Add better error message when mixing exceptions modes. NFC

Depends on llvm/llvm-project#128564 to allow
wasm-ld to report the object file in question.

Fixes: emscripten-core#23732

Author: sbc100

src/lib/libexceptions.js

@@ -257,6 +257,7 @@ var LibraryExceptions = {
   },
 
 #endif
+
 #if WASM_EXCEPTIONS || !DISABLE_EXCEPTION_CATCHING
   $getExceptionMessageCommon__deps: ['__get_exception_message', 'free', '$stackSave', '$stackRestore', '$stackAlloc'],
   $getExceptionMessageCommon: (ptr) => {
@@ -277,17 +278,23 @@ var LibraryExceptions = {
     return [type, message];
   },
 #endif
+
 #if WASM_EXCEPTIONS
+#if ASSERTIONS
+  // Try to give a more useful error message than simply `Undefined reference to `emscripten_longjmp`
+  emscripten_longjmp__deps: [() => error('undefined reference to `emscripten_longjmp`. One or more object files was not compiled with `-fwasm-exceptions`.  Build with `-sASSERTIONS=0` to have wasm-ld report which one.')],
+  emscripten_longjmp: () => {},
+#endif
+
   $getCppExceptionTag: () =>
     // In static linking, tags are defined within the wasm module and are
     // exported, whereas in dynamic linking, tags are defined in library.js in
     // JS code and wasm modules import them.
 #if RELOCATABLE
-    ___cpp_exception // defined in library.js
+    ___cpp_exception, // defined in library.js
 #else
-    wasmExports['__cpp_exception']
+    wasmExports['__cpp_exception'],
 #endif
-  ,
 
 #if EXCEPTION_STACK_TRACES
   // Throw a WebAssembly.Exception object with the C++ tag with a stack trace

src/lib/libexceptions_stub.js

@@ -23,7 +23,7 @@ var LibraryExceptions = {};
   LibraryExceptions[name] = function() { abort(); };
 #if !INCLUDE_FULL_LIBRARY
   // This method of link-time error generation is not compatible with INCLUDE_FULL_LIBRARY
-  LibraryExceptions[name + '__deps'] = [function() {
+  LibraryExceptions[name + '__deps'] = [() => {
     error(`DISABLE_EXCEPTION_THROWING was set (likely due to -fno-exceptions), which means no C++ exception throwing support code is linked in, but such support is required by symbol '${name}'. Either do not set DISABLE_EXCEPTION_THROWING (if you do want exception throwing) or compile all source files with -fno-except (so that no exceptions support code is required); also make sure DISABLE_EXCEPTION_CATCHING is set to the right value - if you want exceptions, it should be off, and vice versa.`);
   }];
 #endif

test/test_other.py

@@ -9438,6 +9438,22 @@ def test_exceptions_c_linker(self):
     stderr = self.expect_fail([EMCC, '-sSTRICT', test_file('other/test_exceptions_c_linker.c')])
     self.assertContained('error: undefined symbol: __cxa_find_matching_catch_1', stderr)
 
+  def test_exceptions_mismatch(self):
+    create_file('main.c', '''
+    #include <setjmp.h>
+    int main() {
+      jmp_buf buf;
+      longjmp(buf, 8);
+    }
+    ''')
+    self.emcc('main.c', ['-c'])
+    err = self.expect_fail([EMCC, 'main.o', '-fwasm-exceptions'])
+    self.assertContained('error: undefined reference to `emscripten_longjmp`. One or more object files was not compiled with `-fwasm-exceptions`.  Build with `-sASSERTIONS=0` to have wasm-ld report which one.', err)
+
+    err = self.expect_fail([EMCC, 'main.o', '-fwasm-exceptions', '-sASSERTIONS=0'])
+    self.assertContained('wasm-ld: error: main.o: undefined symbol: emscripten_longjmp', err)
+
+
   @with_all_eh_sjlj
   def test_exceptions_stack_trace_and_message(self):
     src = r'''