Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 24278 insertions, 0 deletions

diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
new file mode 100644
index 0000000000..809a680d70
--- /dev/null
+++ b/dom/bindings/Codegen.py
@@ -0,0 +1,24278 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# Common codegen classes.
+
+import functools
+import math
+import os
+import re
+import string
+import textwrap
+
+import six
+from Configuration import (
+    Descriptor,
+    MemberIsLegacyUnforgeable,
+    NoSuchDescriptorError,
+    getAllTypes,
+    getTypesFromCallback,
+    getTypesFromDescriptor,
+    getTypesFromDictionary,
+)
+from perfecthash import PerfectHash
+from WebIDL import (
+    BuiltinTypes,
+    IDLAttribute,
+    IDLBuiltinType,
+    IDLDefaultDictionaryValue,
+    IDLDictionary,
+    IDLEmptySequenceValue,
+    IDLInterfaceMember,
+    IDLNullValue,
+    IDLSequenceType,
+    IDLType,
+    IDLUndefinedValue,
+)
+
+AUTOGENERATED_WARNING_COMMENT = (
+    "/* THIS FILE IS AUTOGENERATED BY Codegen.py - DO NOT EDIT */\n\n"
+)
+AUTOGENERATED_WITH_SOURCE_WARNING_COMMENT = (
+    "/* THIS FILE IS AUTOGENERATED FROM %s BY Codegen.py - DO NOT EDIT */\n\n"
+)
+ADDPROPERTY_HOOK_NAME = "_addProperty"
+GETWRAPPERCACHE_HOOK_NAME = "_getWrapperCache"
+FINALIZE_HOOK_NAME = "_finalize"
+OBJECT_MOVED_HOOK_NAME = "_objectMoved"
+CONSTRUCT_HOOK_NAME = "_constructor"
+LEGACYCALLER_HOOK_NAME = "_legacycaller"
+RESOLVE_HOOK_NAME = "_resolve"
+MAY_RESOLVE_HOOK_NAME = "_mayResolve"
+NEW_ENUMERATE_HOOK_NAME = "_newEnumerate"
+ENUM_ENTRY_VARIABLE_NAME = "strings"
+INSTANCE_RESERVED_SLOTS = 1
+
+# This size is arbitrary. It is a power of 2 to make using it as a modulo
+# operand cheap, and is usually around 1/3-1/5th of the set size (sometimes
+# smaller for very large sets).
+GLOBAL_NAMES_PHF_SIZE = 256
+
+
+def memberReservedSlot(member, descriptor):
+    return (
+        "(DOM_INSTANCE_RESERVED_SLOTS + %d)"
+        % member.slotIndices[descriptor.interface.identifier.name]
+    )
+
+
+def memberXrayExpandoReservedSlot(member, descriptor):
+    return (
+        "(xpc::JSSLOT_EXPANDO_COUNT + %d)"
+        % member.slotIndices[descriptor.interface.identifier.name]
+    )
+
+
+def mayUseXrayExpandoSlots(descriptor, attr):
+    assert not attr.getExtendedAttribute("NewObject")
+    # For attributes whose type is a Gecko interface we always use
+    # slots on the reflector for caching.  Also, for interfaces that
+    # don't want Xrays we obviously never use the Xray expando slot.
+    return descriptor.wantsXrays and not attr.type.isGeckoInterface()
+
+
+def toStringBool(arg):
+    """
+    Converts IDL/Python Boolean (True/False) to C++ Boolean (true/false)
+    """
+    return str(not not arg).lower()
+
+
+def toBindingNamespace(arg):
+    return arg + "_Binding"
+
+
+def isTypeCopyConstructible(type):
+    # Nullable and sequence stuff doesn't affect copy-constructibility
+    type = type.unroll()
+    return (
+        type.isUndefined()
+        or type.isPrimitive()
+        or type.isString()
+        or type.isEnum()
+        or (type.isUnion() and CGUnionStruct.isUnionCopyConstructible(type))
+        or (
+            type.isDictionary()
+            and CGDictionary.isDictionaryCopyConstructible(type.inner)
+        )
+        or
+        # Interface types are only copy-constructible if they're Gecko
+        # interfaces.  SpiderMonkey interfaces are not copy-constructible
+        # because of rooting issues.
+        (type.isInterface() and type.isGeckoInterface())
+    )
+
+
+class CycleCollectionUnsupported(TypeError):
+    def __init__(self, message):
+        TypeError.__init__(self, message)
+
+
+def idlTypeNeedsCycleCollection(type):
+    type = type.unroll()  # Takes care of sequences and nullables
+    if (
+        (type.isPrimitive() and type.tag() in builtinNames)
+        or type.isUndefined()
+        or type.isEnum()
+        or type.isString()
+        or type.isAny()
+        or type.isObject()
+        or type.isSpiderMonkeyInterface()
+    ):
+        return False
+    elif type.isCallback() or type.isPromise() or type.isGeckoInterface():
+        return True
+    elif type.isUnion():
+        return any(idlTypeNeedsCycleCollection(t) for t in type.flatMemberTypes)
+    elif type.isRecord():
+        if idlTypeNeedsCycleCollection(type.inner):
+            raise CycleCollectionUnsupported(
+                "Cycle collection for type %s is not supported" % type
+            )
+        return False
+    elif type.isDictionary():
+        return CGDictionary.dictionaryNeedsCycleCollection(type.inner)
+    else:
+        raise CycleCollectionUnsupported(
+            "Don't know whether to cycle-collect type %s" % type
+        )
+
+
+def idlTypeNeedsCallContext(type, descriptor=None, allowTreatNonCallableAsNull=False):
+    """
+    Returns whether the given type needs error reporting via a
+    BindingCallContext for JS-to-C++ conversions.  This will happen when the
+    conversion can throw an exception due to logic in the IDL spec or
+    Gecko-specific security checks.  In particular, a type needs a
+    BindingCallContext if and only if the JS-to-C++ conversion for that type can
+    end up calling ThrowErrorMessage.
+
+    For some types this depends on the descriptor (e.g. because we do certain
+    checks only for some kinds of interfaces).
+
+    The allowTreatNonCallableAsNull optimization is there so we can avoid
+    generating an unnecessary BindingCallContext for all the event handler
+    attribute setters.
+
+    """
+    while True:
+        if type.isSequence():
+            # Sequences can always throw "not an object"
+            return True
+        if type.nullable():
+            # treatNonObjectAsNull() and treatNonCallableAsNull() are
+            # only sane things to test on nullable types, so do that now.
+            if (
+                allowTreatNonCallableAsNull
+                and type.isCallback()
+                and (type.treatNonObjectAsNull() or type.treatNonCallableAsNull())
+            ):
+                # This can't throw. so never needs a method description.
+                return False
+            type = type.inner
+        else:
+            break
+
+    if type.isUndefined():
+        # Clearly doesn't need a method description; we can only get here from
+        # CGHeaders trying to decide whether to include the method description
+        # header.
+        return False
+    # The float check needs to come before the isPrimitive() check,
+    # because floats are primitives too.
+    if type.isFloat():
+        # Floats can throw if restricted.
+        return not type.isUnrestricted()
+    if type.isPrimitive() and type.tag() in builtinNames:
+        # Numbers can throw if enforcing range.
+        return type.hasEnforceRange()
+    if type.isEnum():
+        # Can throw on invalid value.
+        return True
+    if type.isString():
+        # Can throw if it's a ByteString
+        return type.isByteString()
+    if type.isAny():
+        # JS-implemented interfaces do extra security checks so need a
+        # method description here.  If we have no descriptor, this
+        # might be JS-implemented thing, so it will do the security
+        # check and we need the method description.
+        return not descriptor or descriptor.interface.isJSImplemented()
+    if type.isPromise():
+        # JS-to-Promise conversion won't cause us to throw any
+        # specific exceptions, so does not need a method description.
+        return False
+    if (
+        type.isObject()
+        or type.isInterface()
+        or type.isCallback()
+        or type.isDictionary()
+        or type.isRecord()
+        or type.isObservableArray()
+    ):
+        # These can all throw if a primitive is passed in, at the very least.
+        # There are some rare cases when we know we have an object, but those
+        # are not worth the complexity of optimizing for.
+        #
+        # Note that we checked the [LegacyTreatNonObjectAsNull] case already when
+        # unwrapping nullables.
+        return True
+    if type.isUnion():
+        # Can throw if a type not in the union is passed in.
+        return True
+    raise TypeError("Don't know whether type '%s' needs a method description" % type)
+
+
+# TryPreserveWrapper uses the addProperty hook to preserve the wrapper of
+# non-nsISupports cycle collected objects, so if wantsAddProperty is changed
+# to not cover that case then TryPreserveWrapper will need to be changed.
+def wantsAddProperty(desc):
+    return desc.concrete and desc.wrapperCache and not desc.isGlobal()
+
+
+def wantsGetWrapperCache(desc):
+    return (
+        desc.concrete and desc.wrapperCache and not desc.isGlobal() and not desc.proxy
+    )
+
+
+# We'll want to insert the indent at the beginnings of lines, but we
+# don't want to indent empty lines.  So only indent lines that have a
+# non-newline character on them.
+lineStartDetector = re.compile("^(?=[^\n#])", re.MULTILINE)
+
+
+def indent(s, indentLevel=2):
+    """
+    Indent C++ code.
+
+    Weird secret feature: this doesn't indent lines that start with # (such as
+    #include lines or #ifdef/#endif).
+    """
+    if s == "":
+        return s
+    return re.sub(lineStartDetector, indentLevel * " ", s)
+
+
+# dedent() and fill() are often called on the same string multiple
+# times.  We want to memoize their return values so we don't keep
+# recomputing them all the time.
+def memoize(fn):
+    """
+    Decorator to memoize a function of one argument.  The cache just
+    grows without bound.
+    """
+    cache = {}
+
+    @functools.wraps(fn)
+    def wrapper(arg):
+        retval = cache.get(arg)
+        if retval is None:
+            retval = cache[arg] = fn(arg)
+        return retval
+
+    return wrapper
+
+
+@memoize
+def dedent(s):
+    """
+    Remove all leading whitespace from s, and remove a blank line
+    at the beginning.
+    """
+    if s.startswith("\n"):
+        s = s[1:]
+    return textwrap.dedent(s)
+
+
+# This works by transforming the fill()-template to an equivalent
+# string.Template.
+fill_multiline_substitution_re = re.compile(r"( *)\$\*{(\w+)}(\n)?")
+
+
+find_substitutions = re.compile(r"\${")
+
+
+@memoize
+def compile_fill_template(template):
+    """
+    Helper function for fill().  Given the template string passed to fill(),
+    do the reusable part of template processing and return a pair (t,
+    argModList) that can be used every time fill() is called with that
+    template argument.
+
+    argsModList is list of tuples that represent modifications to be
+    made to args.  Each modification has, in order: i) the arg name,
+    ii) the modified name, iii) the indent depth.
+    """
+    t = dedent(template)
+    assert t.endswith("\n") or "\n" not in t
+    argModList = []
+
+    def replace(match):
+        """
+        Replaces a line like '  $*{xyz}\n' with '${xyz_n}',
+        where n is the indent depth, and add a corresponding entry to
+        argModList.
+
+        Note that this needs to close over argModList, so it has to be
+        defined inside compile_fill_template().
+        """
+        indentation, name, nl = match.groups()
+        depth = len(indentation)
+
+        # Check that $*{xyz} appears by itself on a line.
+        prev = match.string[: match.start()]
+        if (prev and not prev.endswith("\n")) or nl is None:
+            raise ValueError(
+                "Invalid fill() template: $*{%s} must appear by itself on a line" % name
+            )
+
+        # Now replace this whole line of template with the indented equivalent.
+        modified_name = name + "_" + str(depth)
+        argModList.append((name, modified_name, depth))
+        return "${" + modified_name + "}"
+
+    t = re.sub(fill_multiline_substitution_re, replace, t)
+    if not re.search(find_substitutions, t):
+        raise TypeError("Using fill() when dedent() would do.")
+    return (string.Template(t), argModList)
+
+
+def fill(template, **args):
+    """
+    Convenience function for filling in a multiline template.
+
+    `fill(template, name1=v1, name2=v2)` is a lot like
+    `string.Template(template).substitute({"name1": v1, "name2": v2})`.
+
+    However, it's shorter, and has a few nice features:
+
+      * If `template` is indented, fill() automatically dedents it!
+        This makes code using fill() with Python's multiline strings
+        much nicer to look at.
+
+      * If `template` starts with a blank line, fill() strips it off.
+        (Again, convenient with multiline strings.)
+
+      * fill() recognizes a special kind of substitution
+        of the form `$*{name}`.
+
+        Use this to paste in, and automatically indent, multiple lines.
+        (Mnemonic: The `*` is for "multiple lines").
+
+        A `$*` substitution must appear by itself on a line, with optional
+        preceding indentation (spaces only). The whole line is replaced by the
+        corresponding keyword argument, indented appropriately.  If the
+        argument is an empty string, no output is generated, not even a blank
+        line.
+    """
+
+    t, argModList = compile_fill_template(template)
+    # Now apply argModList to args
+    for (name, modified_name, depth) in argModList:
+        if not (args[name] == "" or args[name].endswith("\n")):
+            raise ValueError(
+                "Argument %s with value %r is missing a newline" % (name, args[name])
+            )
+        args[modified_name] = indent(args[name], depth)
+
+    return t.substitute(args)
+
+
+class CGThing:
+    """
+    Abstract base class for things that spit out code.
+    """
+
+    def __init__(self):
+        pass  # Nothing for now
+
+    def declare(self):
+        """Produce code for a header file."""
+        assert False  # Override me!
+
+    def define(self):
+        """Produce code for a cpp file."""
+        assert False  # Override me!
+
+    def deps(self):
+        """Produce the deps for a pp file"""
+        assert False  # Override me!
+
+
+class CGStringTable(CGThing):
+    """
+    Generate a function accessor for a WebIDL string table, using the existing
+    concatenated names string and mapping indexes to offsets in that string:
+
+    const char *accessorName(unsigned int index) {
+      static const uint16_t offsets = { ... };
+      return BindingName(offsets[index]);
+    }
+
+    This is more efficient than the more natural:
+
+    const char *table[] = {
+      ...
+    };
+
+    The uint16_t offsets are smaller than the pointer equivalents, and the
+    concatenated string requires no runtime relocations.
+    """
+
+    def __init__(self, accessorName, strings, static=False):
+        CGThing.__init__(self)
+        self.accessorName = accessorName
+        self.strings = strings
+        self.static = static
+
+    def declare(self):
+        if self.static:
+            return ""
+        return "const char *%s(unsigned int aIndex);\n" % self.accessorName
+
+    def define(self):
+        offsets = []
+        for s in self.strings:
+            offsets.append(BindingNamesOffsetEnum(s))
+        return fill(
+            """
+            ${static}const char *${name}(unsigned int aIndex)
+            {
+              static const BindingNamesOffset offsets[] = {
+                $*{offsets}
+              };
+              return BindingName(offsets[aIndex]);
+            }
+            """,
+            static="static " if self.static else "",
+            name=self.accessorName,
+            offsets="".join("BindingNamesOffset::%s,\n" % o for o in offsets),
+        )
+
+
+class CGNativePropertyHooks(CGThing):
+    """
+    Generate a NativePropertyHooks for a given descriptor
+    """
+
+    def __init__(self, descriptor, properties):
+        CGThing.__init__(self)
+        assert descriptor.wantsXrays
+        self.descriptor = descriptor
+        self.properties = properties
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        deleteNamedProperty = "nullptr"
+        if (
+            self.descriptor.concrete
+            and self.descriptor.proxy
+            and not self.descriptor.isMaybeCrossOriginObject()
+        ):
+            resolveOwnProperty = "binding_detail::ResolveOwnProperty"
+            enumerateOwnProperties = "binding_detail::EnumerateOwnProperties"
+            if self.descriptor.needsXrayNamedDeleterHook():
+                deleteNamedProperty = "DeleteNamedProperty"
+        elif self.descriptor.needsXrayResolveHooks():
+            resolveOwnProperty = "ResolveOwnPropertyViaResolve"
+            enumerateOwnProperties = "EnumerateOwnPropertiesViaGetOwnPropertyNames"
+        else:
+            resolveOwnProperty = "nullptr"
+            enumerateOwnProperties = "nullptr"
+        if self.properties.hasNonChromeOnly():
+            regular = "sNativeProperties.Upcast()"
+        else:
+            regular = "nullptr"
+        if self.properties.hasChromeOnly():
+            chrome = "sChromeOnlyNativeProperties.Upcast()"
+        else:
+            chrome = "nullptr"
+        constructorID = "constructors::id::"
+        if self.descriptor.interface.hasInterfaceObject():
+            constructorID += self.descriptor.name
+        else:
+            constructorID += "_ID_Count"
+        prototypeID = "prototypes::id::"
+        if self.descriptor.interface.hasInterfacePrototypeObject():
+            prototypeID += self.descriptor.name
+        else:
+            prototypeID += "_ID_Count"
+
+        if self.descriptor.wantsXrayExpandoClass:
+            expandoClass = "&sXrayExpandoObjectClass"
+        else:
+            expandoClass = "&DefaultXrayExpandoObjectClass"
+
+        return fill(
+            """
+            bool sNativePropertiesInited = false;
+            const NativePropertyHooks sNativePropertyHooks = {
+              ${resolveOwnProperty},
+              ${enumerateOwnProperties},
+              ${deleteNamedProperty},
+              { ${regular}, ${chrome}, &sNativePropertiesInited },
+              ${prototypeID},
+              ${constructorID},
+              ${expandoClass}
+            };
+            """,
+            resolveOwnProperty=resolveOwnProperty,
+            enumerateOwnProperties=enumerateOwnProperties,
+            deleteNamedProperty=deleteNamedProperty,
+            regular=regular,
+            chrome=chrome,
+            prototypeID=prototypeID,
+            constructorID=constructorID,
+            expandoClass=expandoClass,
+        )
+
+
+def NativePropertyHooks(descriptor):
+    return (
+        "&sEmptyNativePropertyHooks"
+        if not descriptor.wantsXrays
+        else "&sNativePropertyHooks"
+    )
+
+
+def DOMClass(descriptor):
+    protoList = ["prototypes::id::" + proto for proto in descriptor.prototypeNameChain]
+    # Pad out the list to the right length with _ID_Count so we
+    # guarantee that all the lists are the same length.  _ID_Count
+    # is never the ID of any prototype, so it's safe to use as
+    # padding.
+    protoList.extend(
+        ["prototypes::id::_ID_Count"]
+        * (descriptor.config.maxProtoChainLength - len(protoList))
+    )
+
+    if descriptor.interface.isSerializable():
+        serializer = "Serialize"
+    else:
+        serializer = "nullptr"
+
+    if wantsGetWrapperCache(descriptor):
+        wrapperCacheGetter = GETWRAPPERCACHE_HOOK_NAME
+    else:
+        wrapperCacheGetter = "nullptr"
+
+    if descriptor.hasOrdinaryObjectPrototype:
+        getProto = "JS::GetRealmObjectPrototypeHandle"
+    else:
+        getProto = "GetProtoObjectHandle"
+
+    return fill(
+        """
+          { ${protoChain} },
+          std::is_base_of_v<nsISupports, ${nativeType}>,
+          ${hooks},
+          FindAssociatedGlobalForNative<${nativeType}>::Get,
+          ${getProto},
+          GetCCParticipant<${nativeType}>::Get(),
+          ${serializer},
+          ${wrapperCacheGetter}
+        """,
+        protoChain=", ".join(protoList),
+        nativeType=descriptor.nativeType,
+        hooks=NativePropertyHooks(descriptor),
+        serializer=serializer,
+        wrapperCacheGetter=wrapperCacheGetter,
+        getProto=getProto,
+    )
+
+
+def InstanceReservedSlots(descriptor):
+    slots = INSTANCE_RESERVED_SLOTS + descriptor.interface.totalMembersInSlots
+    if descriptor.isMaybeCrossOriginObject():
+        # We need a slot for the cross-origin holder too.
