ -16,11 +16,12 @@
 cfg_if::cfg_if! {
     if #[cfg(target_os = "emscripten")] {
         mod emcc;
-    } else if #[cfg(target_env = "msvc")] {
+    } else if #[cfg(any(target_env = "msvc", target_family = "wasm"))] {
         // This is required by the compiler to exist (e.g., it's a lang item),
         // but it's never actually called by the compiler because
-        // _CxxFrameHandler3 is the personality function that is always used.
-        // Hence this is just an aborting stub.
+        // __CxxFrameHandler3 (msvc) / __gxx_wasm_personality_v0 (wasm) is the
+        // personality function that is always used.  Hence this is just an
+        // aborting stub.
         #[lang = "eh_personality"]
         fn rust_eh_personality() {
             core::intrinsics::abort()
@@ -36,7 +37,6 @@ fn rust_eh_personality() {
         mod gcc;
     } else {
         // Targets that don't support unwinding.
-        // - family=wasm
         // - os=none ("bare metal" targets)
         // - os=uefi
         // - os=espidf