[emscripten] Error on LINK_AS_CXX=0 in emdawnwebgpu.port.py

webgpu.cpp uses C++ library functions, so it needs to be linked with C++
support, otherwise it gets a long list of missing C++ STL symbol errors.
Make this an explicit error so people know what to do if they get this
error (because they're using -sSTRICT or -sNO_DEFAULT_TO_CXX).

Initial discussion:
emscripten-core/emscripten#25397 (comment)

Bug: none
Change-Id: I99b766298f47b946e0ba49eacad04fe8b427f6de
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/264434
Reviewed-by: Loko Kung <[email protected]>
Commit-Queue: Kai Ninomiya <[email protected]>

Author: kainino0x

.github/workflows/package-emdawnwebgpu.sh

@@ -113,9 +113,13 @@ LICENSE = "Some files: BSD 3-Clause License. Other files: Emscripten's license (
 DESCRIPTION = "Emdawnwebgpu implements webgpu.h on WebGPU, replacing -sUSE_WEBGPU. **For info on usage and filing feedback, see link below.**"
 URL = 'https://dawn.googlesource.com/dawn/+/${SHA}/src/emdawnwebgpu/pkg/README.md'
 
+
 # Emscripten <4.0.10 won't notice EXTERNAL_PORT and will try to use this.
 def get(ports, settings, shared):
     raise Exception('Remote ports require Emscripten 4.0.10+.')
+
+
+# (Make this look like a port so that the error message above can be hit.)
 def clear(ports, settings, shared):
     pass
 EOF

src/emdawnwebgpu/pkg/emdawnwebgpu.port.py

@@ -181,6 +181,11 @@ def linker_setup(ports, settings):
     if settings.USE_WEBGPU:
         raise Exception('emdawnwebgpu may not be used with -sUSE_WEBGPU=1')
 
+    if not settings.LINK_AS_CXX:
+        raise Exception(
+            'emdawnwebgpu requires C++. Link using em++ (or emcc -sDEFAULT_TO_CXX), instead of emcc.'
+        )
+
     # These will be processed in order; library_webgpu.js must come after
     # the generated files, because it depends on them.
     settings.JS_LIBRARIES += [
@@ -189,6 +194,7 @@ def linker_setup(ports, settings):
         os.path.join(_src_dir, 'library_webgpu_generated_sig_info.js'),
         os.path.join(_src_dir, 'library_webgpu.js'),
     ]
+
     if 'CLOSURE_ARGS' in settings.keys():
         # This works in Emscripten >4.0.7. In <=4.0.7, the user has to pass it.
         settings.CLOSURE_ARGS += [