Auto merge of #24469 - saschanaz:urp-default, r=Manishearth

Support USVString as default value of a union argument

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I don't expect this fixes any test though...

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22543

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- [x] There are tests for these changes

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components/script/dom/bindings/codegen/CodegenRust.py

@@ -732,6 +732,10 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
                 default = "%s::Boolean(%s)" % (
                     union_native_type(type),
                     "true" if defaultValue.value else "false")
+            elif tag is IDLType.Tags.usvstring:
+                default = '%s::USVString(USVString("%s".to_owned()))' % (
+                    union_native_type(type),
+                    defaultValue.value)
             else:
                 raise("We don't currently support default values that aren't null, boolean or default dictionary")
         elif dictionaries: