Allow undefined variants in IDL unions (#37042)

Testing: Includes a test which ensures that unions with a `undefined`
variant compile
Fixes https://github.com/servo/servo/issues/28679
Part of https://github.com/servo/servo/issues/30287

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/script_bindings/codegen/CodegenRust.py

@@ -39,6 +39,7 @@ from WebIDL import (
 from Configuration import (
     Configuration,
     Descriptor,
+    DescriptorProvider,
     MakeNativeName,
     MemberIsLegacyUnforgeable,
     getModuleFromObject,
@@ -617,16 +618,16 @@ class JSToNativeConversionInfo():
 
 
 def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
-                                isDefinitelyObject=False,
+                                isDefinitelyObject: bool = False,
-                                isMember=False,
+                                isMember: bool | str = False,
                                 isArgument=False,
                                 isAutoRooted=False,
                                 invalidEnumValueFatal=True,
                                 defaultValue=None,
                                 exceptionCode=None,
-                                allowTreatNonObjectAsNull=False,
+                                allowTreatNonObjectAsNull: bool = False,
                                 isCallbackReturnValue=False,
-                                sourceDescription="value"):
+                                sourceDescription="value") -> JSToNativeConversionInfo:
     """
     Get a template for converting a JS value to a native object based on the
     given type and descriptor.  If failureCode is given, then we're actually
@@ -5071,7 +5072,7 @@ class CGConstant(CGThing):
         return f"pub const {name}: {const_type} = {value};\n"
 
 
-def getUnionTypeTemplateVars(type, descriptorProvider):
+def getUnionTypeTemplateVars(type, descriptorProvider: DescriptorProvider):
     if type.isGeckoInterface():
         name = type.inner.identifier.name
         typeName = descriptorProvider.getDescriptor(name).returnType
@@ -5108,6 +5109,12 @@ def getUnionTypeTemplateVars(type, descriptorProvider):
     elif type.isCallback():
         name = type.name
         typeName = f"{name}<D>"
+    elif type.isUndefined():
+        return {
+            "name": type.name,
+            "typeName": "()",
+            "jsConversion": CGGeneric("if value.is_undefined() { Ok(Some(())) } else { Ok(None) }")
+        }
     else:
         raise TypeError(f"Can't handle {type} in unions yet")
 
@@ -5173,7 +5180,7 @@ impl{self.generic} Clone for {self.type}{self.genericSuffix} {{
     def manualImpl(self, t, templateVars):
         if t == "Clone":
             return self.manualImplClone(templateVars)
-        raise f"Don't know how to impl {t} for union"
+        raise ValueError(f"Don't know how to impl {t} for union")
 
     def define(self):
         def getTypeWrapper(t):
@@ -5330,31 +5337,44 @@ class CGUnionConversionStruct(CGThing):
         stringTypes = [t for t in memberTypes if t.isString() or t.isEnum()]
         numericTypes = [t for t in memberTypes if t.isNumeric()]
         booleanTypes = [t for t in memberTypes if t.isBoolean()]
-        if stringTypes or numericTypes or booleanTypes:
+        numUndefinedVariants = [t.isUndefined() for t in memberTypes].count(True)
+        if stringTypes or numericTypes or booleanTypes or numUndefinedVariants != 0:
             assert len(stringTypes) <= 1
             assert len(numericTypes) <= 1
             assert len(booleanTypes) <= 1
+            assert numUndefinedVariants <= 1
 
             def getStringOrPrimitiveConversion(memberType):
                 typename = get_name(memberType)
                 return CGGeneric(get_match(typename))
+
             other = []
             stringConversion = list(map(getStringOrPrimitiveConversion, stringTypes))
             numericConversion = list(map(getStringOrPrimitiveConversion, numericTypes))
             booleanConversion = list(map(getStringOrPrimitiveConversion, booleanTypes))
+            undefinedConversion = CGGeneric("return Ok(ConversionResult::Success(Self::Undefined(())));")
+
             if stringConversion:
                 if booleanConversion:
                     other.append(CGIfWrapper("value.get().is_boolean()", booleanConversion[0]))
                 if numericConversion:
                     other.append(CGIfWrapper("value.get().is_number()", numericConversion[0]))
+                if numUndefinedVariants != 0:
+                    other.append(CGIfWrapper("value.get().is_undefined()", undefinedConversion))
                 other.append(stringConversion[0])
             elif numericConversion:
                 if booleanConversion:
                     other.append(CGIfWrapper("value.get().is_boolean()", booleanConversion[0]))
+                if numUndefinedVariants != 0:
+                    other.append(CGIfWrapper("value.get().is_undefined()", undefinedConversion))
                 other.append(numericConversion[0])
-            else:
+            elif booleanConversion:
-                assert booleanConversion
+                if numUndefinedVariants != 0:
+                    other.append(CGIfWrapper("value.get().is_undefined()", undefinedConversion))
                 other.append(booleanConversion[0])
+            else:
+                assert numUndefinedVariants != 0
+                other.append(undefinedConversion)
             conversions.append(CGList(other, "\n\n"))
         conversions.append(CGGeneric(
             f'Ok(ConversionResult::Failure("argument could not be converted to any of: {", ".join(names)}".into()))'
@@ -5376,6 +5396,10 @@ class CGUnionConversionStruct(CGThing):
             post="\n}")
 
     def try_method(self, t):
+        if t.isUndefined():
+            # undefined does not require a conversion method, so we don't generate one
+            return CGGeneric("")
+
         templateVars = getUnionTypeTemplateVars(t, self.descriptorProvider)
         actualType = templateVars["typeName"]
         if type_needs_tracing(t):
@@ -7151,7 +7175,7 @@ impl{self.generic} Clone for {self.makeClassName(self.dictionary)}{self.genericS
     def manualImpl(self, t):
         if t == "Clone":
             return self.manualImplClone()
-        raise f"Don't know how to impl {t} for dicts."
+        raise ValueError(f"Don't know how to impl {t} for dicts.")
 
     def struct(self):
         d = self.dictionary

components/script_bindings/webidls/TestBinding.webidl

@@ -627,3 +627,6 @@ dictionary NotUsedAnyWhereElse {};
 dictionary RecordFieldWithUnionInside {
     record<USVString, (USVString or NotUsedAnyWhereElse)> recordWithUnionField;
 };
+
+// https://github.com/servo/servo/issues/28679
+typedef (USVString or undefined) UnionWithUndefinedVariant;