Add support for wasm exception handling to Emscripten target

Gated behind an unstable `-Z emscripten-wasm-eh` flag

panic_unwind/Cargo.toml

@@ -23,4 +23,4 @@ libc = { version = "0.2", default-features = false }
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-check-cfg = []
+check-cfg = ['cfg(emscripten_wasm_eh)']

panic_unwind/src/lib.rs

@@ -25,13 +25,14 @@
 // `real_imp` is unused with Miri, so silence warnings.
 #![cfg_attr(miri, allow(dead_code))]
 #![allow(internal_features)]
+#![cfg_attr(not(bootstrap), feature(cfg_emscripten_wasm_eh))]
 
 use alloc::boxed::Box;
 use core::any::Any;
 use core::panic::PanicPayload;
 
 cfg_if::cfg_if! {
-    if #[cfg(target_os = "emscripten")] {
+    if #[cfg(all(target_os = "emscripten", not(emscripten_wasm_eh)))] {
         #[path = "emcc.rs"]
         mod imp;
     } else if #[cfg(target_os = "hermit")] {

unwind/Cargo.toml

@@ -37,4 +37,4 @@ system-llvm-libunwind = []
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-check-cfg = []
+check-cfg = ['cfg(emscripten_wasm_eh)']

unwind/src/lib.rs

@@ -4,10 +4,11 @@
 #![feature(staged_api)]
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]
 #![cfg_attr(
-    all(target_family = "wasm", not(target_os = "emscripten")),
+    all(target_family = "wasm", any(not(target_os = "emscripten"), emscripten_wasm_eh)),
     feature(simd_wasm64, wasm_exception_handling_intrinsics)
 )]
 #![allow(internal_features)]
+#![cfg_attr(not(bootstrap), feature(cfg_emscripten_wasm_eh))]
 
 // Force libc to be included even if unused. This is required by many platforms.
 #[cfg(not(all(windows, target_env = "msvc")))]