+        if descriptor.interface.hasChildInterfaces():
+            raise TypeError(
+                "We don't support non-leaf cross-origin interfaces "
+                "like %s" % descriptor.interface.identifier.name
+            )
+        slots += 1
+    return slots
+
+
+class CGDOMJSClass(CGThing):
+    """
+    Generate a DOMJSClass for a given descriptor
+    """
+
+    def __init__(self, descriptor):
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        callHook = (
+            LEGACYCALLER_HOOK_NAME
+            if self.descriptor.operations["LegacyCaller"]
+            else "nullptr"
+        )
+        objectMovedHook = (
+            OBJECT_MOVED_HOOK_NAME if self.descriptor.wrapperCache else "nullptr"
+        )
+        slotCount = InstanceReservedSlots(self.descriptor)
+        classFlags = "JSCLASS_IS_DOMJSCLASS | JSCLASS_FOREGROUND_FINALIZE | "
+        if self.descriptor.isGlobal():
+            classFlags += (
+                "JSCLASS_DOM_GLOBAL | JSCLASS_GLOBAL_FLAGS_WITH_SLOTS(DOM_GLOBAL_SLOTS)"
+            )
+            traceHook = "JS_GlobalObjectTraceHook"
+            reservedSlots = "JSCLASS_GLOBAL_APPLICATION_SLOTS"
+        else:
+            classFlags += "JSCLASS_HAS_RESERVED_SLOTS(%d)" % slotCount
+            traceHook = "nullptr"
+            reservedSlots = slotCount
+        if self.descriptor.interface.hasProbablyShortLivingWrapper():
+            if not self.descriptor.wrapperCache:
+                raise TypeError(
+                    "Need a wrapper cache to support nursery "
+                    "allocation of DOM objects"
+                )
+            classFlags += " | JSCLASS_SKIP_NURSERY_FINALIZE"
+
+        if self.descriptor.interface.getExtendedAttribute("NeedResolve"):
+            resolveHook = RESOLVE_HOOK_NAME
+            mayResolveHook = MAY_RESOLVE_HOOK_NAME
+            newEnumerateHook = NEW_ENUMERATE_HOOK_NAME
+        elif self.descriptor.isGlobal():
+            resolveHook = "mozilla::dom::ResolveGlobal"
+            mayResolveHook = "mozilla::dom::MayResolveGlobal"
+            newEnumerateHook = "mozilla::dom::EnumerateGlobal"
+        else:
+            resolveHook = "nullptr"
+            mayResolveHook = "nullptr"
+            newEnumerateHook = "nullptr"
+
+        return fill(
+            """
+            static const JSClassOps sClassOps = {
+              ${addProperty}, /* addProperty */
+              nullptr,               /* delProperty */
+              nullptr,               /* enumerate */
+              ${newEnumerate}, /* newEnumerate */
+              ${resolve}, /* resolve */
+              ${mayResolve}, /* mayResolve */
+              ${finalize}, /* finalize */
+              ${call}, /* call */
+              nullptr,               /* construct */
+              ${trace}, /* trace */
+            };
+
+            static const js::ClassExtension sClassExtension = {
+              ${objectMoved} /* objectMovedOp */
+            };
+
+            static const DOMJSClass sClass = {
+              { "${name}",
+                ${flags},
+                &sClassOps,
+                JS_NULL_CLASS_SPEC,
+                &sClassExtension,
+                JS_NULL_OBJECT_OPS
+              },
+              $*{descriptor}
+            };
+            static_assert(${instanceReservedSlots} == DOM_INSTANCE_RESERVED_SLOTS,
+                          "Must have the right minimal number of reserved slots.");
+            static_assert(${reservedSlots} >= ${slotCount},
+                          "Must have enough reserved slots.");
+            """,
+            name=self.descriptor.interface.getClassName(),
+            flags=classFlags,
+            addProperty=ADDPROPERTY_HOOK_NAME
+            if wantsAddProperty(self.descriptor)
+            else "nullptr",
+            newEnumerate=newEnumerateHook,
+            resolve=resolveHook,
+            mayResolve=mayResolveHook,
+            finalize=FINALIZE_HOOK_NAME,
+            call=callHook,
+            trace=traceHook,
+            objectMoved=objectMovedHook,
+            descriptor=DOMClass(self.descriptor),
+            instanceReservedSlots=INSTANCE_RESERVED_SLOTS,
+            reservedSlots=reservedSlots,
+            slotCount=slotCount,
+        )
+
+
+class CGDOMProxyJSClass(CGThing):
+    """
+    Generate a DOMJSClass for a given proxy descriptor
+    """
+
+    def __init__(self, descriptor):
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        slotCount = InstanceReservedSlots(self.descriptor)
+        # We need one reserved slot (DOM_OBJECT_SLOT).
+        flags = ["JSCLASS_IS_DOMJSCLASS", "JSCLASS_HAS_RESERVED_SLOTS(%d)" % slotCount]
+        # We don't use an IDL annotation for JSCLASS_EMULATES_UNDEFINED because
+        # we don't want people ever adding that to any interface other than
+        # HTMLAllCollection.  So just hardcode it here.
+        if self.descriptor.interface.identifier.name == "HTMLAllCollection":
+            flags.append("JSCLASS_EMULATES_UNDEFINED")
+        return fill(
+            """
+            static const DOMJSClass sClass = {
+              PROXY_CLASS_DEF("${name}",
+                              ${flags}),
+              $*{descriptor}
+            };
+            """,
+            name=self.descriptor.interface.identifier.name,
+            flags=" | ".join(flags),
+            descriptor=DOMClass(self.descriptor),
+        )
+
+
+class CGXrayExpandoJSClass(CGThing):
+    """
+    Generate a JSClass for an Xray expando object.  This is only
+    needed if we have members in slots (for [Cached] or [StoreInSlot]
+    stuff).
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.interface.totalMembersInSlots != 0
+        assert descriptor.wantsXrays
+        assert descriptor.wantsXrayExpandoClass
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        return fill(
+            """
+            // This may allocate too many slots, because we only really need
+            // slots for our non-interface-typed members that we cache.  But
+            // allocating slots only for those would make the slot index
+            // computations much more complicated, so let's do this the simple
+            // way for now.
+            DEFINE_XRAY_EXPANDO_CLASS(static, sXrayExpandoObjectClass, ${memberSlots});
+            """,
+            memberSlots=self.descriptor.interface.totalMembersInSlots,
+        )
+
+
+def PrototypeIDAndDepth(descriptor):
+    prototypeID = "prototypes::id::"
+    if descriptor.interface.hasInterfacePrototypeObject():
+        prototypeID += descriptor.interface.identifier.name
+        depth = "PrototypeTraits<%s>::Depth" % prototypeID
+    else:
+        prototypeID += "_ID_Count"
+        depth = "0"
+    return (prototypeID, depth)
+
+
+def InterfacePrototypeObjectProtoGetter(descriptor):
+    """
+    Returns a tuple with two elements:
+
+        1) The name of the function to call to get the prototype to use for the
+           interface prototype object as a JSObject*.
+
+        2) The name of the function to call to get the prototype to use for the
+           interface prototype object as a JS::Handle<JSObject*> or None if no
+           such function exists.
+    """
+    parentProtoName = descriptor.parentPrototypeName
+    if descriptor.hasNamedPropertiesObject:
+        protoGetter = "GetNamedPropertiesObject"
+        protoHandleGetter = None
+    elif parentProtoName is None:
+        if descriptor.interface.getExtendedAttribute("ExceptionClass"):
+            protoGetter = "JS::GetRealmErrorPrototype"
+        elif descriptor.interface.isIteratorInterface():
+            protoGetter = "JS::GetRealmIteratorPrototype"
+        elif descriptor.interface.isAsyncIteratorInterface():
+            protoGetter = "JS::GetRealmAsyncIteratorPrototype"
+        else:
+            protoGetter = "JS::GetRealmObjectPrototype"
+        protoHandleGetter = None
+    else:
+        prefix = toBindingNamespace(parentProtoName)
+        protoGetter = prefix + "::GetProtoObject"
+        protoHandleGetter = prefix + "::GetProtoObjectHandle"
+
+    return (protoGetter, protoHandleGetter)
+
+
+class CGPrototypeJSClass(CGThing):
+    def __init__(self, descriptor, properties):
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+        self.properties = properties
+
+    def declare(self):
+        # We're purely for internal consumption
+        return ""
+
+    def define(self):
+        prototypeID, depth = PrototypeIDAndDepth(self.descriptor)
+        slotCount = "DOM_INTERFACE_PROTO_SLOTS_BASE"
+        # Globals handle unforgeables directly in Wrap() instead of
+        # via a holder.
+        if (
+            self.descriptor.hasLegacyUnforgeableMembers
+            and not self.descriptor.isGlobal()
+        ):
+            slotCount += (
+                " + 1 /* slot for the JSObject holding the unforgeable properties */"
+            )
+        (protoGetter, _) = InterfacePrototypeObjectProtoGetter(self.descriptor)
+        type = (
+            "eGlobalInterfacePrototype"
+            if self.descriptor.isGlobal()
+            else "eInterfacePrototype"
+        )
+        return fill(
+            """
+            static const DOMIfaceAndProtoJSClass sPrototypeClass = {
+              {
+                "${name}Prototype",
+                JSCLASS_IS_DOMIFACEANDPROTOJSCLASS | JSCLASS_HAS_RESERVED_SLOTS(${slotCount}),
+                JS_NULL_CLASS_OPS,
+                JS_NULL_CLASS_SPEC,
+                JS_NULL_CLASS_EXT,
+                JS_NULL_OBJECT_OPS
+              },
+              ${type},
+              false,
+              ${prototypeID},
+              ${depth},
+              ${hooks},
+              nullptr,
+              ${protoGetter}
+            };
+            """,
+            name=self.descriptor.interface.getClassName(),
+            slotCount=slotCount,
+            type=type,
+            hooks=NativePropertyHooks(self.descriptor),
+            prototypeID=prototypeID,
+            depth=depth,
+            protoGetter=protoGetter,
+        )
+
+
+def InterfaceObjectProtoGetter(descriptor, forXrays=False):
+    """
+    Returns a tuple with two elements:
+
+        1) The name of the function to call to get the prototype to use for the
+           interface object as a JSObject*.
+
+        2) The name of the function to call to get the prototype to use for the
+           interface prototype as a JS::Handle<JSObject*> or None if no such
+           function exists.
+    """
+    parentInterface = descriptor.interface.parent
+    if parentInterface:
+        assert not descriptor.interface.isNamespace()
+        parentIfaceName = parentInterface.identifier.name
+        parentDesc = descriptor.getDescriptor(parentIfaceName)
+        prefix = toBindingNamespace(parentDesc.name)
+        protoGetter = prefix + "::GetConstructorObject"
+        protoHandleGetter = prefix + "::GetConstructorObjectHandle"
+    elif descriptor.interface.isNamespace():
+        if forXrays or not descriptor.interface.getExtendedAttribute("ProtoObjectHack"):
+            protoGetter = "JS::GetRealmObjectPrototype"
+        else:
+            protoGetter = "GetHackedNamespaceProtoObject"
+        protoHandleGetter = None
+    else:
+        protoGetter = "JS::GetRealmFunctionPrototype"
+        protoHandleGetter = None
+    return (protoGetter, protoHandleGetter)
+
+
+class CGInterfaceObjectJSClass(CGThing):
+    def __init__(self, descriptor, properties):
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+        self.properties = properties
+
+    def declare(self):
+        # We're purely for internal consumption
+        return ""
+
+    def define(self):
+        if self.descriptor.interface.ctor():
+            assert not self.descriptor.interface.isNamespace()
+            ctorname = CONSTRUCT_HOOK_NAME
+        elif self.descriptor.interface.isNamespace():
+            ctorname = "nullptr"
+        else:
+            ctorname = "ThrowingConstructor"
+        needsHasInstance = self.descriptor.interface.hasInterfacePrototypeObject()
+
+        prototypeID, depth = PrototypeIDAndDepth(self.descriptor)
+        slotCount = "DOM_INTERFACE_SLOTS_BASE"
+        if len(self.descriptor.interface.legacyFactoryFunctions) > 0:
+            slotCount += " + %i /* slots for the legacy factory functions */" % len(
+                self.descriptor.interface.legacyFactoryFunctions
+            )
+        (protoGetter, _) = InterfaceObjectProtoGetter(self.descriptor, forXrays=True)
+
+        if ctorname == "ThrowingConstructor":
+            ret = ""
+            classOpsPtr = "&sBoringInterfaceObjectClassClassOps"
+        elif ctorname == "nullptr":
+            ret = ""
+            classOpsPtr = "JS_NULL_CLASS_OPS"
+        else:
+            ret = fill(
+                """
+                static const JSClassOps sInterfaceObjectClassOps = {
+                    nullptr,               /* addProperty */
+                    nullptr,               /* delProperty */
+                    nullptr,               /* enumerate */
+                    nullptr,               /* newEnumerate */
+                    nullptr,               /* resolve */
+                    nullptr,               /* mayResolve */
+                    nullptr,               /* finalize */
+                    ${ctorname}, /* call */
+                    ${ctorname}, /* construct */
+                    nullptr,               /* trace */
+                };
+
+                """,
+                ctorname=ctorname,
+            )
+            classOpsPtr = "&sInterfaceObjectClassOps"
+
+        if self.descriptor.interface.isNamespace():
+            classString = self.descriptor.interface.getExtendedAttribute("ClassString")
+            if classString is None:
+                classString = self.descriptor.interface.identifier.name
+            else:
+                classString = classString[0]
+            funToString = "nullptr"
+            objectOps = "JS_NULL_OBJECT_OPS"
+        else:
+            classString = "Function"
+            funToString = (
+                '"function %s() {\\n    [native code]\\n}"'
+                % self.descriptor.interface.identifier.name
+            )
+            # We need non-default ObjectOps so we can actually make
+            # use of our funToString.
+            objectOps = "&sInterfaceObjectClassObjectOps"
+
+        ret = ret + fill(
+            """
+            static const DOMIfaceAndProtoJSClass sInterfaceObjectClass = {
+              {
+                "${classString}",
+                JSCLASS_IS_DOMIFACEANDPROTOJSCLASS | JSCLASS_HAS_RESERVED_SLOTS(${slotCount}),
+                ${classOpsPtr},
+                JS_NULL_CLASS_SPEC,
+                JS_NULL_CLASS_EXT,
+                ${objectOps}
+              },
+              ${type},
+              ${needsHasInstance},
+              ${prototypeID},
+              ${depth},
+              ${hooks},
+              ${funToString},
+              ${protoGetter}
+            };
+            """,
+            classString=classString,
+            slotCount=slotCount,
+            classOpsPtr=classOpsPtr,
+            hooks=NativePropertyHooks(self.descriptor),
+            objectOps=objectOps,
+            type="eNamespace"
+            if self.descriptor.interface.isNamespace()
+            else "eInterface",
+            needsHasInstance=toStringBool(needsHasInstance),
+            prototypeID=prototypeID,
+            depth=depth,
+            funToString=funToString,
+            protoGetter=protoGetter,
+        )
+        return ret
+
+
+class CGList(CGThing):
+    """
+    Generate code for a list of GCThings.  Just concatenates them together, with
+    an optional joiner string.  "\n" is a common joiner.
+    """
+
+    def __init__(self, children, joiner=""):
+        CGThing.__init__(self)
+        # Make a copy of the kids into a list, because if someone passes in a
+        # generator we won't be able to both declare and define ourselves, or
+        # define ourselves more than once!
+        self.children = list(children)
+        self.joiner = joiner
+
+    def append(self, child):
+        self.children.append(child)
+
+    def prepend(self, child):
+        self.children.insert(0, child)
+
+    def extend(self, kids):
+        self.children.extend(kids)
+
+    def join(self, iterable):
+        return self.joiner.join(s for s in iterable if len(s) > 0)
+
+    def declare(self):
+        return self.join(
+            child.declare() for child in self.children if child is not None
+        )
+
+    def define(self):
+        return self.join(child.define() for child in self.children if child is not None)
+
+    def deps(self):
+        deps = set()
+        for child in self.children:
+            if child is None:
+                continue
+            deps = deps.union(child.deps())
+        return deps
+
+    def __len__(self):
+        return len(self.children)
+
+
+class CGGeneric(CGThing):
+    """
+    A class that spits out a fixed string into the codegen.  Can spit out a
+    separate string for the declaration too.
+    """
+
+    def __init__(self, define="", declare=""):
+        self.declareText = declare
+        self.defineText = define
+
+    def declare(self):
+        return self.declareText
+
+    def define(self):
+        return self.defineText
+
+    def deps(self):
+        return set()
+
+
+class CGIndenter(CGThing):
+    """
+    A class that takes another CGThing and generates code that indents that
+    CGThing by some number of spaces.  The default indent is two spaces.
+    """
+
+    def __init__(self, child, indentLevel=2, declareOnly=False):
+        assert isinstance(child, CGThing)
+        CGThing.__init__(self)
+        self.child = child
+        self.indentLevel = indentLevel
+        self.declareOnly = declareOnly
+
+    def declare(self):
+        return indent(self.child.declare(), self.indentLevel)
+
+    def define(self):
+        defn = self.child.define()
+        if self.declareOnly:
+            return defn
+        else:
+            return indent(defn, self.indentLevel)
+
+
+class CGWrapper(CGThing):
+    """
+    Generic CGThing that wraps other CGThings with pre and post text.
+    """
+
+    def __init__(
+        self,
+        child,
+        pre="",
+        post="",
+        declarePre=None,
+        declarePost=None,
+        definePre=None,
+        definePost=None,
+        declareOnly=False,
+        defineOnly=False,
+        reindent=False,
+    ):
+        CGThing.__init__(self)
+        self.child = child
+        self.declarePre = declarePre or pre
+        self.declarePost = declarePost or post
+        self.definePre = definePre or pre
+        self.definePost = definePost or post
+        self.declareOnly = declareOnly
+        self.defineOnly = defineOnly
+        self.reindent = reindent
+
+    def declare(self):
+        if self.defineOnly:
+            return ""
+        decl = self.child.declare()
+        if self.reindent:
+            decl = self.reindentString(decl, self.declarePre)
+        return self.declarePre + decl + self.declarePost
+
+    def define(self):
+        if self.declareOnly:
+            return ""
+        defn = self.child.define()
+        if self.reindent:
+            defn = self.reindentString(defn, self.definePre)
+        return self.definePre + defn + self.definePost
+
+    @staticmethod
+    def reindentString(stringToIndent, widthString):
+        # We don't use lineStartDetector because we don't want to
+        # insert whitespace at the beginning of our _first_ line.
+        # Use the length of the last line of width string, in case
+        # it is a multiline string.
+        lastLineWidth = len(widthString.splitlines()[-1])
+        return stripTrailingWhitespace(
+            stringToIndent.replace("\n", "\n" + (" " * lastLineWidth))
+        )
+
+    def deps(self):
+        return self.child.deps()
+
+
+class CGIfWrapper(CGList):
+    def __init__(self, child, condition):
+        CGList.__init__(
+            self,
+            [
+                CGWrapper(
+                    CGGeneric(condition), pre="if (", post=") {\n", reindent=True
+                ),
+                CGIndenter(child),
+                CGGeneric("}\n"),
+            ],
+        )
+
+
+class CGIfElseWrapper(CGList):
+    def __init__(self, condition, ifTrue, ifFalse):
+        CGList.__init__(
+            self,
+            [
+                CGWrapper(
+                    CGGeneric(condition), pre="if (", post=") {\n", reindent=True
+                ),
+                CGIndenter(ifTrue),
+                CGGeneric("} else {\n"),
+                CGIndenter(ifFalse),
+                CGGeneric("}\n"),
+            ],
+        )
+
+
+class CGElseChain(CGThing):
+    """
+    Concatenate if statements in an if-else-if-else chain.
+    """
+
+    def __init__(self, children):
+        self.children = [c for c in children if c is not None]
+
+    def declare(self):
+        assert False
+
+    def define(self):
+        if not self.children:
+            return ""
+        s = self.children[0].define()
+        assert s.endswith("\n")
+        for child in self.children[1:]:
+            code = child.define()
+            assert code.startswith("if") or code.startswith("{")
+            assert code.endswith("\n")
+            s = s.rstrip() + " else " + code
+        return s
+
+
+class CGTemplatedType(CGWrapper):
+    def __init__(self, templateName, child, isConst=False, isReference=False):
+        if isinstance(child, list):
+            child = CGList(child, ", ")
+        const = "const " if isConst else ""
+        pre = "%s%s<" % (const, templateName)
+        ref = "&" if isReference else ""
+        post = ">%s" % ref
+        CGWrapper.__init__(self, child, pre=pre, post=post)
+
+
+class CGNamespace(CGThing):
+    """
+    Generates namespace block that wraps other CGThings.
