Make (dictionary)::empty() safe

It currently works by constructing from null (which will throw a runtime
error if there are non-defaultable members).

This changes it so that we no longer need a JSContext to construct this,
so it can be safely constructed. In the case of non-defaultable members,
this method simply does not exist.

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

@@ -744,7 +744,7 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
             if defaultValue:
                 assert isinstance(defaultValue, IDLNullValue)
                 dictionary, = dictionaries
-                default = "%s::%s(%s::%s::empty(cx))" % (
+                default = "%s::%s(%s::%s::empty())" % (
                     union_native_type(type),
                     dictionary.name,
                     CGDictionary.makeModuleName(dictionary.inner),
@@ -1148,7 +1148,7 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
         typeName = "%s::%s" % (CGDictionary.makeModuleName(type.inner),
                                CGDictionary.makeDictionaryName(type.inner))
         declType = CGGeneric(typeName)
-        empty = "%s::empty(cx)" % typeName
+        empty = "%s::empty()" % typeName
 
         if type_needs_tracing(type):
             declType = CGTemplatedType("RootedTraceableBox", declType)
@@ -6274,12 +6274,7 @@ class CGDictionary(CGThing):
 
         return string.Template(
             "impl ${selfName} {\n"
-            "    pub unsafe fn empty(cx: *mut JSContext) -> ${actualType} {\n"
+            "${empty}\n"
-            "        match ${selfName}::new(cx, HandleValue::null()) {\n"
-            "            Ok(ConversionResult::Success(v)) => v,\n"
-            "            _ => unreachable!(),\n"
-            "        }\n"
-            "    }\n"
             "    pub unsafe fn new(cx: *mut JSContext, val: HandleValue) \n"
             "                      -> Result<ConversionResult<${actualType}>, ()> {\n"
             "        let object = if val.get().is_null_or_undefined() {\n"
@@ -6315,6 +6310,7 @@ class CGDictionary(CGThing):
             "}\n").substitute({
                 "selfName": selfName,
                 "actualType": actualType,
+                "empty": CGIndenter(CGGeneric(self.makeEmpty()), indentLevel=4).define(),
                 "initParent": CGIndenter(CGGeneric(initParent), indentLevel=12).define(),
                 "initMembers": CGIndenter(memberInits, indentLevel=12).define(),
                 "insertMembers": CGIndenter(memberInserts, indentLevel=8).define(),
@@ -6380,6 +6376,50 @@ class CGDictionary(CGThing):
 
         return CGGeneric(conversion)
 
+    def makeEmpty(self):
+        if self.hasRequiredFields(self.dictionary):
+            return ""
+        parentTemplate = "parent: %s::%s::empty(),\n"
+        fieldTemplate = "%s: %s,\n"
+        functionTemplate = (
+            "pub fn empty() -> Self {\n"
+            "    Self {\n"
+            "%s"
+            "    }\n"
+            "}"
+        )
+        if self.membersNeedTracing():
+            parentTemplate = "dictionary.parent = %s::%s::empty();\n"
+            fieldTemplate = "dictionary.%s = %s;\n"
+            functionTemplate = (
+                "pub fn empty() -> RootedTraceableBox<Self> {\n"
+                "    let mut dictionary = RootedTraceableBox::new(Self::default());\n"
+                "%s"
+                "    dictionary\n"
+                "}"
+            )
+        s = ""
+        if self.dictionary.parent:
+            s += parentTemplate % (self.makeModuleName(self.dictionary.parent),
+                                   self.makeClassName(self.dictionary.parent))
+        for member, info in self.memberInfo:
+            if not member.optional:
+                return ""
+            default = info.default
+            if not default:
+                default = "None"
+            s += fieldTemplate % (self.makeMemberName(member.identifier.name), default)
+        return functionTemplate % CGIndenter(CGGeneric(s), 12).define()
+
+    def hasRequiredFields(self, dictionary):
+        if dictionary.parent:
+            if self.hasRequiredFields(dictionary.parent):
+                return True
+        for member in dictionary.members:
+            if not member.optional:
+                return True
+        return False
+
     @staticmethod
     def makeMemberName(name):
         # Can't use Rust keywords as member names.

components/script/dom/bindings/iterable.rs

@@ -115,7 +115,7 @@ fn dict_return(cx: *mut JSContext,
                mut result: MutableHandleObject,
                done: bool,
                value: HandleValue) -> Fallible<()> {
-    let mut dict = unsafe { IterableKeyOrValueResult::empty(cx) };
+    let mut dict = IterableKeyOrValueResult::empty();
     dict.done = done;
     dict.value.set(value.get());
     rooted!(in(cx) let mut dict_value = UndefinedValue());
@@ -130,7 +130,7 @@ fn key_and_value_return(cx: *mut JSContext,
                         mut result: MutableHandleObject,
                         key: HandleValue,
                         value: HandleValue) -> Fallible<()> {
-    let mut dict = unsafe { IterableKeyAndValueResult::empty(cx) };
+    let mut dict = IterableKeyAndValueResult::empty();
     dict.done = false;
     dict.value = Some(vec![key, value]
         .into_iter()