Emscripten: Turn wasm-eh on by default

As specified by MCP:
rust-lang/compiler-team#920
Resolves #148309

Author: hoodmane

compiler/rustc_interface/src/tests.rs

@@ -796,7 +796,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(embed_metadata, false);
     tracked!(embed_source, true);
     tracked!(emit_thin_lto, false);
-    tracked!(emscripten_wasm_eh, true);
+    tracked!(emscripten_wasm_eh, false);
     tracked!(export_executable_symbols, true);
     tracked!(fewer_names, Some(true));
     tracked!(fixed_x18, true);

compiler/rustc_session/src/options.rs

@@ -2307,7 +2307,7 @@ 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],
+    emscripten_wasm_eh: bool = (true, 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"),

src/doc/rustc/src/platform-support/wasm32-unknown-emscripten.md

@@ -95,10 +95,11 @@ This target can be cross-compiled from any host.
 ## Emscripten ABI Compatibility
 
 The Emscripten compiler toolchain does not follow a semantic versioning scheme
-that clearly indicates when breaking changes to the ABI can be made. Additionally,
-Emscripten offers many different ABIs even for a single version of Emscripten
-depending on the linker flags used, e.g. `-fexceptions` and `-sWASM_BIGINT`. If
-the ABIs mismatch, your code may exhibit undefined behaviour.
+that clearly indicates when breaking changes to the ABI can be made.
+Additionally, Emscripten offers many different ABIs even for a single version of
+Emscripten depending on the linker flags used, e.g. `-fwasm-exceptions` and
+`-sWASM_BIGINT`. If the ABIs do not match, your code may exhibit undefined
+behaviour.
 
 To ensure that the ABIs of your Rust code, of the Rust standard library, and of
 other code compiled for Emscripten all match, you should rebuild the Rust standard
@@ -158,9 +159,9 @@ features can be disabled, and how Rust code can be conditionally compiled based
 which features are enabled.
 
 Note that Rust code compiled for `wasm32-unknown-emscripten` currently enables
-`-fexceptions` (JS exceptions) by default unless the Rust code is compiled with
-`-Cpanic=abort`. `-fwasm-exceptions` (WASM exceptions) is not yet currently supported,
-see <https://github.com/rust-lang/rust/issues/112195>.
+`-fwasm-exceptions` (legacy WASM exceptions) by default unless the Rust code is
+compiled with `-Cpanic=abort`. It is possible to use JS exceptions by passing
+the flag ``-Z emscripten-wasm-eh=false`` but this will be removed in the future.
 
 Please refer to the [Emscripten ABI compatibility](#emscripten-abi-compatibility)
 section to ensure that the features that are enabled do not cause an ABI mismatch

tests/ui/feature-gates/feature-gate-cfg-emscripten-wasm-eh.rs

@@ -1,4 +1,4 @@
 //@ compile-flags: --check-cfg=cfg(emscripten_wasm_eh)
-#[cfg(not(emscripten_wasm_eh))]
+#[cfg(emscripten_wasm_eh)]
 //~^ ERROR `cfg(emscripten_wasm_eh)` is experimental
 fn main() {}