+    """
+
+    def __init__(self, namespace, child):
+        CGThing.__init__(self)
+        self.child = child
+        self.pre = "namespace %s {\n" % namespace
+        self.post = "} // namespace %s\n" % namespace
+
+    def declare(self):
+        decl = self.child.declare()
+        if len(decl.strip()) == 0:
+            return ""
+        return self.pre + decl + self.post
+
+    def define(self):
+        defn = self.child.define()
+        if len(defn.strip()) == 0:
+            return ""
+        return self.pre + defn + self.post
+
+    def deps(self):
+        return self.child.deps()
+
+    @staticmethod
+    def build(namespaces, child):
+        """
+        Static helper method to build multiple wrapped namespaces.
+        """
+        if not namespaces:
+            return CGWrapper(child)
+        return CGNamespace("::".join(namespaces), child)
+
+
+class CGIncludeGuard(CGWrapper):
+    """
+    Generates include guards for a header.
+    """
+
+    def __init__(self, prefix, child):
+        """|prefix| is the filename without the extension."""
+        define = "mozilla_dom_%s_h" % prefix
+        CGWrapper.__init__(
+            self,
+            child,
+            declarePre="#ifndef %s\n#define %s\n\n" % (define, define),
+            declarePost="\n#endif // %s\n" % define,
+        )
+
+
+class CGHeaders(CGWrapper):
+    """
+    Generates the appropriate include statements.
+    """
+
+    def __init__(
+        self,
+        descriptors,
+        dictionaries,
+        callbacks,
+        callbackDescriptors,
+        declareIncludes,
+        defineIncludes,
+        prefix,
+        child,
+        config=None,
+        jsImplementedDescriptors=[],
+    ):
+        """
+        Builds a set of includes to cover |descriptors|.
+
+        Also includes the files in |declareIncludes| in the header
+        file and the files in |defineIncludes| in the .cpp.
+
+        |prefix| contains the basename of the file that we generate include
+        statements for.
+        """
+
+        # Determine the filenames for which we need headers.
+        interfaceDeps = [d.interface for d in descriptors]
+        ancestors = []
+        for iface in interfaceDeps:
+            if iface.parent:
+                # We're going to need our parent's prototype, to use as the
+                # prototype of our prototype object.
+                ancestors.append(iface.parent)
+                # And if we have an interface object, we'll need the nearest
+                # ancestor with an interface object too, so we can use its
+                # interface object as the proto of our interface object.
+                if iface.hasInterfaceObject():
+                    parent = iface.parent
+                    while parent and not parent.hasInterfaceObject():
+                        parent = parent.parent
+                    if parent:
+                        ancestors.append(parent)
+        interfaceDeps.extend(ancestors)
+
+        # Include parent interface headers needed for default toJSON code.
+        jsonInterfaceParents = []
+        for desc in descriptors:
+            if not desc.hasDefaultToJSON:
+                continue
+            parent = desc.interface.parent
+            while parent:
+                parentDesc = desc.getDescriptor(parent.identifier.name)
+                if parentDesc.hasDefaultToJSON:
+                    jsonInterfaceParents.append(parentDesc.interface)
+                parent = parent.parent
+        interfaceDeps.extend(jsonInterfaceParents)
+
+        bindingIncludes = set(self.getDeclarationFilename(d) for d in interfaceDeps)
+
+        # Grab all the implementation declaration files we need.
+        implementationIncludes = set(
+            d.headerFile for d in descriptors if d.needsHeaderInclude()
+        )
+
+        # Now find all the things we'll need as arguments because we
+        # need to wrap or unwrap them.
+        bindingHeaders = set()
+        declareIncludes = set(declareIncludes)
+
+        def addHeadersForType(typeAndPossibleOriginType):
+            """
+            Add the relevant headers for this type.  We use its origin type, if
+            passed, to decide what to do with interface types.
+            """
+            t, originType = typeAndPossibleOriginType
+            isFromDictionary = originType and originType.isDictionary()
+            isFromCallback = originType and originType.isCallback()
+            # Dictionaries have members that need to be actually
+            # declared, not just forward-declared.
+            # Callbacks have nullable union arguments that need to be actually
+            # declared, not just forward-declared.
+            if isFromDictionary:
+                headerSet = declareIncludes
+            elif isFromCallback and t.nullable() and t.isUnion():
+                headerSet = declareIncludes
+            else:
+                headerSet = bindingHeaders
+            # Strip off outer layers and add headers they might require.  (This
+            # is conservative: only nullable non-pointer types need Nullable.h;
+            # only sequences or observable arrays outside unions need
+            # ForOfIterator.h; only functions that return, and attributes that
+            # are, sequences or observable arrays in interfaces need Array.h, &c.)
+            unrolled = t
+            while True:
+                if idlTypeNeedsCallContext(unrolled):
+                    bindingHeaders.add("mozilla/dom/BindingCallContext.h")
+                if unrolled.nullable():
+                    headerSet.add("mozilla/dom/Nullable.h")
+                elif unrolled.isSequence() or unrolled.isObservableArray():
+                    bindingHeaders.add("js/Array.h")
+                    bindingHeaders.add("js/ForOfIterator.h")
+                    if unrolled.isObservableArray():
+                        bindingHeaders.add("mozilla/dom/ObservableArrayProxyHandler.h")
+                else:
+                    break
+                unrolled = unrolled.inner
+            if unrolled.isUnion():
+                headerSet.add(self.getUnionDeclarationFilename(config, unrolled))
+                for t in unrolled.flatMemberTypes:
+                    addHeadersForType((t, None))
+            elif unrolled.isPromise():
+                # See comment in the isInterface() case for why we add
+                # Promise.h to headerSet, not bindingHeaders.
+                headerSet.add("mozilla/dom/Promise.h")
+                # We need ToJSValue to do the Promise to JS conversion.
+                bindingHeaders.add("mozilla/dom/ToJSValue.h")
+            elif unrolled.isInterface():
+                if unrolled.isSpiderMonkeyInterface():
+                    bindingHeaders.add("jsfriendapi.h")
+                    if jsImplementedDescriptors:
+                        # Since we can't forward-declare typed array types
+                        # (because they're typedefs), we have to go ahead and
+                        # just include their header if we need to have functions
+                        # taking references to them declared in that header.
+                        headerSet = declareIncludes
+                    headerSet.add("mozilla/dom/TypedArray.h")
+                else:
+                    try:
+                        typeDesc = config.getDescriptor(unrolled.inner.identifier.name)
+                    except NoSuchDescriptorError:
+                        return
+                    # Dictionaries with interface members rely on the
+                    # actual class definition of that interface member
+                    # being visible in the binding header, because they
+                    # store them in RefPtr and have inline
+                    # constructors/destructors.
+                    #
+                    # XXXbz maybe dictionaries with interface members
+                    # should just have out-of-line constructors and
+                    # destructors?
+                    headerSet.add(typeDesc.headerFile)
+            elif unrolled.isDictionary():
+                headerSet.add(self.getDeclarationFilename(unrolled.inner))
+                # And if it needs rooting, we need RootedDictionary too
+                if typeNeedsRooting(unrolled):
+                    headerSet.add("mozilla/dom/RootedDictionary.h")
+            elif unrolled.isCallback():
+                headerSet.add(self.getDeclarationFilename(unrolled.callback))
+            elif unrolled.isFloat() and not unrolled.isUnrestricted():
+                # Restricted floats are tested for finiteness
+                bindingHeaders.add("mozilla/FloatingPoint.h")
+                bindingHeaders.add("mozilla/dom/PrimitiveConversions.h")
+            elif unrolled.isEnum():
+                filename = self.getDeclarationFilename(unrolled.inner)
+                declareIncludes.add(filename)
+            elif unrolled.isPrimitive():
+                bindingHeaders.add("mozilla/dom/PrimitiveConversions.h")
+            elif unrolled.isRecord():
+                if isFromDictionary or jsImplementedDescriptors:
+                    declareIncludes.add("mozilla/dom/Record.h")
+                else:
+                    bindingHeaders.add("mozilla/dom/Record.h")
+                # Also add headers for the type the record is
+                # parametrized over, if needed.
+                addHeadersForType((t.inner, originType if isFromDictionary else None))
+
+        for t in getAllTypes(
+            descriptors + callbackDescriptors, dictionaries, callbacks
+        ):
+            addHeadersForType(t)
+
+        def addHeaderForFunc(func, desc):
+            if func is None:
+                return
+            # Include the right class header, which we can only do
+            # if this is a class member function.
+            if desc is not None and not desc.headerIsDefault:
+                # An explicit header file was provided, assume that we know
+                # what we're doing.
+                return
+
+            if "::" in func:
+                # Strip out the function name and convert "::" to "/"
+                bindingHeaders.add("/".join(func.split("::")[:-1]) + ".h")
+
+        # Now for non-callback descriptors make sure we include any
+        # headers needed by Func declarations and other things like that.
+        for desc in descriptors:
+            # If this is an iterator or an async iterator interface generated
+            # for a separate iterable interface, skip generating type includes,
+            # as we have what we need in IterableIterator.h
+            if (
+                desc.interface.isIteratorInterface()
+                or desc.interface.isAsyncIteratorInterface()
+            ):
+                continue
+
+            for m in desc.interface.members:
+                addHeaderForFunc(PropertyDefiner.getStringAttr(m, "Func"), desc)
+                staticTypeOverride = PropertyDefiner.getStringAttr(
+                    m, "StaticClassOverride"
+                )
+                if staticTypeOverride:
+                    bindingHeaders.add("/".join(staticTypeOverride.split("::")) + ".h")
+            # getExtendedAttribute() returns a list, extract the entry.
+            funcList = desc.interface.getExtendedAttribute("Func")
+            if funcList is not None:
+                addHeaderForFunc(funcList[0], desc)
+
+            if desc.interface.maplikeOrSetlikeOrIterable:
+                # We need ToJSValue.h for maplike/setlike type conversions
+                bindingHeaders.add("mozilla/dom/ToJSValue.h")
+                # Add headers for the key and value types of the
+                # maplike/setlike/iterable, since they'll be needed for
+                # convenience functions
+                if desc.interface.maplikeOrSetlikeOrIterable.hasKeyType():
+                    addHeadersForType(
+                        (desc.interface.maplikeOrSetlikeOrIterable.keyType, None)
+                    )
+                if desc.interface.maplikeOrSetlikeOrIterable.hasValueType():
+                    addHeadersForType(
+                        (desc.interface.maplikeOrSetlikeOrIterable.valueType, None)
+                    )
+
+        for d in dictionaries:
+            if d.parent:
+                declareIncludes.add(self.getDeclarationFilename(d.parent))
+            bindingHeaders.add(self.getDeclarationFilename(d))
+            for m in d.members:
+                addHeaderForFunc(PropertyDefiner.getStringAttr(m, "Func"), None)
+            # No need to worry about Func on members of ancestors, because that
+            # will happen automatically in whatever files those ancestors live
+            # in.
+
+        for c in callbacks:
+            bindingHeaders.add(self.getDeclarationFilename(c))
+
+        for c in callbackDescriptors:
+            bindingHeaders.add(self.getDeclarationFilename(c.interface))
+
+        if len(callbacks) != 0:
+            # We need CallbackFunction to serve as our parent class
+            declareIncludes.add("mozilla/dom/CallbackFunction.h")
+            # And we need ToJSValue.h so we can wrap "this" objects
+            declareIncludes.add("mozilla/dom/ToJSValue.h")
+
+        if len(callbackDescriptors) != 0 or len(jsImplementedDescriptors) != 0:
+            # We need CallbackInterface to serve as our parent class
+            declareIncludes.add("mozilla/dom/CallbackInterface.h")
+            # And we need ToJSValue.h so we can wrap "this" objects
+            declareIncludes.add("mozilla/dom/ToJSValue.h")
+
+        # Also need to include the headers for ancestors of
+        # JS-implemented interfaces.
+        for jsImplemented in jsImplementedDescriptors:
+            jsParent = jsImplemented.interface.parent
+            if jsParent:
+                parentDesc = jsImplemented.getDescriptor(jsParent.identifier.name)
+                declareIncludes.add(parentDesc.jsImplParentHeader)
+
+        # Now make sure we're not trying to include the header from inside itself
+        declareIncludes.discard(prefix + ".h")
+
+        # Let the machinery do its thing.
+        def _includeString(includes):
+            def headerName(include):
+                # System headers are specified inside angle brackets.
+                if include.startswith("<"):
+                    return include
+                # Non-system headers need to be placed in quotes.
+                return '"%s"' % include
+
+            return "".join(["#include %s\n" % headerName(i) for i in includes]) + "\n"
+
+        CGWrapper.__init__(
+            self,
+            child,
+            declarePre=_includeString(sorted(declareIncludes)),
+            definePre=_includeString(
+                sorted(
+                    set(defineIncludes)
+                    | bindingIncludes
+                    | bindingHeaders
+                    | implementationIncludes
+                )
+            ),
+        )
+
+    @staticmethod
+    def getDeclarationFilename(decl):
+        # Use our local version of the header, not the exported one, so that
+        # test bindings, which don't export, will work correctly.
+        basename = os.path.basename(decl.filename())
+        return basename.replace(".webidl", "Binding.h")
+
+    @staticmethod
+    def getUnionDeclarationFilename(config, unionType):
+        assert unionType.isUnion()
+        assert unionType.unroll() == unionType
+        # If a union is "defined" in multiple files, it goes in UnionTypes.h.
+        if len(config.filenamesPerUnion[unionType.name]) > 1:
+            return "mozilla/dom/UnionTypes.h"
+        # If a union is defined by a built-in typedef, it also goes in
+        # UnionTypes.h.
+        assert len(config.filenamesPerUnion[unionType.name]) == 1
+        if "<unknown>" in config.filenamesPerUnion[unionType.name]:
+            return "mozilla/dom/UnionTypes.h"
+        return CGHeaders.getDeclarationFilename(unionType)
+
+
+def SortedDictValues(d):
+    """
+    Returns a list of values from the dict sorted by key.
+    """
+    return [v for k, v in sorted(d.items())]
+
+
+def UnionsForFile(config, webIDLFile):
+    """
+    Returns a list of union types for all union types that are only used in
+    webIDLFile. If webIDLFile is None this will return the list of tuples for
+    union types that are used in more than one WebIDL file.
+    """
+    return config.unionsPerFilename.get(webIDLFile, [])
+
+
+def UnionTypes(unionTypes, config):
+    """
+    The unionTypes argument should be a list of union types. This is typically
+    the list generated by UnionsForFile.
+
+    Returns a tuple containing a set of header filenames to include in
+    the header for the types in unionTypes, a set of header filenames to
+    include in the implementation file for the types in unionTypes, a set
+    of tuples containing a type declaration and a boolean if the type is a
+    struct for member types of the union, a list of traverse methods,
+    unlink methods and a list of union types. These last three lists only
+    contain unique union types.
+    """
+
+    headers = set()
+    implheaders = set()
+    declarations = set()
+    unionStructs = dict()
+    traverseMethods = dict()
+    unlinkMethods = dict()
+
+    for t in unionTypes:
+        name = str(t)
+        if name not in unionStructs:
+            unionStructs[name] = t
+
+            def addHeadersForType(f):
+                if f.nullable():
+                    headers.add("mozilla/dom/Nullable.h")
+                isSequence = f.isSequence()
+                if isSequence:
+                    # Dealing with sequences requires for-of-compatible
+                    # iteration.
+                    implheaders.add("js/ForOfIterator.h")
+                    # Sequences can always throw "not an object" exceptions.
+                    implheaders.add("mozilla/dom/BindingCallContext.h")
+                    if typeNeedsRooting(f):
+                        headers.add("mozilla/dom/RootedSequence.h")
+                f = f.unroll()
+                if idlTypeNeedsCallContext(f):
+                    implheaders.add("mozilla/dom/BindingCallContext.h")
+                if f.isPromise():
+                    headers.add("mozilla/dom/Promise.h")
+                    # We need ToJSValue to do the Promise to JS conversion.
+                    headers.add("mozilla/dom/ToJSValue.h")
+                elif f.isInterface():
+                    if f.isSpiderMonkeyInterface():
+                        headers.add("js/RootingAPI.h")
+                        headers.add("js/Value.h")
+                        headers.add("mozilla/dom/TypedArray.h")
+                    else:
+                        try:
+                            typeDesc = config.getDescriptor(f.inner.identifier.name)
+                        except NoSuchDescriptorError:
+                            return
+                        if typeDesc.interface.isCallback() or isSequence:
+                            # Callback interfaces always use strong refs, so
+                            # we need to include the right header to be able
+                            # to Release() in our inlined code.
+                            #
+                            # Similarly, sequences always contain strong
+                            # refs, so we'll need the header to handler
+                            # those.
+                            headers.add(typeDesc.headerFile)
+                        elif typeDesc.interface.identifier.name == "WindowProxy":
+                            # In UnionTypes.h we need to see the declaration of the
+                            # WindowProxyHolder that we use to store the WindowProxy, so
+                            # we have its sizeof and know how big to make our union.
+                            headers.add(typeDesc.headerFile)
+                        else:
+                            declarations.add((typeDesc.nativeType, False))
+                            implheaders.add(typeDesc.headerFile)
+                elif f.isDictionary():
+                    # For a dictionary, we need to see its declaration in
+                    # UnionTypes.h so we have its sizeof and know how big to
+                    # make our union.
+                    headers.add(CGHeaders.getDeclarationFilename(f.inner))
+                    # And if it needs rooting, we need RootedDictionary too
+                    if typeNeedsRooting(f):
+                        headers.add("mozilla/dom/RootedDictionary.h")
+                elif f.isFloat() and not f.isUnrestricted():
+                    # Restricted floats are tested for finiteness
+                    implheaders.add("mozilla/FloatingPoint.h")
+                    implheaders.add("mozilla/dom/PrimitiveConversions.h")
+                elif f.isEnum():
+                    # Need to see the actual definition of the enum,
+                    # unfortunately.
+                    headers.add(CGHeaders.getDeclarationFilename(f.inner))
+                elif f.isPrimitive():
+                    implheaders.add("mozilla/dom/PrimitiveConversions.h")
+                elif f.isCallback():
+                    # Callbacks always use strong refs, so we need to include
+                    # the right header to be able to Release() in our inlined
+                    # code.
+                    headers.add(CGHeaders.getDeclarationFilename(f.callback))
+                elif f.isRecord():
+                    headers.add("mozilla/dom/Record.h")
+                    # And add headers for the type we're parametrized over
+                    addHeadersForType(f.inner)
+                    # And if it needs rooting, we need RootedRecord too
+                    if typeNeedsRooting(f):
+                        headers.add("mozilla/dom/RootedRecord.h")
+
+            implheaders.add(CGHeaders.getUnionDeclarationFilename(config, t))
+            for f in t.flatMemberTypes:
+                assert not f.nullable()
+                addHeadersForType(f)
+
+            if idlTypeNeedsCycleCollection(t):
+                declarations.add(
+                    ("mozilla::dom::%s" % CGUnionStruct.unionTypeName(t, True), False)
+                )
+                traverseMethods[name] = CGCycleCollectionTraverseForOwningUnionMethod(t)
+                unlinkMethods[name] = CGCycleCollectionUnlinkForOwningUnionMethod(t)
+
+    # The order of items in CGList is important.
+    # Since the union structs friend the unlinkMethods, the forward-declaration
+    # for these methods should come before the class declaration. Otherwise
+    # some compilers treat the friend declaration as a forward-declaration in
+    # the class scope.
