Add -Z emscritpen-wasm-eh flag

Don't make it do anything yet

Author: hoodmane

compiler/rustc_interface/src/tests.rs

@@ -782,6 +782,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(dwarf_version, Some(5));
     tracked!(embed_source, true);
     tracked!(emit_thin_lto, false);
+    tracked!(emscripten_wasm_eh)
     tracked!(export_executable_symbols, true);
     tracked!(fewer_names, Some(true));
     tracked!(fixed_x18, true);

compiler/rustc_session/src/options.rs

@@ -1771,6 +1771,8 @@ options! {
         "emit a section containing stack size metadata (default: no)"),
     emit_thin_lto: bool = (true, parse_bool, [TRACKED],
         "emit the bc module with thin LTO info (default: yes)"),
+    emscripten_wasm_eh: bool = (false, parse_bool, [TRACKED],
+        "Use WebAssembly error handling for wasm32-unknown-emscripten"),
     enforce_type_length_limit: bool = (false, parse_bool, [TRACKED],
         "enforce the type length limit when monomorphizing instances in codegen"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],

src/doc/unstable-book/src/compiler-flags/emscripten-wasm-eh.md

@@ -0,0 +1,6 @@
+# `emscripten-wasm-eh`
+
+Use the WebAssembly exception handling ABI to unwind for the
+`wasm32-unknown-emscripten`. If compiling with this setting, the `emcc` linker
+should be invoked with `-fwasm-exceptions`. If linking with C/C++ files, the
+C/C++ files should also be compiled with `-fwasm-exceptions`.