trycatch: do not try catch when no exceptions is enabled

For WASM targets the `cc` crate changed to always set `-fno-exceptions`,
before only `-fignore-exceptions` was being set elsewhere, so now programs
fail to build when using `cxx` if any `throw` or `try ... catch` are used.

> error: cannot use 'try' with exceptions disabled

Instead for trycatch just call `func()`.

book/src/binding/result.md

@@ -137,10 +137,16 @@ you'd like for the Rust error to have.
 ...namespace behavior {
 ...
 template <typename Try, typename Fail>
-static void trycatch(Try &&func, Fail &&fail) noexcept try {
+static void trycatch(Try &&func, Fail &&fail) noexcept {
+#if defined(RUST_CXX_NO_EXCEPTIONS)
   func();
-} catch (const std::exception &e) {
-  fail(e.what());
+#else
+  try {
+    func();
+  } catch (const std::exception &e) {
+    fail(e.what());
+  }
+#endif
 }
 ...
 ...} // namespace behavior

gen/src/builtin.rs

@@ -410,10 +410,16 @@ pub(super) fn write(out: &mut OutFile) {
             "    ::std::is_same<decltype(trycatch(::std::declval<Try>(), ::std::declval<Fail>())),",
         );
         writeln!(out, "                 missing>::value>::type");
-        writeln!(out, "trycatch(Try &&func, Fail &&fail) noexcept try {{");
+        writeln!(out, "trycatch(Try &&func, Fail &&fail) noexcept {{");
+        writeln!(out, "#if defined(RUST_CXX_NO_EXCEPTIONS)");
         writeln!(out, "  func();");
-        writeln!(out, "}} catch (::std::exception const &e) {{");
-        writeln!(out, "  fail(e.what());");
+        writeln!(out, "#else");
+        writeln!(out, "  try {{");
+        writeln!(out, "    func();");
+        writeln!(out, "  }} catch (::std::exception const &e) {{");
+        writeln!(out, "    fail(e.what());");
+        writeln!(out, "  }}");
+        writeln!(out, "#endif");
         writeln!(out, "}}");
         out.end_block(Block::Namespace("behavior"));
     }