+    return (
+        headers,
+        implheaders,
+        declarations,
+        SortedDictValues(traverseMethods),
+        SortedDictValues(unlinkMethods),
+        SortedDictValues(unionStructs),
+    )
+
+
+class Argument:
+    """
+    A class for outputting the type and name of an argument
+    """
+
+    def __init__(self, argType, name, default=None):
+        self.argType = argType
+        self.name = name
+        self.default = default
+
+    def declare(self):
+        string = self.argType + " " + self.name
+        if self.default is not None:
+            string += " = " + self.default
+        return string
+
+    def define(self):
+        return self.argType + " " + self.name
+
+
+class CGAbstractMethod(CGThing):
+    """
+    An abstract class for generating code for a method.  Subclasses
+    should override definition_body to create the actual code.
+
+    descriptor is the descriptor for the interface the method is associated with
+
+    name is the name of the method as a string
+
+    returnType is the IDLType of the return value
+
+    args is a list of Argument objects
+
+    inline should be True to generate an inline method, whose body is
+    part of the declaration.
+
+    alwaysInline should be True to generate an inline method annotated with
+    MOZ_ALWAYS_INLINE.
+
+    static should be True to generate a static method, which only has
+    a definition.
+
+    If templateArgs is not None it should be a list of strings containing
+    template arguments, and the function will be templatized using those
+    arguments.
+
+    canRunScript should be True to generate a MOZ_CAN_RUN_SCRIPT annotation.
+
+    signatureOnly should be True to only declare the signature (either in
+                  the header, or if static is True in the cpp file).
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        name,
+        returnType,
+        args,
+        inline=False,
+        alwaysInline=False,
+        static=False,
+        templateArgs=None,
+        canRunScript=False,
+        signatureOnly=False,
+    ):
+        CGThing.__init__(self)
+        self.descriptor = descriptor
+        self.name = name
+        self.returnType = returnType
+        self.args = args
+        self.inline = inline
+        self.alwaysInline = alwaysInline
+        self.static = static
+        self.templateArgs = templateArgs
+        self.canRunScript = canRunScript
+        self.signatureOnly = signatureOnly
+
+    def _argstring(self, declare):
+        return ", ".join([a.declare() if declare else a.define() for a in self.args])
+
+    def _template(self):
+        if self.templateArgs is None:
+            return ""
+        return "template <%s>\n" % ", ".join(self.templateArgs)
+
+    def _decorators(self):
+        decorators = []
+        if self.canRunScript:
+            decorators.append("MOZ_CAN_RUN_SCRIPT")
+        if self.alwaysInline:
+            decorators.append("MOZ_ALWAYS_INLINE")
+        elif self.inline:
+            decorators.append("inline")
+        if self.static:
+            decorators.append("static")
+        decorators.append(self.returnType)
+        maybeNewline = " " if self.inline else "\n"
+        return " ".join(decorators) + maybeNewline
+
+    def signature(self):
+        return "%s%s%s(%s);\n" % (
+            self._template(),
+            self._decorators(),
+            self.name,
+            self._argstring(True),
+        )
+
+    def declare(self):
+        if self.static:
+            return ""
+        if self.inline:
+            return self._define(True)
+        return self.signature()
+
+    def indent_body(self, body):
+        """
+        Indent the code returned by self.definition_body(). Most classes
+        simply indent everything two spaces. This is here for
+        CGRegisterProtos, which needs custom indentation.
+        """
+        return indent(body)
+
+    def _define(self, fromDeclare=False):
+        return (
+            self.definition_prologue(fromDeclare)
+            + self.indent_body(self.definition_body())
+            + self.definition_epilogue()
+        )
+
+    def define(self):
+        if self.signatureOnly:
+            if self.static:
+                # self.static makes us not output anything in the header, so output the signature here.
+                return self.signature()
+            return ""
+        return "" if (self.inline and not self.static) else self._define()
+
+    def definition_prologue(self, fromDeclare):
+        error_reporting_label = self.error_reporting_label()
+        if error_reporting_label:
+            # We're going to want a BindingCallContext.  Rename our JSContext*
+            # arg accordingly.
+            i = 0
+            while i < len(self.args):
+                arg = self.args[i]
+                if arg.argType == "JSContext*":
+                    cxname = arg.name
+                    self.args[i] = Argument(arg.argType, "cx_", arg.default)
+                    break
+                i += 1
+            if i == len(self.args):
+                raise TypeError("Must have a JSContext* to create a BindingCallContext")
+
+        prologue = "%s%s%s(%s)\n{\n" % (
+            self._template(),
+            self._decorators(),
+            self.name,
+            self._argstring(fromDeclare),
+        )
+        if error_reporting_label:
+            prologue += indent(
+                fill(
+                    """
+                BindingCallContext ${cxname}(cx_, "${label}");
+                """,
+                    cxname=cxname,
+                    label=error_reporting_label,
+                )
+            )
+
+        profiler_label = self.auto_profiler_label()
+        if profiler_label:
+            prologue += indent(profiler_label) + "\n"
+
+        return prologue
+
+    def definition_epilogue(self):
+        return "}\n"
+
+    def definition_body(self):
+        assert False  # Override me!
+
+    """
+    Override this method to return a pair of (descriptive string, name of a
+    JSContext* variable) in order to generate a profiler label for this method.
+    """
+
+    def auto_profiler_label(self):
+        return None  # Override me!
+
+    """
+    Override this method to return a string to be used as the label for a
+    BindingCallContext.  If this does not return None, one of the arguments of
+    this method must be of type 'JSContext*'.  Its name will be replaced with
+    'cx_' and a BindingCallContext named 'cx' will be instantiated with the
+    given label.
+    """
+
+    def error_reporting_label(self):
+        return None  # Override me!
+
+
+class CGAbstractStaticMethod(CGAbstractMethod):
+    """
+    Abstract base class for codegen of implementation-only (no
+    declaration) static methods.
+    """
+
+    def __init__(self, descriptor, name, returnType, args, canRunScript=False):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            name,
+            returnType,
+            args,
+            inline=False,
+            static=True,
+            canRunScript=canRunScript,
+        )
+
+
+class CGAbstractClassHook(CGAbstractStaticMethod):
+    """
+    Meant for implementing JSClass hooks, like Finalize or Trace. Does very raw
+    'this' unwrapping as it assumes that the unwrapped type is always known.
+    """
+
+    def __init__(self, descriptor, name, returnType, args):
+        CGAbstractStaticMethod.__init__(self, descriptor, name, returnType, args)
+
+    def definition_body_prologue(self):
+        return "%s* self = UnwrapPossiblyNotInitializedDOMObject<%s>(obj);\n" % (
+            self.descriptor.nativeType,
+            self.descriptor.nativeType,
+        )
+
+    def definition_body(self):
+        return self.definition_body_prologue() + self.generate_code()
+
+    def generate_code(self):
+        assert False  # Override me!
+
+
+class CGAddPropertyHook(CGAbstractClassHook):
+    """
+    A hook for addProperty, used to preserve our wrapper from GC.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::Handle<JS::Value>", "val"),
+        ]
+        CGAbstractClassHook.__init__(
+            self, descriptor, ADDPROPERTY_HOOK_NAME, "bool", args
+        )
+
+    def generate_code(self):
+        assert self.descriptor.wrapperCache
+        # This hook is also called by TryPreserveWrapper on non-nsISupports
+        # cycle collected objects, so if addProperty is ever changed to do
+        # anything more or less than preserve the wrapper, TryPreserveWrapper
+        # will need to be changed.
+        return dedent(
+            """
+            // We don't want to preserve if we don't have a wrapper, and we
+            // obviously can't preserve if we're not initialized.
+            if (self && self->GetWrapperPreserveColor()) {
+              PreserveWrapper(self);
+            }
+            return true;
+            """
+        )
+
+
+class CGGetWrapperCacheHook(CGAbstractClassHook):
+    """
+    A hook for GetWrapperCache, used by HasReleasedWrapper to get the
+    nsWrapperCache pointer for a non-nsISupports object.
+    """
+
+    def __init__(self, descriptor):
+        args = [Argument("JS::Handle<JSObject*>", "obj")]
+        CGAbstractClassHook.__init__(
+            self, descriptor, GETWRAPPERCACHE_HOOK_NAME, "nsWrapperCache*", args
+        )
+
+    def generate_code(self):
+        assert self.descriptor.wrapperCache
+        return dedent(
+            """
+            return self;
+            """
+        )
+
+
+def finalizeHook(descriptor, hookName, gcx, obj):
+    finalize = "JS::SetReservedSlot(%s, DOM_OBJECT_SLOT, JS::UndefinedValue());\n" % obj
+    if descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+        finalize += fill(
+            """
+            // Either our proxy created an expando object or not.  If it did,
+            // then we would have preserved ourselves, and hence if we're going
+            // away so is our C++ object and we should reset its expando value.
+            // It's possible that in this situation the C++ object's reflector
+            // pointer has been nulled out, but if not it's pointing to us.  If
+            // our proxy did _not_ create an expando object then it's possible
+            // that we're no longer the reflector for our C++ object (and
+            // incremental finalization is finally getting to us), and that in
+            // the meantime the new reflector has created an expando object.
+            // In that case we do NOT want to clear the expando pointer in the
+            // C++ object.
+            //
+            // It's important to do this before we ClearWrapper, of course.
+            JSObject* reflector = self->GetWrapperMaybeDead();
+            if (!reflector || reflector == ${obj}) {
+              self->mExpandoAndGeneration.expando = JS::UndefinedValue();
+            }
+            """,
+            obj=obj,
+        )
+    for m in descriptor.interface.members:
+        if m.isAttr() and m.type.isObservableArray():
+            finalize += fill(
+                """
+                {
+                  JS::Value val = JS::GetReservedSlot(obj, ${slot});
+                  if (!val.isUndefined()) {
+                    JSObject* obj = &val.toObject();
+                    js::SetProxyReservedSlot(obj, OBSERVABLE_ARRAY_DOM_INTERFACE_SLOT, JS::UndefinedValue());
+                  }
+                }
+                """,
+                slot=memberReservedSlot(m, descriptor),
+            )
+    if descriptor.wrapperCache:
+        finalize += "ClearWrapper(self, self, %s);\n" % obj
+    if descriptor.isGlobal():
+        finalize += "mozilla::dom::FinalizeGlobal(%s, %s);\n" % (gcx, obj)
+    finalize += fill(
+        """
+        if (size_t mallocBytes = BindingJSObjectMallocBytes(self)) {
+          JS::RemoveAssociatedMemory(${obj}, mallocBytes,
+                                     JS::MemoryUse::DOMBinding);
+        }
+        """,
+        obj=obj,
+    )
+    finalize += "AddForDeferredFinalization<%s>(self);\n" % descriptor.nativeType
+    return CGIfWrapper(CGGeneric(finalize), "self")
+
+
+class CGClassFinalizeHook(CGAbstractClassHook):
+    """
+    A hook for finalize, used to release our native object.
+    """
+
+    def __init__(self, descriptor):
+        args = [Argument("JS::GCContext*", "gcx"), Argument("JSObject*", "obj")]
+        CGAbstractClassHook.__init__(self, descriptor, FINALIZE_HOOK_NAME, "void", args)
+
+    def generate_code(self):
+        return finalizeHook(
+            self.descriptor, self.name, self.args[0].name, self.args[1].name
+        ).define()
+
+
+def objectMovedHook(descriptor, hookName, obj, old):
+    assert descriptor.wrapperCache
+    return fill(
+        """
+        if (self) {
+          UpdateWrapper(self, self, ${obj}, ${old});
+        }
+
+        return 0;
+        """,
+        obj=obj,
+        old=old,
+    )
+
+
+class CGClassObjectMovedHook(CGAbstractClassHook):
+    """
+    A hook for objectMovedOp, used to update the wrapper cache when an object it
+    is holding moves.
+    """
+
+    def __init__(self, descriptor):
+        args = [Argument("JSObject*", "obj"), Argument("JSObject*", "old")]
+        CGAbstractClassHook.__init__(
+            self, descriptor, OBJECT_MOVED_HOOK_NAME, "size_t", args
+        )
+
+    def generate_code(self):
+        return objectMovedHook(
+            self.descriptor, self.name, self.args[0].name, self.args[1].name
+        )
+
+
+def JSNativeArguments():
+    return [
+        Argument("JSContext*", "cx"),
+        Argument("unsigned", "argc"),
+        Argument("JS::Value*", "vp"),
+    ]
+
+
+class CGClassConstructor(CGAbstractStaticMethod):
+    """
+    JS-visible constructor for our objects
+    """
+
+    def __init__(self, descriptor, ctor, name=CONSTRUCT_HOOK_NAME):
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", JSNativeArguments()
+        )
+        self._ctor = ctor
+
+    def define(self):
+        if not self._ctor:
+            return ""
+        return CGAbstractStaticMethod.define(self)
+
+    def definition_body(self):
+        return self.generate_code()
+
+    def generate_code(self):
+        if self._ctor.isHTMLConstructor():
+            # We better have a prototype object.  Otherwise our proto
+            # id won't make sense.
+            assert self.descriptor.interface.hasInterfacePrototypeObject()
+            # We also better have a constructor object, if this is
+            # getting called!
+            assert self.descriptor.interface.hasInterfaceObject()
+            # We can't just pass null for the CreateInterfaceObjects callback,
+            # because our newTarget might be in a different compartment, in
+            # which case we'll need to look up constructor objects in that
+            # compartment.
+            return fill(
+                """
+                return HTMLConstructor(cx, argc, vp,
+                                       constructors::id::${name},
+                                       prototypes::id::${name},
+                                       CreateInterfaceObjects);
+                """,
+                name=self.descriptor.name,
+            )
+
+        # If the interface is already SecureContext, notify getConditionList to skip that check,
+        # because the constructor won't be exposed in non-secure contexts to start with.
+        alreadySecureContext = self.descriptor.interface.getExtendedAttribute(
+            "SecureContext"
+        )
+
+        # We want to throw if any of the conditions returned by getConditionList are false.
+        conditionsCheck = ""
+        rawConditions = getRawConditionList(
+            self._ctor, "cx", "obj", alreadySecureContext
+        )
+        if len(rawConditions) > 0:
+            notConditions = " ||\n".join("!" + cond for cond in rawConditions)
+            failedCheckAction = CGGeneric("return ThrowingConstructor(cx, argc, vp);\n")
+            conditionsCheck = (
+                CGIfWrapper(failedCheckAction, notConditions).define() + "\n"
+            )
+
+        # Additionally, we want to throw if a caller does a bareword invocation
+        # of a constructor without |new|.
+        ctorName = GetConstructorNameForReporting(self.descriptor, self._ctor)
+
+        preamble = fill(
+            """
+            JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+            JS::Rooted<JSObject*> obj(cx, &args.callee());
+            $*{conditionsCheck}
+            if (!args.isConstructing()) {
+              return ThrowConstructorWithoutNew(cx, "${ctorName}");
+            }
+
+            JS::Rooted<JSObject*> desiredProto(cx);
+            if (!GetDesiredProto(cx, args,
+                                 prototypes::id::${name},
+                                 CreateInterfaceObjects,
+                                 &desiredProto)) {
+              return false;
+            }
+            """,
+            conditionsCheck=conditionsCheck,
+            ctorName=ctorName,
+            name=self.descriptor.name,
+        )
+
+        name = self._ctor.identifier.name
+        nativeName = MakeNativeName(self.descriptor.binaryNameFor(name, True))
+        callGenerator = CGMethodCall(
+            nativeName, True, self.descriptor, self._ctor, isConstructor=True
+        )
+        return preamble + "\n" + callGenerator.define()
+
+    def auto_profiler_label(self):
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${ctorName}", "constructor", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            ctorName=GetConstructorNameForReporting(self.descriptor, self._ctor),
+        )
+
+    def error_reporting_label(self):
+        return CGSpecializedMethod.error_reporting_label_helper(
+            self.descriptor, self._ctor, isConstructor=True
+        )
+
+
+def LegacyFactoryFunctionName(m):
+    return "_" + m.identifier.name
+
+
+class CGLegacyFactoryFunctions(CGThing):
+    def __init__(self, descriptor):
+        self.descriptor = descriptor
+        CGThing.__init__(self)
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        if len(self.descriptor.interface.legacyFactoryFunctions) == 0:
+            return ""
+
+        constructorID = "constructors::id::"
+        if self.descriptor.interface.hasInterfaceObject():
+            constructorID += self.descriptor.name
+        else:
+            constructorID += "_ID_Count"
+
+        namedConstructors = ""
+        for n in self.descriptor.interface.legacyFactoryFunctions:
+            namedConstructors += (
+                '{ "%s", { %s, &sLegacyFactoryFunctionNativePropertyHooks }, %i },\n'
+                % (n.identifier.name, LegacyFactoryFunctionName(n), methodLength(n))
+            )
+
+        return fill(
+            """
+            bool sLegacyFactoryFunctionNativePropertiesInited = true;
+            const NativePropertyHooks sLegacyFactoryFunctionNativePropertyHooks = {
+                nullptr,
+                nullptr,
+                nullptr,
+                { nullptr, nullptr, &sLegacyFactoryFunctionNativePropertiesInited },
+                prototypes::id::${name},
+                ${constructorID},
+                nullptr
+            };
+
+            static const LegacyFactoryFunction namedConstructors[] = {
+              $*{namedConstructors}
+              { nullptr, { nullptr, nullptr }, 0 }
+            };
+            """,
+            name=self.descriptor.name,
+            constructorID=constructorID,
+            namedConstructors=namedConstructors,
+        )
+
+
+def isChromeOnly(m):
+    return m.getExtendedAttribute("ChromeOnly")
+
+
+def prefIdentifier(pref):
+    return pref.replace(".", "_").replace("-", "_")
+
+
+def prefHeader(pref):
+    return "mozilla/StaticPrefs_%s.h" % pref.partition(".")[0]
+
+
+class MemberCondition:
+    """
+    An object representing the condition for a member to actually be
+    exposed.  Any of the arguments can be None.  If not
+    None, they should have the following types:
+
+    pref: The name of the preference.
+    func: The name of the function.
+    secureContext: A bool indicating whether a secure context is required.
+    nonExposedGlobals: A set of names of globals.  Can be empty, in which case
+                       it's treated the same way as None.
+    trial: The name of the origin trial.
+    """
+
+    def __init__(
+        self,
+        pref=None,
+        func=None,
+        secureContext=False,
+        nonExposedGlobals=None,
+        trial=None,
+    ):
+        assert pref is None or isinstance(pref, str)
+        assert func is None or isinstance(func, str)
+        assert trial is None or isinstance(trial, str)
+        assert isinstance(secureContext, bool)
+        assert nonExposedGlobals is None or isinstance(nonExposedGlobals, set)
+        self.pref = pref
+        if self.pref:
+            identifier = prefIdentifier(self.pref)
+            self.prefFuncIndex = "WebIDLPrefIndex::" + identifier
+        else:
+            self.prefFuncIndex = "WebIDLPrefIndex::NoPref"
+
+        self.secureContext = secureContext
+
+        def toFuncPtr(val):
+            if val is None:
+                return "nullptr"
+            return "&" + val
+
+        self.func = toFuncPtr(func)
+
+        if nonExposedGlobals:
+            # Nonempty set
+            self.nonExposedGlobals = " | ".join(
+                map(lambda g: "GlobalNames::%s" % g, sorted(nonExposedGlobals))
+            )
+        else:
+            self.nonExposedGlobals = "0"
+
+        if trial:
+            self.trial = "OriginTrial::" + trial
+        else:
+            self.trial = "OriginTrial(0)"
+
+    def __eq__(self, other):
+        return (
+            self.pref == other.pref
+            and self.func == other.func
+            and self.secureContext == other.secureContext
+            and self.nonExposedGlobals == other.nonExposedGlobals
+            and self.trial == other.trial
+        )
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
+    def hasDisablers(self):
+        return (
+            self.pref is not None
+            or self.secureContext
+            or self.func != "nullptr"
+            or self.nonExposedGlobals != "0"
+            or self.trial != "OriginTrial(0)"
+        )
+
+
+class PropertyDefiner:
+    """
+    A common superclass for defining things on prototype objects.
+
+    Subclasses should implement generateArray to generate the actual arrays of
+    things we're defining.  They should also set self.chrome to the list of
+    things only exposed to chrome and self.regular to the list of things exposed
+    to both chrome and web pages.
+    """
+
+    def __init__(self, descriptor, name):
+        self.descriptor = descriptor
+        self.name = name
+
+    def hasChromeOnly(self):
+        return len(self.chrome) > 0
+
+    def hasNonChromeOnly(self):
+        return len(self.regular) > 0
+
+    def variableName(self, chrome):
+        if chrome:
+            if self.hasChromeOnly():
+                return "sChrome" + self.name
+        else:
+            if self.hasNonChromeOnly():
+                return "s" + self.name
+        return "nullptr"
+
+    def usedForXrays(self):
+        return self.descriptor.wantsXrays
+
+    def length(self, chrome):
+        return len(self.chrome) if chrome else len(self.regular)
+
+    def __str__(self):
+        # We only need to generate id arrays for things that will end
+        # up used via ResolveProperty or EnumerateProperties.
+        str = self.generateArray(self.regular, self.variableName(False))
+        if self.hasChromeOnly():
+            str += self.generateArray(self.chrome, self.variableName(True))
+        return str
+
+    @staticmethod
+    def getStringAttr(member, name):
+        attr = member.getExtendedAttribute(name)
+        if attr is None:
+            return None
+        # It's a list of strings
+        assert len(attr) == 1
+        assert attr[0] is not None
+        return attr[0]
+
+    @staticmethod
+    def getControllingCondition(interfaceMember, descriptor):
+        interface = descriptor.interface
+        nonExposureSet = interface.exposureSet - interfaceMember.exposureSet
+
+        trial = PropertyDefiner.getStringAttr(interfaceMember, "Trial")
+        if trial and interface.identifier.name in ["Window", "Document"]:
+            raise TypeError(
+                "[Trial] not yet supported for %s.%s, see bug 1757935"
+                % (interface.identifier.name, interfaceMember.identifier.name)
+            )
+
+        return MemberCondition(
+            PropertyDefiner.getStringAttr(interfaceMember, "Pref"),
+            PropertyDefiner.getStringAttr(interfaceMember, "Func"),
+            interfaceMember.getExtendedAttribute("SecureContext") is not None,
+            nonExposureSet,
+            trial,
+        )
+
+    @staticmethod
+    def generatePrefableArrayValues(
+        array,
+        descriptor,
+        specFormatter,
+        specTerminator,
+        getCondition,
+        getDataTuple,
+        switchToCondition=None,
+    ):
+        """
+        This method generates an array of spec entries for interface members. It returns
+          a tuple containing the array of spec entries and the maximum of the number of
+          spec entries per condition.
+
+        array is an array of interface members.
+
+        descriptor is the descriptor for the interface that array contains members of.
+
+        specFormatter is a function that takes a single argument, a tuple,
+          and returns a string, a spec array entry.
+
+        specTerminator is a terminator for the spec array (inserted every time
+          our controlling pref changes and at the end of the array).
+
+        getCondition is a callback function that takes an array entry and
+          returns the corresponding MemberCondition.
+
+        getDataTuple is a callback function that takes an array entry and
+          returns a tuple suitable to be passed to specFormatter.
+
+        switchToCondition is a function that takes a MemberCondition and an array of
+          previously generated spec entries. If None is passed for this function then all
+          the interface members should return the same value from getCondition.
+        """
+
+        def unsupportedSwitchToCondition(condition, specs):
+            # If no specs have been added yet then this is just the first call to
+            # switchToCondition that we call to avoid putting a specTerminator at the
+            # front of the list.
+            if len(specs) == 0:
+                return
+            raise "Not supported"
+
+        if switchToCondition is None:
+            switchToCondition = unsupportedSwitchToCondition
+
+        specs = []
+        numSpecsInCurPrefable = 0
+        maxNumSpecsInPrefable = 0
+
+        # So we won't put a specTerminator at the very front of the list:
+        lastCondition = getCondition(array[0], descriptor)
+
+        switchToCondition(lastCondition, specs)
+
+        for member in array:
+            curCondition = getCondition(member, descriptor)
+            if lastCondition != curCondition:
+                # Terminate previous list
+                specs.append(specTerminator)
+                if numSpecsInCurPrefable > maxNumSpecsInPrefable:
+                    maxNumSpecsInPrefable = numSpecsInCurPrefable
+                numSpecsInCurPrefable = 0
+                # And switch to our new condition
+                switchToCondition(curCondition, specs)
+                lastCondition = curCondition
+            # And the actual spec
+            specs.append(specFormatter(getDataTuple(member, descriptor)))
+            numSpecsInCurPrefable += 1
+        if numSpecsInCurPrefable > maxNumSpecsInPrefable:
+            maxNumSpecsInPrefable = numSpecsInCurPrefable
+        specs.append(specTerminator)
+
+        return (specs, maxNumSpecsInPrefable)
+
+    def generatePrefableArray(
+        self,
+        array,
+        name,
+        specFormatter,
+        specTerminator,
+        specType,
+        getCondition,
+        getDataTuple,
+    ):
+        """
+        This method generates our various arrays.
+
+        array is an array of interface members as passed to generateArray
+
+        name is the name as passed to generateArray
+
+        specFormatter is a function that takes a single argument, a tuple,
+          and returns a string, a spec array entry
+
+        specTerminator is a terminator for the spec array (inserted every time
+          our controlling pref changes and at the end of the array)
+
+        specType is the actual typename of our spec
+
+        getCondition is a callback function that takes an array entry and
+          returns the corresponding MemberCondition.
+
+        getDataTuple is a callback function that takes an array entry and
+          returns a tuple suitable to be passed to specFormatter.
+        """
+
+        # We want to generate a single list of specs, but with specTerminator
+        # inserted at every point where the pref name controlling the member
+        # changes.  That will make sure the order of the properties as exposed
+        # on the interface and interface prototype objects does not change when
+        # pref control is added to members while still allowing us to define all
+        # the members in the smallest number of JSAPI calls.
+        assert len(array) != 0
+
+        disablers = []
+        prefableSpecs = []
+
+        disablersTemplate = dedent(
+            """
+            static const PrefableDisablers %s_disablers%d = {
+              %s, %s, %s, %s, %s
+            };
+            """
+        )
+        prefableWithDisablersTemplate = "  { &%s_disablers%d, &%s_specs[%d] }"
+        prefableWithoutDisablersTemplate = "  { nullptr, &%s_specs[%d] }"
+
+        def switchToCondition(condition, specs):
+            # Set up pointers to the new sets of specs inside prefableSpecs
+            if condition.hasDisablers():
+                prefableSpecs.append(
+                    prefableWithDisablersTemplate % (name, len(specs), name, len(specs))
+                )
+                disablers.append(
+                    disablersTemplate
+                    % (
+                        name,
+                        len(specs),
+                        condition.prefFuncIndex,
+                        condition.nonExposedGlobals,
+                        toStringBool(condition.secureContext),
+                        condition.trial,
+                        condition.func,
+                    )
+                )
+            else:
+                prefableSpecs.append(
+                    prefableWithoutDisablersTemplate % (name, len(specs))
+                )
+
+        specs, maxNumSpecsInPrefable = self.generatePrefableArrayValues(
+            array,
+            self.descriptor,
+            specFormatter,
+            specTerminator,
+            getCondition,
+            getDataTuple,
+            switchToCondition,
+        )
+        prefableSpecs.append("  { nullptr, nullptr }")
+
+        specType = "const " + specType
+        arrays = fill(
+            """
+            static ${specType} ${name}_specs[] = {
+            ${specs}
+            };
+
+            ${disablers}
+            static const Prefable<${specType}> ${name}[] = {
+            ${prefableSpecs}
+            };
+
+            """,
+            specType=specType,
+            name=name,
+            disablers="\n".join(disablers),
+            specs=",\n".join(specs),
+            prefableSpecs=",\n".join(prefableSpecs),
+        )
+
+        if self.usedForXrays():
+            arrays = fill(
+                """
+                $*{arrays}
+                static_assert(${numPrefableSpecs} <= 1ull << NUM_BITS_PROPERTY_INFO_PREF_INDEX,
+                    "We have a prefable index that is >= (1 << NUM_BITS_PROPERTY_INFO_PREF_INDEX)");
+                static_assert(${maxNumSpecsInPrefable} <= 1ull << NUM_BITS_PROPERTY_INFO_SPEC_INDEX,
+                    "We have a spec index that is >= (1 << NUM_BITS_PROPERTY_INFO_SPEC_INDEX)");
+
+                """,
+                arrays=arrays,
+                # Minus 1 because there's a list terminator in prefableSpecs.
+                numPrefableSpecs=len(prefableSpecs) - 1,
+                maxNumSpecsInPrefable=maxNumSpecsInPrefable,
+            )
+
+        return arrays
+
+
+# The length of a method is the minimum of the lengths of the
+# argument lists of all its overloads.
+def overloadLength(arguments):
+    i = len(arguments)
+    while i > 0 and arguments[i - 1].optional:
+        i -= 1
+    return i
+
+
+def methodLength(method):
+    signatures = method.signatures()
+    return min(overloadLength(arguments) for retType, arguments in signatures)
+
+
+def clearableCachedAttrs(descriptor):
+    return (
+        m
+        for m in descriptor.interface.members
+        if m.isAttr() and
+        # Constants should never need clearing!
+        m.dependsOn != "Nothing" and m.slotIndices is not None
+    )
+
+
+def MakeClearCachedValueNativeName(member):
+    return "ClearCached%sValue" % MakeNativeName(member.identifier.name)
+
+
+def IDLToCIdentifier(name):
+    return name.replace("-", "_")
+
+
+def EnumerabilityFlags(member):
+    if member.getExtendedAttribute("NonEnumerable"):
+        return "0"
+    return "JSPROP_ENUMERATE"
+
+
+class MethodDefiner(PropertyDefiner):
+    """
+    A class for defining methods on a prototype object.
+    """
+
+    def __init__(self, descriptor, name, crossOriginOnly, static, unforgeable=False):
+        assert not (static and unforgeable)
+        PropertyDefiner.__init__(self, descriptor, name)
+
+        # FIXME https://bugzilla.mozilla.org/show_bug.cgi?id=772822
+        #       We should be able to check for special operations without an
+        #       identifier. For now we check if the name starts with __
+
+        # Ignore non-static methods for interfaces without a proto object
+        if descriptor.interface.hasInterfacePrototypeObject() or static:
+            methods = [
+                m
+                for m in descriptor.interface.members
+                if m.isMethod()
+                and m.isStatic() == static
+                and MemberIsLegacyUnforgeable(m, descriptor) == unforgeable
+                and (
+                    not crossOriginOnly or m.getExtendedAttribute("CrossOriginCallable")
+                )
+                and not m.isIdentifierLess()
+                and not m.getExtendedAttribute("Unexposed")
+            ]
+        else:
+            methods = []
+        self.chrome = []
+        self.regular = []
+        for m in methods:
+            method = self.methodData(m, descriptor)
+
+            if m.isStatic():
+                method["nativeName"] = CppKeywords.checkMethodName(
+                    IDLToCIdentifier(m.identifier.name)
+                )
+
+            if isChromeOnly(m):
+                self.chrome.append(method)
+            else:
+                self.regular.append(method)
+
+        # TODO: Once iterable is implemented, use tiebreak rules instead of
+        # failing. Also, may be more tiebreak rules to implement once spec bug
+        # is resolved.
+        # https://www.w3.org/Bugs/Public/show_bug.cgi?id=28592
+        def hasIterator(methods, regular):
+            return any("@@iterator" in m.aliases for m in methods) or any(
+                "@@iterator" == r["name"] for r in regular
+            )
+
+        # Check whether we need to output an @@iterator due to having an indexed
+        # getter.  We only do this while outputting non-static and
+        # non-unforgeable methods, since the @@iterator function will be
+        # neither.
+        if not static and not unforgeable and descriptor.supportsIndexedProperties():
+            if hasIterator(methods, self.regular):
+                raise TypeError(
+                    "Cannot have indexed getter/attr on "
+                    "interface %s with other members "
+                    "that generate @@iterator, such as "
+                    "maplike/setlike or aliased functions."
+                    % self.descriptor.interface.identifier.name
+                )
+            self.regular.append(
+                {
+                    "name": "@@iterator",
+                    "methodInfo": False,
+                    "selfHostedName": "$ArrayValues",
+                    "length": 0,
+                    "flags": "0",  # Not enumerable, per spec.
+                    "condition": MemberCondition(),
+                }
+            )
+
+        # Generate the keys/values/entries aliases for value iterables.
+        maplikeOrSetlikeOrIterable = descriptor.interface.maplikeOrSetlikeOrIterable
+        if (
+            not static
+            and not unforgeable
+            and maplikeOrSetlikeOrIterable
+            and maplikeOrSetlikeOrIterable.isIterable()
+            and maplikeOrSetlikeOrIterable.isValueIterator()
+        ):
+            # Add our keys/values/entries/forEach
+            self.regular.append(
+                {
+                    "name": "keys",
+                    "methodInfo": False,
+                    "selfHostedName": "ArrayKeys",
+                    "length": 0,
+                    "flags": "JSPROP_ENUMERATE",
+                    "condition": PropertyDefiner.getControllingCondition(
+                        maplikeOrSetlikeOrIterable, descriptor
+                    ),
+                }
+            )
+            self.regular.append(
+                {
+                    "name": "values",
+                    "methodInfo": False,
+                    "selfHostedName": "$ArrayValues",
+                    "length": 0,
+                    "flags": "JSPROP_ENUMERATE",
+                    "condition": PropertyDefiner.getControllingCondition(
+                        maplikeOrSetlikeOrIterable, descriptor
+                    ),
+                }
+            )
+            self.regular.append(
+                {
+                    "name": "entries",
+                    "methodInfo": False,
+                    "selfHostedName": "ArrayEntries",
+                    "length": 0,
+                    "flags": "JSPROP_ENUMERATE",
+                    "condition": PropertyDefiner.getControllingCondition(
+                        maplikeOrSetlikeOrIterable, descriptor
+                    ),
+                }
+            )
+            self.regular.append(
+                {
+                    "name": "forEach",
+                    "methodInfo": False,
+                    "selfHostedName": "ArrayForEach",
+                    "length": 1,
+                    "flags": "JSPROP_ENUMERATE",
+                    "condition": PropertyDefiner.getControllingCondition(
+                        maplikeOrSetlikeOrIterable, descriptor
+                    ),
+                }
+            )
+
+        if not static:
+            stringifier = descriptor.operations["Stringifier"]
+            if stringifier and unforgeable == MemberIsLegacyUnforgeable(
+                stringifier, descriptor
+            ):
+                toStringDesc = {
+                    "name": GetWebExposedName(stringifier, descriptor),
+                    "nativeName": stringifier.identifier.name,
+                    "length": 0,
+                    "flags": "JSPROP_ENUMERATE",
+                    "condition": PropertyDefiner.getControllingCondition(
+                        stringifier, descriptor
+                    ),
+                }
+                if isChromeOnly(stringifier):
+                    self.chrome.append(toStringDesc)
+                else:
+                    self.regular.append(toStringDesc)
+            if unforgeable and descriptor.interface.getExtendedAttribute(
+                "LegacyUnforgeable"
+            ):
+                # Synthesize our valueOf method
+                self.regular.append(
+                    {
+                        "name": "valueOf",
+                        "selfHostedName": "Object_valueOf",
+                        "methodInfo": False,
+                        "length": 0,
+                        "flags": "0",  # readonly/permanent added automatically.
+                        "condition": MemberCondition(),
+                    }
+                )
+
+        if descriptor.interface.isJSImplemented():
+            if static:
+                if descriptor.interface.hasInterfaceObject():
+                    self.chrome.append(
+                        {
+                            "name": "_create",
+                            "nativeName": ("%s::_Create" % descriptor.name),
+                            "methodInfo": False,
+                            "length": 2,
+                            "flags": "0",
+                            "condition": MemberCondition(),
+                        }
+                    )
+
+        self.unforgeable = unforgeable
+
+        if static:
+            if not descriptor.interface.hasInterfaceObject():
+                # static methods go on the interface object
+                assert not self.hasChromeOnly() and not self.hasNonChromeOnly()
+        else:
+            if not descriptor.interface.hasInterfacePrototypeObject():
+                # non-static methods go on the interface prototype object
+                assert not self.hasChromeOnly() and not self.hasNonChromeOnly()
+
+    @staticmethod
+    def methodData(m, descriptor, overrideFlags=None):
+        return {
+            "name": m.identifier.name,
+            "methodInfo": not m.isStatic(),
+            "length": methodLength(m),
+            "flags": EnumerabilityFlags(m)
+            if (overrideFlags is None)
+            else overrideFlags,
+            "condition": PropertyDefiner.getControllingCondition(m, descriptor),
+            "allowCrossOriginThis": m.getExtendedAttribute("CrossOriginCallable"),
+            "returnsPromise": m.returnsPromise(),
+            "hasIteratorAlias": "@@iterator" in m.aliases,
+        }
+
+    @staticmethod
+    def formatSpec(fields):
+        if fields[0].startswith("@@"):
+            fields = (fields[0][2:],) + fields[1:]
+            return "  JS_SYM_FNSPEC(%s, %s, %s, %s, %s, %s)" % fields
+        return '  JS_FNSPEC("%s", %s, %s, %s, %s, %s)' % fields
+
+    @staticmethod
+    def specData(m, descriptor, unforgeable=False):
+        def flags(m, unforgeable):
+            unforgeable = " | JSPROP_PERMANENT | JSPROP_READONLY" if unforgeable else ""
+            return m["flags"] + unforgeable
+
+        if "selfHostedName" in m:
+            selfHostedName = '"%s"' % m["selfHostedName"]
+            assert not m.get("methodInfo", True)
+            accessor = "nullptr"
+            jitinfo = "nullptr"
+        else:
+            selfHostedName = "nullptr"
+            # When defining symbols, function name may not match symbol name
+            methodName = m.get("methodName", m["name"])
+            accessor = m.get("nativeName", IDLToCIdentifier(methodName))
+            if m.get("methodInfo", True):
+                if m.get("returnsPromise", False):
+                    exceptionPolicy = "ConvertExceptionsToPromises"
+                else:
+                    exceptionPolicy = "ThrowExceptions"
+
+                # Cast this in case the methodInfo is a
+                # JSTypedMethodJitInfo.
+                jitinfo = (
+                    "reinterpret_cast<const JSJitInfo*>(&%s_methodinfo)" % accessor
+                )
+                if m.get("allowCrossOriginThis", False):
+                    accessor = (
+                        "(GenericMethod<CrossOriginThisPolicy, %s>)" % exceptionPolicy
+                    )
+                elif descriptor.interface.hasDescendantWithCrossOriginMembers:
+                    accessor = (
+                        "(GenericMethod<MaybeCrossOriginObjectThisPolicy, %s>)"
+                        % exceptionPolicy
+                    )
+                elif descriptor.interface.isOnGlobalProtoChain():
+                    accessor = (
+                        "(GenericMethod<MaybeGlobalThisPolicy, %s>)" % exceptionPolicy
+                    )
+                else:
+                    accessor = "(GenericMethod<NormalThisPolicy, %s>)" % exceptionPolicy
+            else:
+                if m.get("returnsPromise", False):
+                    jitinfo = "&%s_methodinfo" % accessor
+                    accessor = "StaticMethodPromiseWrapper"
+                else:
+                    jitinfo = "nullptr"
+
+        return (
+            m["name"],
+            accessor,
+            jitinfo,
+            m["length"],
+            flags(m, unforgeable),
+            selfHostedName,
+        )
+
+    @staticmethod
+    def condition(m, d):
+        return m["condition"]
+
+    def generateArray(self, array, name):
+        if len(array) == 0:
+            return ""
+
+        return self.generatePrefableArray(
+            array,
+            name,
+            self.formatSpec,
+            "  JS_FS_END",
+            "JSFunctionSpec",
+            self.condition,
+            functools.partial(self.specData, unforgeable=self.unforgeable),
+        )
+
+
+class AttrDefiner(PropertyDefiner):
+    def __init__(self, descriptor, name, crossOriginOnly, static, unforgeable=False):
+        assert not (static and unforgeable)
+        PropertyDefiner.__init__(self, descriptor, name)
+        self.name = name
+        # Ignore non-static attributes for interfaces without a proto object
+        if descriptor.interface.hasInterfacePrototypeObject() or static:
+            idlAttrs = [
+                m
+                for m in descriptor.interface.members
+                if m.isAttr()
+                and m.isStatic() == static
+                and MemberIsLegacyUnforgeable(m, descriptor) == unforgeable
+                and (
+                    not crossOriginOnly
+                    or m.getExtendedAttribute("CrossOriginReadable")
+                    or m.getExtendedAttribute("CrossOriginWritable")
+                )
+            ]
+        else:
+            idlAttrs = []
+
+        attributes = []
+        for attr in idlAttrs:
+            attributes.extend(self.attrData(attr, unforgeable))
+        self.chrome = [m for m in attributes if isChromeOnly(m["attr"])]
+        self.regular = [m for m in attributes if not isChromeOnly(m["attr"])]
+        self.static = static
+
+        if static:
+            if not descriptor.interface.hasInterfaceObject():
+                # static attributes go on the interface object
+                assert not self.hasChromeOnly() and not self.hasNonChromeOnly()
+        else:
+            if not descriptor.interface.hasInterfacePrototypeObject():
+                # non-static attributes go on the interface prototype object
+                assert not self.hasChromeOnly() and not self.hasNonChromeOnly()
+
+    @staticmethod
+    def attrData(attr, unforgeable=False, overrideFlags=None):
+        if overrideFlags is None:
+            permanent = " | JSPROP_PERMANENT" if unforgeable else ""
+            flags = EnumerabilityFlags(attr) + permanent
+        else:
+            flags = overrideFlags
+        return (
+            {"name": name, "attr": attr, "flags": flags}
+            for name in [attr.identifier.name] + attr.bindingAliases
+        )
+
+    @staticmethod
+    def condition(m, d):
+        return PropertyDefiner.getControllingCondition(m["attr"], d)
+
+    @staticmethod
+    def specData(entry, descriptor, static=False, crossOriginOnly=False):
+        def getter(attr):
+            if crossOriginOnly and not attr.getExtendedAttribute("CrossOriginReadable"):
+                return "nullptr, nullptr"
+            if static:
+                if attr.type.isPromise():
+                    raise TypeError(
+                        "Don't know how to handle "
+                        "static Promise-returning "
+                        "attribute %s.%s" % (descriptor.name, attr.identifier.name)
+                    )
+                accessor = "get_" + IDLToCIdentifier(attr.identifier.name)
+                jitinfo = "nullptr"
+            else:
+                if attr.type.isPromise():
+                    exceptionPolicy = "ConvertExceptionsToPromises"
+                else:
+                    exceptionPolicy = "ThrowExceptions"
+
+                if attr.hasLegacyLenientThis():
+                    if attr.getExtendedAttribute("CrossOriginReadable"):
+                        raise TypeError(
+                            "Can't handle lenient cross-origin "
+                            "readable attribute %s.%s"
+                            % (descriptor.name, attr.identifier.name)
+                        )
+                    if descriptor.interface.hasDescendantWithCrossOriginMembers:
+                        accessor = (
+                            "GenericGetter<MaybeCrossOriginObjectLenientThisPolicy, %s>"
+                            % exceptionPolicy
+                        )
+                    else:
+                        accessor = (
+                            "GenericGetter<LenientThisPolicy, %s>" % exceptionPolicy
+                        )
+                elif attr.getExtendedAttribute("CrossOriginReadable"):
+                    accessor = (
+                        "GenericGetter<CrossOriginThisPolicy, %s>" % exceptionPolicy
+                    )
+                elif descriptor.interface.hasDescendantWithCrossOriginMembers:
+                    accessor = (
+                        "GenericGetter<MaybeCrossOriginObjectThisPolicy, %s>"
+                        % exceptionPolicy
+                    )
+                elif descriptor.interface.isOnGlobalProtoChain():
+                    accessor = (
+                        "GenericGetter<MaybeGlobalThisPolicy, %s>" % exceptionPolicy
+                    )
+                else:
+                    accessor = "GenericGetter<NormalThisPolicy, %s>" % exceptionPolicy
+                jitinfo = "&%s_getterinfo" % IDLToCIdentifier(attr.identifier.name)
+            return "%s, %s" % (accessor, jitinfo)
+
+        def setter(attr):
+            if (
+                attr.readonly
+                and attr.getExtendedAttribute("PutForwards") is None
+                and attr.getExtendedAttribute("Replaceable") is None
+                and attr.getExtendedAttribute("LegacyLenientSetter") is None
+            ):
+                return "nullptr, nullptr"
+            if crossOriginOnly and not attr.getExtendedAttribute("CrossOriginWritable"):
+                return "nullptr, nullptr"
+            if static:
+                accessor = "set_" + IDLToCIdentifier(attr.identifier.name)
+                jitinfo = "nullptr"
+            else:
+                if attr.hasLegacyLenientThis():
+                    if attr.getExtendedAttribute("CrossOriginWritable"):
+                        raise TypeError(
+                            "Can't handle lenient cross-origin "
+                            "writable attribute %s.%s"
+                            % (descriptor.name, attr.identifier.name)
+                        )
+                    if descriptor.interface.hasDescendantWithCrossOriginMembers:
+                        accessor = (
+                            "GenericSetter<MaybeCrossOriginObjectLenientThisPolicy>"
+                        )
+                    else:
+                        accessor = "GenericSetter<LenientThisPolicy>"
+                elif attr.getExtendedAttribute("CrossOriginWritable"):
+                    accessor = "GenericSetter<CrossOriginThisPolicy>"
+                elif descriptor.interface.hasDescendantWithCrossOriginMembers:
+                    accessor = "GenericSetter<MaybeCrossOriginObjectThisPolicy>"
+                elif descriptor.interface.isOnGlobalProtoChain():
+                    accessor = "GenericSetter<MaybeGlobalThisPolicy>"
+                else:
+                    accessor = "GenericSetter<NormalThisPolicy>"
+                jitinfo = "&%s_setterinfo" % IDLToCIdentifier(attr.identifier.name)
+            return "%s, %s" % (accessor, jitinfo)
+
+        name, attr, flags = entry["name"], entry["attr"], entry["flags"]
+        return (name, flags, getter(attr), setter(attr))
+
+    @staticmethod
+    def formatSpec(fields):
+        return '  JSPropertySpec::nativeAccessors("%s", %s, %s, %s)' % fields
+
+    def generateArray(self, array, name):
+        if len(array) == 0:
+            return ""
+
+        return self.generatePrefableArray(
+            array,
+            name,
+            self.formatSpec,
+            "  JS_PS_END",
+            "JSPropertySpec",
+            self.condition,
+            functools.partial(self.specData, static=self.static),
+        )
+
+
+class ConstDefiner(PropertyDefiner):
+    """
+    A class for definining constants on the interface object
+    """
+
+    def __init__(self, descriptor, name):
+        PropertyDefiner.__init__(self, descriptor, name)
+        self.name = name
+        constants = [m for m in descriptor.interface.members if m.isConst()]
+        self.chrome = [m for m in constants if isChromeOnly(m)]
+        self.regular = [m for m in constants if not isChromeOnly(m)]
+
+    def generateArray(self, array, name):
+        if len(array) == 0:
+            return ""
+
+        def specData(const, descriptor):
+            return (const.identifier.name, convertConstIDLValueToJSVal(const.value))
+
+        return self.generatePrefableArray(
+            array,
+            name,
+            lambda fields: '  { "%s", %s }' % fields,
+            "  { 0, JS::UndefinedValue() }",
+            "ConstantSpec",
+            PropertyDefiner.getControllingCondition,
+            specData,
+        )
+
+
+class PropertyArrays:
+    def __init__(self, descriptor, crossOriginOnly=False):
+        self.staticMethods = MethodDefiner(
+            descriptor, "StaticMethods", crossOriginOnly, static=True
+        )
+        self.staticAttrs = AttrDefiner(
+            descriptor, "StaticAttributes", crossOriginOnly, static=True
+        )
+        self.methods = MethodDefiner(
+            descriptor, "Methods", crossOriginOnly, static=False
+        )
+        self.attrs = AttrDefiner(
+            descriptor, "Attributes", crossOriginOnly, static=False
+        )
+        self.unforgeableMethods = MethodDefiner(
+            descriptor,
+            "UnforgeableMethods",
+            crossOriginOnly,
+            static=False,
+            unforgeable=True,
+        )
+        self.unforgeableAttrs = AttrDefiner(
+            descriptor,
+            "UnforgeableAttributes",
+            crossOriginOnly,
+            static=False,
+            unforgeable=True,
+        )
+        self.consts = ConstDefiner(descriptor, "Constants")
+
+    @staticmethod
+    def arrayNames():
+        return [
+            "staticMethods",
+            "staticAttrs",
+            "methods",
+            "attrs",
+            "unforgeableMethods",
+            "unforgeableAttrs",
+            "consts",
+        ]
+
+    def hasChromeOnly(self):
+        return any(getattr(self, a).hasChromeOnly() for a in self.arrayNames())
+
+    def hasNonChromeOnly(self):
+        return any(getattr(self, a).hasNonChromeOnly() for a in self.arrayNames())
+
+    def __str__(self):
+        define = ""
+        for array in self.arrayNames():
+            define += str(getattr(self, array))
+        return define
+
+
+class CGConstDefinition(CGThing):
+    """
+    Given a const member of an interface, return the C++ static const definition
+    for the member. Should be part of the interface namespace in the header
+    file.
+    """
+
+    def __init__(self, member):
+        assert (
+            member.isConst()
+            and member.value.type.isPrimitive()
+            and not member.value.type.nullable()
+        )
+
+        name = CppKeywords.checkMethodName(IDLToCIdentifier(member.identifier.name))
+        tag = member.value.type.tag()
+        value = member.value.value
+        if tag == IDLType.Tags.bool:
+            value = toStringBool(member.value.value)
+        self.const = "static const %s %s = %s;" % (builtinNames[tag], name, value)
+
+    def declare(self):
+        return self.const
+
+    def define(self):
+        return ""
+
+    def deps(self):
+        return []
+
+
+class CGNativeProperties(CGList):
+    def __init__(self, descriptor, properties):
+        def generateNativeProperties(name, chrome):
+            def check(p):
+                return p.hasChromeOnly() if chrome else p.hasNonChromeOnly()
+
+            nativePropsInts = []
+            nativePropsPtrs = []
+            nativePropsDuos = []
+
+            duosOffset = 0
+            idsOffset = 0
+            for array in properties.arrayNames():
+                propertyArray = getattr(properties, array)
+                if check(propertyArray):
+                    varName = propertyArray.variableName(chrome)
+                    bitfields = "true,  %d /* %s */" % (duosOffset, varName)
+                    duosOffset += 1
+                    nativePropsInts.append(CGGeneric(bitfields))
+
+                    if propertyArray.usedForXrays():
+                        ids = "&%s_propertyInfos[%d]" % (name, idsOffset)
+                        idsOffset += propertyArray.length(chrome)
+                    else:
+                        ids = "nullptr"
+                    duo = "{ %s, %s }" % (varName, ids)
+                    nativePropsDuos.append(CGGeneric(duo))
+                else:
+                    bitfields = "false, 0"
+                    nativePropsInts.append(CGGeneric(bitfields))
+
+            iteratorAliasIndex = -1
+            for index, item in enumerate(properties.methods.regular):
+                if item.get("hasIteratorAlias"):
+                    iteratorAliasIndex = index
+                    break
+            nativePropsInts.append(CGGeneric(str(iteratorAliasIndex)))
+
+            nativePropsDuos = [
+                CGWrapper(
+                    CGIndenter(CGList(nativePropsDuos, ",\n")), pre="{\n", post="\n}"
+                )
+            ]
+
+            pre = "static const NativePropertiesN<%d> %s = {\n" % (duosOffset, name)
+            post = "\n};\n"
+            if descriptor.wantsXrays:
+                pre = fill(
+                    """
+                    static uint16_t ${name}_sortedPropertyIndices[${size}];
+                    static PropertyInfo ${name}_propertyInfos[${size}];
+
+                    $*{pre}
+                    """,
+                    name=name,
+                    size=idsOffset,
+                    pre=pre,
+                )
+                if iteratorAliasIndex > 0:
+                    # The iteratorAliasMethodIndex is a signed integer, so the
+                    # max value it can store is 2^(nbits-1)-1.
+                    post = fill(
+                        """
+                        $*{post}
+                        static_assert(${iteratorAliasIndex} < 1ull << (CHAR_BIT * sizeof(${name}.iteratorAliasMethodIndex) - 1),
+                            "We have an iterator alias index that is oversized");
+                        """,
+                        post=post,
+                        iteratorAliasIndex=iteratorAliasIndex,
+                        name=name,
+                    )
+                post = fill(
+                    """
+                    $*{post}
+                    static_assert(${propertyInfoCount} < 1ull << (CHAR_BIT * sizeof(${name}.propertyInfoCount)),
+                        "We have a property info count that is oversized");
+                    """,
+                    post=post,
+                    propertyInfoCount=idsOffset,
+                    name=name,
+                )
+                nativePropsInts.append(CGGeneric("%d" % idsOffset))
+                nativePropsPtrs.append(CGGeneric("%s_sortedPropertyIndices" % name))
+            else:
+                nativePropsInts.append(CGGeneric("0"))
+                nativePropsPtrs.append(CGGeneric("nullptr"))
+            nativeProps = nativePropsInts + nativePropsPtrs + nativePropsDuos
+            return CGWrapper(CGIndenter(CGList(nativeProps, ",\n")), pre=pre, post=post)
+
+        nativeProperties = []
+        if properties.hasNonChromeOnly():
+            nativeProperties.append(
+                generateNativeProperties("sNativeProperties", False)
+            )
+        if properties.hasChromeOnly():
+            nativeProperties.append(
+                generateNativeProperties("sChromeOnlyNativeProperties", True)
+            )
+
+        CGList.__init__(self, nativeProperties, "\n")
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        return CGList.define(self)
+
+
+class CGCollectJSONAttributesMethod(CGAbstractMethod):
+    """
+    Generate the CollectJSONAttributes method for an interface descriptor
+    """
+
+    def __init__(self, descriptor, toJSONMethod):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("%s*" % descriptor.nativeType, "self"),
+            Argument("JS::Rooted<JSObject*>&", "result"),
+        ]
+        CGAbstractMethod.__init__(
+            self, descriptor, "CollectJSONAttributes", "bool", args, canRunScript=True
+        )
+        self.toJSONMethod = toJSONMethod
+
+    def definition_body(self):
+        ret = ""
+        interface = self.descriptor.interface
+        toJSONCondition = PropertyDefiner.getControllingCondition(
+            self.toJSONMethod, self.descriptor
+        )
+        needUnwrappedObj = False
+        for m in interface.members:
+            if m.isAttr() and not m.isStatic() and m.type.isJSONType():
+                getAndDefine = fill(
+                    """
+                    JS::Rooted<JS::Value> temp(cx);
+                    if (!get_${name}(cx, obj, self, JSJitGetterCallArgs(&temp))) {
+                      return false;
+                    }
+                    if (!JS_DefineProperty(cx, result, "${name}", temp, JSPROP_ENUMERATE)) {
+                      return false;
+                    }
+                    """,
+                    name=IDLToCIdentifier(m.identifier.name),
+                )
+                # Make sure we don't include things which are supposed to be
+                # disabled.  Things that either don't have disablers or whose
+                # disablers match the disablers for our toJSON method can't
+                # possibly be disabled, but other things might be.
+                condition = PropertyDefiner.getControllingCondition(m, self.descriptor)
+                if condition.hasDisablers() and condition != toJSONCondition:
+                    needUnwrappedObj = True
+                    ret += fill(
+                        """
+                        // This is unfortunately a linear scan through sAttributes, but we
+                        // only do it for things which _might_ be disabled, which should
+                        // help keep the performance problems down.
+                        if (IsGetterEnabled(cx, unwrappedObj, (JSJitGetterOp)get_${name}, sAttributes)) {
+                          $*{getAndDefine}
+                        }
+                        """,
+                        name=IDLToCIdentifier(m.identifier.name),
+                        getAndDefine=getAndDefine,
+                    )
+                else:
+                    ret += fill(
+                        """
+                        { // scope for "temp"
+                          $*{getAndDefine}
+                        }
+                        """,
+                        getAndDefine=getAndDefine,
+                    )
+        ret += "return true;\n"
+
+        if needUnwrappedObj:
+            # If we started allowing cross-origin objects here, we'd need to
+            # use CheckedUnwrapDynamic and figure out whether it makes sense.
+            # But in practice no one is trying to add toJSON methods to those,
+            # so let's just guard against it.
+            assert not self.descriptor.isMaybeCrossOriginObject()
+            ret = fill(
+                """
+                JS::Rooted<JSObject*> unwrappedObj(cx, js::CheckedUnwrapStatic(obj));
+                if (!unwrappedObj) {
+                  // How did that happen?  We managed to get called with that
+                  // object as "this"!  Just give up on sanity.
+                  return false;
+                }
+
+                $*{ret}
+                """,
+                ret=ret,
+            )
+
+        return ret
+
+
+class CGCreateInterfaceObjectsMethod(CGAbstractMethod):
+    """
+    Generate the CreateInterfaceObjects method for an interface descriptor.
+
+    properties should be a PropertyArrays instance.
+    """
+
+    def __init__(
+        self, descriptor, properties, haveUnscopables, haveLegacyWindowAliases, static
+    ):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("JS::Handle<JSObject*>", "aGlobal"),
+            Argument("ProtoAndIfaceCache&", "aProtoAndIfaceCache"),
+            Argument("bool", "aDefineOnGlobal"),
+        ]
+        CGAbstractMethod.__init__(
+            self, descriptor, "CreateInterfaceObjects", "void", args, static=static
+        )
+        self.properties = properties
+        self.haveUnscopables = haveUnscopables
+        self.haveLegacyWindowAliases = haveLegacyWindowAliases
+
+    def definition_body(self):
+        (protoGetter, protoHandleGetter) = InterfacePrototypeObjectProtoGetter(
+            self.descriptor
+        )
+        if protoHandleGetter is None:
+            parentProtoType = "Rooted"
+            getParentProto = "aCx, " + protoGetter
+        else:
+            parentProtoType = "Handle"
+            getParentProto = protoHandleGetter
+        getParentProto = getParentProto + "(aCx)"
+
+        (protoGetter, protoHandleGetter) = InterfaceObjectProtoGetter(self.descriptor)
+        if protoHandleGetter is None:
+            getConstructorProto = "aCx, " + protoGetter
+            constructorProtoType = "Rooted"
+        else:
+            getConstructorProto = protoHandleGetter
+            constructorProtoType = "Handle"
+        getConstructorProto += "(aCx)"
+
+        needInterfaceObject = self.descriptor.interface.hasInterfaceObject()
+        needInterfacePrototypeObject = (
+            self.descriptor.interface.hasInterfacePrototypeObject()
+        )
+
+        # if we don't need to create anything, why are we generating this?
+        assert needInterfaceObject or needInterfacePrototypeObject
+
+        getParentProto = fill(
+            """
+            JS::${type}<JSObject*> parentProto(${getParentProto});
+            if (!parentProto) {
+              return;
+            }
+            """,
+            type=parentProtoType,
+            getParentProto=getParentProto,
+        )
+
+        getConstructorProto = fill(
+            """
+            JS::${type}<JSObject*> constructorProto(${getConstructorProto});
+            if (!constructorProto) {
+              return;
+            }
+            """,
+            type=constructorProtoType,
+            getConstructorProto=getConstructorProto,
+        )
+
+        if self.descriptor.interface.ctor():
+            constructArgs = methodLength(self.descriptor.interface.ctor())
+            isConstructorChromeOnly = isChromeOnly(self.descriptor.interface.ctor())
+        else:
+            constructArgs = 0
+            isConstructorChromeOnly = False
+        if len(self.descriptor.interface.legacyFactoryFunctions) > 0:
+            namedConstructors = "namedConstructors"
+        else:
+            namedConstructors = "nullptr"
+
+        if needInterfacePrototypeObject:
+            protoClass = "&sPrototypeClass.mBase"
+            protoCache = (
+                "&aProtoAndIfaceCache.EntrySlotOrCreate(prototypes::id::%s)"
+                % self.descriptor.name
+            )
+            parentProto = "parentProto"
+            getParentProto = CGGeneric(getParentProto)
+        else:
+            protoClass = "nullptr"
+            protoCache = "nullptr"
+            parentProto = "nullptr"
+            getParentProto = None
+
+        if needInterfaceObject:
+            interfaceClass = "&sInterfaceObjectClass.mBase"
+            interfaceCache = (
+                "&aProtoAndIfaceCache.EntrySlotOrCreate(constructors::id::%s)"
+                % self.descriptor.name
+            )
+            getConstructorProto = CGGeneric(getConstructorProto)
+            constructorProto = "constructorProto"
+        else:
+            # We don't have slots to store the legacy factory functions.
+            assert len(self.descriptor.interface.legacyFactoryFunctions) == 0
+            interfaceClass = "nullptr"
+            interfaceCache = "nullptr"
+            getConstructorProto = None
+            constructorProto = "nullptr"
+
+        isGlobal = self.descriptor.isGlobal() is not None
+        if self.properties.hasNonChromeOnly():
+            properties = "sNativeProperties.Upcast()"
+        else:
+            properties = "nullptr"
+        if self.properties.hasChromeOnly():
+            chromeProperties = "sChromeOnlyNativeProperties.Upcast()"
+        else:
+            chromeProperties = "nullptr"
+
+        # We use getClassName here. This should be the right thing to pass as
+        # the name argument to CreateInterfaceObjects. This is generally the
+        # interface identifier, except for the synthetic interfaces created for
+        # the default iterator objects. If needInterfaceObject is true then
+        # we'll use the name to install a property on the global object, so
+        # there shouldn't be any spaces in the name.
+        name = self.descriptor.interface.getClassName()
+        assert not (needInterfaceObject and " " in name)
+
+        call = fill(
+            """
+            JS::Heap<JSObject*>* protoCache = ${protoCache};
+            JS::Heap<JSObject*>* interfaceCache = ${interfaceCache};
+            dom::CreateInterfaceObjects(aCx, aGlobal, ${parentProto},
+                                        ${protoClass}, protoCache,
+                                        ${constructorProto}, ${interfaceClass}, ${constructArgs}, ${isConstructorChromeOnly}, ${namedConstructors},
+                                        interfaceCache,
+                                        ${properties},
+                                        ${chromeProperties},
+                                        "${name}", aDefineOnGlobal,
+                                        ${unscopableNames},
+                                        ${isGlobal},
+                                        ${legacyWindowAliases},
+                                        ${isNamespace});
+            """,
+            protoClass=protoClass,
+            parentProto=parentProto,
+            protoCache=protoCache,
+            constructorProto=constructorProto,
+            interfaceClass=interfaceClass,
+            constructArgs=constructArgs,
+            isConstructorChromeOnly=toStringBool(isConstructorChromeOnly),
+            namedConstructors=namedConstructors,
+            interfaceCache=interfaceCache,
+            properties=properties,
+            chromeProperties=chromeProperties,
+            name=name,
+            unscopableNames="unscopableNames" if self.haveUnscopables else "nullptr",
+            isGlobal=toStringBool(isGlobal),
+            legacyWindowAliases="legacyWindowAliases"
+            if self.haveLegacyWindowAliases
+            else "nullptr",
+            isNamespace=toStringBool(self.descriptor.interface.isNamespace()),
+        )
+
+        # If we fail after here, we must clear interface and prototype caches
+        # using this code: intermediate failure must not expose the interface in
+        # partially-constructed state.  Note that every case after here needs an
+        # interface prototype object.
+        failureCode = dedent(
+            """
+            *protoCache = nullptr;
+            if (interfaceCache) {
+              *interfaceCache = nullptr;
+            }
+            return;
+            """
+        )
+
+        needProtoVar = False
+
+        aliasedMembers = [
+            m for m in self.descriptor.interface.members if m.isMethod() and m.aliases
+        ]
+        if aliasedMembers:
+            assert needInterfacePrototypeObject
+
+            def defineAlias(alias):
+                if alias == "@@iterator" or alias == "@@asyncIterator":
+                    name = alias[2:]
+
+                    symbolJSID = (
+                        "JS::GetWellKnownSymbolKey(aCx, JS::SymbolCode::%s)" % name
+                    )
+                    prop = "%sId" % name
+                    getSymbolJSID = CGGeneric(
+                        fill(
+                            "JS::Rooted<jsid> ${prop}(aCx, ${symbolJSID});",
+                            prop=prop,
+                            symbolJSID=symbolJSID,
+                        )
+                    )
+                    defineFn = "JS_DefinePropertyById"
+                    enumFlags = "0"  # Not enumerable, per spec.
+                elif alias.startswith("@@"):
+                    raise TypeError(
+                        "Can't handle any well-known Symbol other than @@iterator and @@asyncIterator"
+                    )
+                else:
+                    getSymbolJSID = None
+                    defineFn = "JS_DefineProperty"
+                    prop = '"%s"' % alias
+                    # XXX If we ever create non-enumerable properties that can
+                    #     be aliased, we should consider making the aliases
+                    #     match the enumerability of the property being aliased.
+                    enumFlags = "JSPROP_ENUMERATE"
+                return CGList(
+                    [
+                        getSymbolJSID,
+                        CGGeneric(
+                            fill(
+                                """
+                        if (!${defineFn}(aCx, proto, ${prop}, aliasedVal, ${enumFlags})) {
+                          $*{failureCode}
+                        }
+                        """,
+                                defineFn=defineFn,
+                                prop=prop,
+                                enumFlags=enumFlags,
+                                failureCode=failureCode,
+                            )
+                        ),
+                    ],
+                    "\n",
+                )
+
+            def defineAliasesFor(m):
+                return CGList(
+                    [
+                        CGGeneric(
+                            fill(
+                                """
+                        if (!JS_GetProperty(aCx, proto, \"${prop}\", &aliasedVal)) {
+                          $*{failureCode}
+                        }
+                        """,
+                                failureCode=failureCode,
+                                prop=m.identifier.name,
+                            )
+                        )
+                    ]
+                    + [defineAlias(alias) for alias in sorted(m.aliases)]
+                )
+
+            defineAliases = CGList(
+                [
+                    CGGeneric(
+                        dedent(
+                            """
+                    // Set up aliases on the interface prototype object we just created.
+                    """
+                        )
+                    ),
+                    CGGeneric("JS::Rooted<JS::Value> aliasedVal(aCx);\n\n"),
+                ]
+                + [
+                    defineAliasesFor(m)
+                    for m in sorted(aliasedMembers, key=lambda m: m.identifier.name)
+                ]
+            )
+            needProtoVar = True
+        else:
+            defineAliases = None
+
+        # Globals handle unforgeables directly in Wrap() instead of
+        # via a holder.
+        if (
+            self.descriptor.hasLegacyUnforgeableMembers
+            and not self.descriptor.isGlobal()
+        ):
+            assert needInterfacePrototypeObject
+
+            # We want to use the same JSClass and prototype as the object we'll
+            # end up defining the unforgeable properties on in the end, so that
+            # we can use JS_InitializePropertiesFromCompatibleNativeObject to do
+            # a fast copy.  In the case of proxies that's null, because the
+            # expando object is a vanilla object, but in the case of other DOM
+            # objects it's whatever our class is.
+            if self.descriptor.proxy:
+                holderClass = "nullptr"
+                holderProto = "nullptr"
+            else:
+                holderClass = "sClass.ToJSClass()"
+                holderProto = "proto"
+                needProtoVar = True
+            createUnforgeableHolder = CGGeneric(
+                fill(
+                    """
+                JS::Rooted<JSObject*> unforgeableHolder(
+                    aCx, JS_NewObjectWithoutMetadata(aCx, ${holderClass}, ${holderProto}));
+                if (!unforgeableHolder) {
+                  $*{failureCode}
+                }
+                """,
+                    holderProto=holderProto,
+                    holderClass=holderClass,
+                    failureCode=failureCode,
+                )
+            )
+            defineUnforgeables = InitUnforgeablePropertiesOnHolder(
+                self.descriptor, self.properties, failureCode
+            )
+            createUnforgeableHolder = CGList(
+                [createUnforgeableHolder, defineUnforgeables]
+            )
+
+            installUnforgeableHolder = CGGeneric(
+                dedent(
+                    """
+                if (*protoCache) {
+                  JS::SetReservedSlot(*protoCache, DOM_INTERFACE_PROTO_SLOTS_BASE,
+                                      JS::ObjectValue(*unforgeableHolder));
+                }
+                """
+                )
+            )
+
+            unforgeableHolderSetup = CGList(
+                [createUnforgeableHolder, installUnforgeableHolder], "\n"
+            )
+        else:
+            unforgeableHolderSetup = None
+
+        # FIXME Unclear whether this is needed for hasOrdinaryObjectPrototype
+        if (
+            self.descriptor.interface.isOnGlobalProtoChain()
+            and needInterfacePrototypeObject
+            and not self.descriptor.hasOrdinaryObjectPrototype
+        ):
+            makeProtoPrototypeImmutable = CGGeneric(
+                fill(
+                    """
+                {
+                  bool succeeded;
+                  if (!JS_SetImmutablePrototype(aCx, proto, &succeeded)) {
+                    $*{failureCode}
+                  }
+
+                  MOZ_ASSERT(succeeded,
+                             "making a fresh prototype object's [[Prototype]] "
+                             "immutable can internally fail, but it should "
+                             "never be unsuccessful");
+                }
+                """,
+                    protoCache=protoCache,
+                    failureCode=failureCode,
+                )
+            )
+            needProtoVar = True
+        else:
+            makeProtoPrototypeImmutable = None
+
+        if needProtoVar:
+            defineProtoVar = CGGeneric(
+                fill(
+                    """
+                JS::AssertObjectIsNotGray(*protoCache);
+                JS::Handle<JSObject*> proto = JS::Handle<JSObject*>::fromMarkedLocation(protoCache->address());
+                if (!proto) {
+                  $*{failureCode}
+                }
+                """,
+                    failureCode=failureCode,
+                )
+            )
+        else:
+            defineProtoVar = None
+        return CGList(
+            [
+                getParentProto,
+                getConstructorProto,
+                CGGeneric(call),
+                defineProtoVar,
+                defineAliases,
+                unforgeableHolderSetup,
+                makeProtoPrototypeImmutable,
+            ],
+            "\n",
+        ).define()
+
+
+class CGGetProtoObjectHandleMethod(CGAbstractMethod):
+    """
+    A method for getting the interface prototype object.
+    """
+
+    def __init__(self, descriptor, static, signatureOnly=False):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "GetProtoObjectHandle",
+            "JS::Handle<JSObject*>",
+            [Argument("JSContext*", "aCx")],
+            inline=True,
+            static=static,
+            signatureOnly=signatureOnly,
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            /* Get the interface prototype object for this class.  This will create the
+               object as needed. */
+            return GetPerInterfaceObjectHandle(aCx, prototypes::id::${name},
+                                               &CreateInterfaceObjects,
+                                               /* aDefineOnGlobal = */ true);
+
+            """,
+            name=self.descriptor.name,
+        )
+
+
+class CGGetProtoObjectMethod(CGAbstractMethod):
+    """
+    A method for getting the interface prototype object.
+    """
+
+    def __init__(self, descriptor):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "GetProtoObject",
+            "JSObject*",
+            [Argument("JSContext*", "aCx")],
+        )
+
+    def definition_body(self):
+        return "return GetProtoObjectHandle(aCx);\n"
+
+
+class CGGetConstructorObjectHandleMethod(CGAbstractMethod):
+    """
+    A method for getting the interface constructor object.
+    """
+
+    def __init__(self, descriptor):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "GetConstructorObjectHandle",
+            "JS::Handle<JSObject*>",
+            [
+                Argument("JSContext*", "aCx"),
+                Argument("bool", "aDefineOnGlobal", "true"),
+            ],
+            inline=True,
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            /* Get the interface object for this class.  This will create the object as
+               needed. */
+
+            return GetPerInterfaceObjectHandle(aCx, constructors::id::${name},
+                                               &CreateInterfaceObjects,
+                                               aDefineOnGlobal);
+            """,
+            name=self.descriptor.name,
+        )
+
+
+class CGGetConstructorObjectMethod(CGAbstractMethod):
+    """
+    A method for getting the interface constructor object.
+    """
+
+    def __init__(self, descriptor):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "GetConstructorObject",
+            "JSObject*",
+            [Argument("JSContext*", "aCx")],
+        )
+
+    def definition_body(self):
+        return "return GetConstructorObjectHandle(aCx);\n"
+
+
+class CGGetNamedPropertiesObjectMethod(CGAbstractStaticMethod):
+    def __init__(self, descriptor):
+        args = [Argument("JSContext*", "aCx")]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, "GetNamedPropertiesObject", "JSObject*", args
+        )
+
+    def definition_body(self):
+        parentProtoName = self.descriptor.parentPrototypeName
+        if parentProtoName is None:
+            getParentProto = ""
+            parentProto = "nullptr"
+        else:
+            getParentProto = fill(
+                """
+                JS::Rooted<JSObject*> parentProto(aCx, ${parent}::GetProtoObjectHandle(aCx));
+                if (!parentProto) {
+                  return nullptr;
+                }
+                """,
+                parent=toBindingNamespace(parentProtoName),
+            )
+            parentProto = "parentProto"
+        return fill(
+            """
+            /* Make sure our global is sane.  Hopefully we can remove this sometime */
+            JSObject* global = JS::CurrentGlobalOrNull(aCx);
+            if (!(JS::GetClass(global)->flags & JSCLASS_DOM_GLOBAL)) {
+              return nullptr;
+            }
+
+            /* Check to see whether the named properties object has already been created */
+            ProtoAndIfaceCache& protoAndIfaceCache = *GetProtoAndIfaceCache(global);
+
+            JS::Heap<JSObject*>& namedPropertiesObject = protoAndIfaceCache.EntrySlotOrCreate(namedpropertiesobjects::id::${ifaceName});
+            if (!namedPropertiesObject) {
+              $*{getParentProto}
+              namedPropertiesObject = ${nativeType}::CreateNamedPropertiesObject(aCx, ${parentProto});
+              DebugOnly<const DOMIfaceAndProtoJSClass*> clasp =
+                DOMIfaceAndProtoJSClass::FromJSClass(JS::GetClass(namedPropertiesObject));
+              MOZ_ASSERT(clasp->mType == eNamedPropertiesObject,
+                         "Expected ${nativeType}::CreateNamedPropertiesObject to return a named properties object");
+              MOZ_ASSERT(clasp->mNativeHooks,
+                         "The named properties object for ${nativeType} should have NativePropertyHooks.");
+              MOZ_ASSERT(!clasp->mNativeHooks->mResolveOwnProperty,
+                         "Shouldn't resolve the properties of the named properties object for ${nativeType} for Xrays.");
+              MOZ_ASSERT(!clasp->mNativeHooks->mEnumerateOwnProperties,
+                         "Shouldn't enumerate the properties of the named properties object for ${nativeType} for Xrays.");
+            }
+            return namedPropertiesObject.get();
+            """,
+            getParentProto=getParentProto,
+            ifaceName=self.descriptor.name,
+            parentProto=parentProto,
+            nativeType=self.descriptor.nativeType,
+        )
+
+
+def getRawConditionList(idlobj, cxName, objName, ignoreSecureContext=False):
+    """
+    Get the list of conditions for idlobj (to be used in "is this enabled"
+    checks).  This will be returned as a CGList with " &&\n" as the separator,
+    for readability.
+
+    objName is the name of the object that we're working with, because some of
+    our test functions want that.
+
+    ignoreSecureContext is used only for constructors in which the WebIDL interface
+    itself is already marked as [SecureContext]. There is no need to do the work twice.
+    """
+    conditions = []
+    pref = idlobj.getExtendedAttribute("Pref")
+    if pref:
+        assert isinstance(pref, list) and len(pref) == 1
+        conditions.append("StaticPrefs::%s()" % prefIdentifier(pref[0]))
+    if isChromeOnly(idlobj):
+        conditions.append("nsContentUtils::ThreadsafeIsSystemCaller(%s)" % cxName)
+    func = idlobj.getExtendedAttribute("Func")
+    if func:
+        assert isinstance(func, list) and len(func) == 1
+        conditions.append("%s(%s, %s)" % (func[0], cxName, objName))
+    trial = idlobj.getExtendedAttribute("Trial")
+    if trial:
+        assert isinstance(trial, list) and len(trial) == 1
+        conditions.append(
+            "OriginTrials::IsEnabled(%s, %s, OriginTrial::%s)"
+            % (cxName, objName, trial[0])
+        )
+    if not ignoreSecureContext and idlobj.getExtendedAttribute("SecureContext"):
+        conditions.append(
+            "mozilla::dom::IsSecureContextOrObjectIsFromSecureContext(%s, %s)"
+            % (cxName, objName)
+        )
+    return conditions
+
+
+def getConditionList(idlobj, cxName, objName, ignoreSecureContext=False):
+    """
+    Get the list of conditions from getRawConditionList
+    See comment on getRawConditionList above for more info about arguments.
+
+    The return value is a possibly-empty conjunctive CGList of conditions.
+    """
+    conditions = getRawConditionList(idlobj, cxName, objName, ignoreSecureContext)
+    return CGList((CGGeneric(cond) for cond in conditions), " &&\n")
+
+
+class CGConstructorEnabled(CGAbstractMethod):
+    """
+    A method for testing whether we should be exposing this interface object.
+    This can perform various tests depending on what conditions are specified
+    on the interface.
+    """
+
+    def __init__(self, descriptor):
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "ConstructorEnabled",
+            "bool",
+            [Argument("JSContext*", "aCx"), Argument("JS::Handle<JSObject*>", "aObj")],
+        )
+
+    def definition_body(self):
+        body = CGList([], "\n")
+
+        iface = self.descriptor.interface
+
+        if not iface.isExposedInWindow():
+            exposedInWindowCheck = dedent(
+                """
+                MOZ_ASSERT(!NS_IsMainThread(), "Why did we even get called?");
+                """
+            )
+            body.append(CGGeneric(exposedInWindowCheck))
+
+        if iface.isExposedInSomeButNotAllWorkers():
+            workerGlobals = sorted(iface.getWorkerExposureSet())
+            workerCondition = CGList(
+                (
+                    CGGeneric('strcmp(name, "%s")' % workerGlobal)
+                    for workerGlobal in workerGlobals
+                ),
+                " && ",
+            )
+            exposedInWorkerCheck = fill(
+                """
+                const char* name = JS::GetClass(aObj)->name;
+                if (${workerCondition}) {
+                  return false;
+                }
+                """,
+                workerCondition=workerCondition.define(),
+            )
+            exposedInWorkerCheck = CGGeneric(exposedInWorkerCheck)
+            if iface.isExposedInWindow():
+                exposedInWorkerCheck = CGIfWrapper(
+                    exposedInWorkerCheck, "!NS_IsMainThread()"
+                )
+            body.append(exposedInWorkerCheck)
+
+        conditions = getConditionList(iface, "aCx", "aObj")
+
+        # We should really have some conditions
+        assert len(body) or len(conditions)
+
+        conditionsWrapper = ""
+        if len(conditions):
+            conditionsWrapper = CGWrapper(
+                conditions, pre="return ", post=";\n", reindent=True
+            )
+        else:
+            conditionsWrapper = CGGeneric("return true;\n")
+
+        body.append(conditionsWrapper)
+        return body.define()
+
+
+def StructuredCloneTag(name):
+    return "SCTAG_DOM_%s" % name.upper()
+
+
+class CGSerializer(CGAbstractStaticMethod):
+    """
+    Implementation of serialization for things marked [Serializable].
+    This gets stored in our DOMJSClass, so it can be static.
+
+    The caller is expected to pass in the object whose DOMJSClass it
+    used to get the serializer.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("JSStructuredCloneWriter*", "aWriter"),
+            Argument("JS::Handle<JSObject*>", "aObj"),
+        ]
+        CGAbstractStaticMethod.__init__(self, descriptor, "Serialize", "bool", args)
+
+    def definition_body(self):
+        return fill(
+            """
+            MOZ_ASSERT(IsDOMObject(aObj), "Non-DOM object passed");
+            MOZ_ASSERT(GetDOMClass(aObj)->mSerializer == &Serialize,
+                       "Wrong object passed");
+            return JS_WriteUint32Pair(aWriter, ${tag}, 0) &&
+                   UnwrapDOMObject<${type}>(aObj)->WriteStructuredClone(aCx, aWriter);
+            """,
+            tag=StructuredCloneTag(self.descriptor.name),
+            type=self.descriptor.nativeType,
+        )
+
+
+class CGDeserializer(CGAbstractMethod):
+    """
+    Implementation of deserialization for things marked [Serializable].
+    This will need to be accessed from WebIDLSerializable, so can't be static.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("nsIGlobalObject*", "aGlobal"),
+            Argument("JSStructuredCloneReader*", "aReader"),
+        ]
+        CGAbstractMethod.__init__(self, descriptor, "Deserialize", "JSObject*", args)
+
+    def definition_body(self):
+        # WrapObject has different signatures depending on whether
+        # the object is wrappercached.
+        if self.descriptor.wrapperCache:
+            wrapCall = dedent(
+                """
+                result = obj->WrapObject(aCx, nullptr);
+                if (!result) {
+                  return nullptr;
+                }
+                """
+            )
+        else:
+            wrapCall = dedent(
+                """
+                if (!obj->WrapObject(aCx, nullptr, &result)) {
+                  return nullptr;
+                }
+                """
+            )
+
+        return fill(
+            """
+            // Protect the result from a moving GC in ~RefPtr
+            JS::Rooted<JSObject*> result(aCx);
+            {  // Scope for the RefPtr
+              RefPtr<${type}> obj = ${type}::ReadStructuredClone(aCx, aGlobal, aReader);
+              if (!obj) {
+                return nullptr;
+              }
+              $*{wrapCall}
+            }
+            return result;
+            """,
+            type=self.descriptor.nativeType,
+            wrapCall=wrapCall,
+        )
+
+
+def CreateBindingJSObject(descriptor):
+    objDecl = "BindingJSObjectCreator<%s> creator(aCx);\n" % descriptor.nativeType
+
+    # We don't always need to root obj, but there are a variety
+    # of cases where we do, so for simplicity, just always root it.
+    if descriptor.proxy:
+        if descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+            assert not descriptor.isMaybeCrossOriginObject()
+            create = dedent(
+                """
+                aObject->mExpandoAndGeneration.expando.setUndefined();
+                JS::Rooted<JS::Value> expandoValue(aCx, JS::PrivateValue(&aObject->mExpandoAndGeneration));
+                creator.CreateProxyObject(aCx, &sClass.mBase, DOMProxyHandler::getInstance(),
+                                          proto, /* aLazyProto = */ false, aObject,
+                                          expandoValue, aReflector);
+                """
+            )
+        else:
+            if descriptor.isMaybeCrossOriginObject():
+                proto = "nullptr"
+                lazyProto = "true"
+            else:
+                proto = "proto"
+                lazyProto = "false"
+            create = fill(
+                """
+                creator.CreateProxyObject(aCx, &sClass.mBase, DOMProxyHandler::getInstance(),
+                                          ${proto}, /* aLazyProto = */ ${lazyProto},
+                                          aObject, JS::UndefinedHandleValue, aReflector);
+                """,
+                proto=proto,
+                lazyProto=lazyProto,
+            )
+    else:
+        create = dedent(
+            """
+            creator.CreateObject(aCx, sClass.ToJSClass(), proto, aObject, aReflector);
+            """
+        )
+    return (
+        objDecl
+        + create
+        + dedent(
+            """
+        if (!aReflector) {
+          return false;
+        }
+        """
+        )
+    )
+
+
+def InitUnforgeablePropertiesOnHolder(
+    descriptor, properties, failureCode, holderName="unforgeableHolder"
+):
+    """
+    Define the unforgeable properties on the unforgeable holder for
+    the interface represented by descriptor.
+
+    properties is a PropertyArrays instance.
+
+    """
+    assert (
+        properties.unforgeableAttrs.hasNonChromeOnly()
+        or properties.unforgeableAttrs.hasChromeOnly()
+        or properties.unforgeableMethods.hasNonChromeOnly()
+        or properties.unforgeableMethods.hasChromeOnly()
+    )
+
+    unforgeables = []
+
+    defineUnforgeableAttrs = fill(
+        """
+        if (!DefineLegacyUnforgeableAttributes(aCx, ${holderName}, %s)) {
+          $*{failureCode}
+        }
+        """,
+        failureCode=failureCode,
+        holderName=holderName,
+    )
+    defineUnforgeableMethods = fill(
+        """
+        if (!DefineLegacyUnforgeableMethods(aCx, ${holderName}, %s)) {
+          $*{failureCode}
+        }
+        """,
+        failureCode=failureCode,
+        holderName=holderName,
+    )
+
+    unforgeableMembers = [
+        (defineUnforgeableAttrs, properties.unforgeableAttrs),
+        (defineUnforgeableMethods, properties.unforgeableMethods),
+    ]
+    for (template, array) in unforgeableMembers:
+        if array.hasNonChromeOnly():
+            unforgeables.append(CGGeneric(template % array.variableName(False)))
+        if array.hasChromeOnly():
+            unforgeables.append(
+                CGIfWrapper(
+                    CGGeneric(template % array.variableName(True)),
+                    "nsContentUtils::ThreadsafeIsSystemCaller(aCx)",
+                )
+            )
+
+    if descriptor.interface.getExtendedAttribute("LegacyUnforgeable"):
+        # We do our undefined toPrimitive here, not as a regular property
+        # because we don't have a concept of value props anywhere in IDL.
+        unforgeables.append(
+            CGGeneric(
+                fill(
+                    """
+            JS::Rooted<JS::PropertyKey> toPrimitive(aCx,
+              JS::GetWellKnownSymbolKey(aCx, JS::SymbolCode::toPrimitive));
+            if (!JS_DefinePropertyById(aCx, ${holderName}, toPrimitive,
+                                       JS::UndefinedHandleValue,
+                                       JSPROP_READONLY | JSPROP_PERMANENT)) {
+              $*{failureCode}
+            }
+            """,
+                    failureCode=failureCode,
+                    holderName=holderName,
+                )
+            )
+        )
+
+    return CGWrapper(CGList(unforgeables), pre="\n")
+
+
+def CopyUnforgeablePropertiesToInstance(descriptor, failureCode):
+    """
+    Copy the unforgeable properties from the unforgeable holder for
+    this interface to the instance object we have.
+    """
+    assert not descriptor.isGlobal()
+
+    if not descriptor.hasLegacyUnforgeableMembers:
+        return ""
+
+    copyCode = [
+        CGGeneric(
+            dedent(
+                """
+            // Important: do unforgeable property setup after we have handed
+            // over ownership of the C++ object to obj as needed, so that if
+            // we fail and it ends up GCed it won't have problems in the
+            // finalizer trying to drop its ownership of the C++ object.
+            """
+            )
+        )
+    ]
+
+    # For proxies, we want to define on the expando object, not directly on the
+    # reflector, so we can make sure we don't get confused by named getters.
+    if descriptor.proxy:
+        copyCode.append(
+            CGGeneric(
+                fill(
+                    """
+            JS::Rooted<JSObject*> expando(aCx,
+              DOMProxyHandler::EnsureExpandoObject(aCx, aReflector));
+            if (!expando) {
+              $*{failureCode}
+            }
+            """,
+                    failureCode=failureCode,
+                )
+            )
+        )
+        obj = "expando"
+    else:
+        obj = "aReflector"
+
+    copyCode.append(
+        CGGeneric(
+            fill(
+                """
+        JS::Rooted<JSObject*> unforgeableHolder(aCx,
+          &JS::GetReservedSlot(canonicalProto, DOM_INTERFACE_PROTO_SLOTS_BASE).toObject());
+        if (!JS_InitializePropertiesFromCompatibleNativeObject(aCx, ${obj}, unforgeableHolder)) {
+          $*{failureCode}
+        }
+        """,
+                obj=obj,
+                failureCode=failureCode,
+            )
+        )
+    )
+
+    return CGWrapper(CGList(copyCode), pre="\n").define()
+
+
+def AssertInheritanceChain(descriptor):
+    # We can skip the reinterpret_cast check for the descriptor's nativeType
+    # if aObject is a pointer of that type.
+    asserts = fill(
+        """
+        static_assert(std::is_same_v<decltype(aObject), ${nativeType}*>);
+        """,
+        nativeType=descriptor.nativeType,
+    )
+    iface = descriptor.interface
+    while iface.parent:
+        iface = iface.parent
+        desc = descriptor.getDescriptor(iface.identifier.name)
+        asserts += (
+            "MOZ_ASSERT(static_cast<%s*>(aObject) == \n"
+            "           reinterpret_cast<%s*>(aObject),\n"
+            '           "Multiple inheritance for %s is broken.");\n'
+            % (desc.nativeType, desc.nativeType, desc.nativeType)
+        )
+    asserts += "MOZ_ASSERT(ToSupportsIsCorrect(aObject));\n"
+    return asserts
+
+
+def InitMemberSlots(descriptor, failureCode):
+    """
+    Initialize member slots on our JS object if we're supposed to have some.
+
+    Note that this is called after the SetWrapper() call in the
+    wrapperCache case, since that can affect how our getters behave
+    and we plan to invoke them here.  So if we fail, we need to
+    ClearWrapper.
+    """
+    if not descriptor.interface.hasMembersInSlots():
+        return ""
+    return fill(
+        """
+        if (!UpdateMemberSlots(aCx, aReflector, aObject)) {
+          $*{failureCode}
+        }
+        """,
+        failureCode=failureCode,
+    )
+
+
+def DeclareProto(descriptor, noGivenProto=False):
+    """
+    Declare the canonicalProto and proto we have for our wrapping operation.
+    """
+    getCanonical = dedent(
+        """
+        JS::Handle<JSObject*> ${canonicalProto} = GetProtoObjectHandle(aCx);
+        if (!${canonicalProto}) {
+          return false;
+        }
+        """
+    )
+
+    if noGivenProto:
+        return fill(getCanonical, canonicalProto="proto")
+
+    getCanonical = fill(getCanonical, canonicalProto="canonicalProto")
+
+    preamble = getCanonical + dedent(
+        """
+        JS::Rooted<JSObject*> proto(aCx);
+        """
+    )
+    if descriptor.isMaybeCrossOriginObject():
+        return preamble + dedent(
+            """
+            MOZ_ASSERT(!aGivenProto,
+                       "Shouldn't have constructors on cross-origin objects");
+            // Set proto to canonicalProto to avoid preserving our wrapper if
+            // we don't have to.
+            proto = canonicalProto;
+            """
+        )
+
+    return preamble + dedent(
+        """
+        if (aGivenProto) {
+          proto = aGivenProto;
+          // Unfortunately, while aGivenProto was in the compartment of aCx
+          // coming in, we changed compartments to that of "parent" so may need
+          // to wrap the proto here.
+          if (js::GetContextCompartment(aCx) != JS::GetCompartment(proto)) {
+            if (!JS_WrapObject(aCx, &proto)) {
+              return false;
+            }
+          }
+        } else {
+          proto = canonicalProto;
+        }
+        """
+    )
+
+
+class CGWrapWithCacheMethod(CGAbstractMethod):
+    """
+    Create a wrapper JSObject for a given native that implements nsWrapperCache.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.interface.hasInterfacePrototypeObject()
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument(descriptor.nativeType + "*", "aObject"),
+            Argument("nsWrapperCache*", "aCache"),
+            Argument("JS::Handle<JSObject*>", "aGivenProto"),
+            Argument("JS::MutableHandle<JSObject*>", "aReflector"),
+        ]
+        CGAbstractMethod.__init__(self, descriptor, "Wrap", "bool", args)
+
+    def definition_body(self):
+        failureCode = dedent(
+            """
+            aCache->ReleaseWrapper(aObject);
+            aCache->ClearWrapper();
+            return false;
+            """
+        )
+
+        if self.descriptor.proxy:
+            finalize = "DOMProxyHandler::getInstance()->finalize"
+        else:
+            finalize = FINALIZE_HOOK_NAME
+
+        return fill(
+            """
+            static_assert(!std::is_base_of_v<NonRefcountedDOMObject, ${nativeType}>,
+                          "Shouldn't have wrappercached things that are not refcounted.");
+            $*{assertInheritance}
+            MOZ_ASSERT_IF(aGivenProto, js::IsObjectInContextCompartment(aGivenProto, aCx));
+            MOZ_ASSERT(!aCache->GetWrapper(),
+                       "You should probably not be using Wrap() directly; use "
+                       "GetOrCreateDOMReflector instead");
+
+            MOZ_ASSERT(ToSupportsIsOnPrimaryInheritanceChain(aObject, aCache),
+                       "nsISupports must be on our primary inheritance chain");
+
+            // If the wrapper cache contains a dead reflector then finalize that
+            // now, ensuring that the finalizer for the old reflector always
+            // runs before the new reflector is created and attached. This
+            // avoids the awkward situation where there are multiple reflector
+            // objects that contain pointers to the same native.
+
+            if (JSObject* oldReflector = aCache->GetWrapperMaybeDead()) {
+              ${finalize}(nullptr /* unused */, oldReflector);
+              MOZ_ASSERT(!aCache->GetWrapperMaybeDead());
+            }
+
+            JS::Rooted<JSObject*> global(aCx, FindAssociatedGlobal(aCx, aObject->GetParentObject()));
+            if (!global) {
+              return false;
+            }
+            MOZ_ASSERT(JS_IsGlobalObject(global));
+            JS::AssertObjectIsNotGray(global);
+
+            // That might have ended up wrapping us already, due to the wonders
+            // of XBL.  Check for that, and bail out as needed.
+            aReflector.set(aCache->GetWrapper());
+            if (aReflector) {
+            #ifdef DEBUG
+              AssertReflectorHasGivenProto(aCx, aReflector, aGivenProto);
+            #endif // DEBUG
+              return true;
+            }
+
+            JSAutoRealm ar(aCx, global);
+            $*{declareProto}
+
+            $*{createObject}
+
+            aCache->SetWrapper(aReflector);
+            $*{unforgeable}
+            $*{slots}
+            creator.InitializationSucceeded();
+
+            MOZ_ASSERT(aCache->GetWrapperPreserveColor() &&
+                       aCache->GetWrapperPreserveColor() == aReflector);
+            // If proto != canonicalProto, we have to preserve our wrapper;
+            // otherwise we won't be able to properly recreate it later, since
+            // we won't know what proto to use.  Note that we don't check
+            // aGivenProto here, since it's entirely possible (and even
+            // somewhat common) to have a non-null aGivenProto which is the
+            // same as canonicalProto.
+            if (proto != canonicalProto) {
+              PreserveWrapper(aObject);
+            }
+
+            return true;
+            """,
+            nativeType=self.descriptor.nativeType,
+            assertInheritance=AssertInheritanceChain(self.descriptor),
+            declareProto=DeclareProto(self.descriptor),
+            createObject=CreateBindingJSObject(self.descriptor),
+            unforgeable=CopyUnforgeablePropertiesToInstance(
+                self.descriptor, failureCode
+            ),
+            slots=InitMemberSlots(self.descriptor, failureCode),
+            finalize=finalize,
+        )
+
+
+class CGWrapMethod(CGAbstractMethod):
+    def __init__(self, descriptor):
+        # XXX can we wrap if we don't have an interface prototype object?
+        assert descriptor.interface.hasInterfacePrototypeObject()
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("T*", "aObject"),
+            Argument("JS::Handle<JSObject*>", "aGivenProto"),
+        ]
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "Wrap",
+            "JSObject*",
+            args,
+            inline=True,
+            templateArgs=["class T"],
+        )
+
+    def definition_body(self):
+        return dedent(
+            """
+            JS::Rooted<JSObject*> reflector(aCx);
+            return Wrap(aCx, aObject, aObject, aGivenProto, &reflector) ? reflector.get() : nullptr;
+            """
+        )
+
+
+class CGWrapNonWrapperCacheMethod(CGAbstractMethod):
+    """
+    Create a wrapper JSObject for a given native that does not implement
+    nsWrapperCache.
+    """
+
+    def __init__(self, descriptor, static=False, signatureOnly=False):
+        # XXX can we wrap if we don't have an interface prototype object?
+        assert descriptor.interface.hasInterfacePrototypeObject()
+        self.noGivenProto = (
+            descriptor.interface.isIteratorInterface()
+            or descriptor.interface.isAsyncIteratorInterface()
+        )
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument(descriptor.nativeType + "*", "aObject"),
+        ]
+        if not self.noGivenProto:
+            args.append(Argument("JS::Handle<JSObject*>", "aGivenProto"))
+        args.append(Argument("JS::MutableHandle<JSObject*>", "aReflector"))
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "Wrap",
+            "bool",
+            args,
+            static=static,
+            signatureOnly=signatureOnly,
+        )
+
+    def definition_body(self):
+        failureCode = "return false;\n"
+
+        declareProto = DeclareProto(self.descriptor, noGivenProto=self.noGivenProto)
+        if self.noGivenProto:
+            assertGivenProto = ""
+        else:
+            assertGivenProto = dedent(
+                """
+                MOZ_ASSERT_IF(aGivenProto, js::IsObjectInContextCompartment(aGivenProto, aCx));
+                """
+            )
+        return fill(
+            """
+            $*{assertions}
+            $*{assertGivenProto}
+
+            JS::Rooted<JSObject*> global(aCx, JS::CurrentGlobalOrNull(aCx));
+            $*{declareProto}
+
+            $*{createObject}
+
+            $*{unforgeable}
+
+            $*{slots}
+
+            creator.InitializationSucceeded();
+            return true;
+            """,
+            assertions=AssertInheritanceChain(self.descriptor),
+            assertGivenProto=assertGivenProto,
+            declareProto=declareProto,
+            createObject=CreateBindingJSObject(self.descriptor),
+            unforgeable=CopyUnforgeablePropertiesToInstance(
+                self.descriptor, failureCode
+            ),
+            slots=InitMemberSlots(self.descriptor, failureCode),
+        )
+
+
+class CGWrapGlobalMethod(CGAbstractMethod):
+    """
+    Create a wrapper JSObject for a global.  The global must implement
+    nsWrapperCache.
+
+    properties should be a PropertyArrays instance.
+    """
+
+    def __init__(self, descriptor, properties):
+        assert descriptor.interface.hasInterfacePrototypeObject()
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument(descriptor.nativeType + "*", "aObject"),
+            Argument("nsWrapperCache*", "aCache"),
+            Argument("JS::RealmOptions&", "aOptions"),
+            Argument("JSPrincipals*", "aPrincipal"),
+            Argument("bool", "aInitStandardClasses"),
+            Argument("JS::MutableHandle<JSObject*>", "aReflector"),
+        ]
+        CGAbstractMethod.__init__(self, descriptor, "Wrap", "bool", args)
+        self.descriptor = descriptor
+        self.properties = properties
+
+    def definition_body(self):
+        if self.properties.hasNonChromeOnly():
+            properties = "sNativeProperties.Upcast()"
+        else:
+            properties = "nullptr"
+        if self.properties.hasChromeOnly():
+            chromeProperties = "nsContentUtils::ThreadsafeIsSystemCaller(aCx) ? sChromeOnlyNativeProperties.Upcast() : nullptr"
+        else:
+            chromeProperties = "nullptr"
+
+        failureCode = dedent(
+            """
+            aCache->ReleaseWrapper(aObject);
+            aCache->ClearWrapper();
+            return false;
+            """
+        )
+
+        if self.descriptor.hasLegacyUnforgeableMembers:
+            unforgeable = InitUnforgeablePropertiesOnHolder(
+                self.descriptor, self.properties, failureCode, "aReflector"
+            ).define()
+        else:
+            unforgeable = ""
+
+        if self.descriptor.hasOrdinaryObjectPrototype:
+            getProto = "JS::GetRealmObjectPrototypeHandle"
+        else:
+            getProto = "GetProtoObjectHandle"
+        return fill(
+            """
+            $*{assertions}
+            MOZ_ASSERT(ToSupportsIsOnPrimaryInheritanceChain(aObject, aCache),
+                       "nsISupports must be on our primary inheritance chain");
+
+            if (!CreateGlobal<${nativeType}, ${getProto}>(aCx,
+                                             aObject,
+                                             aCache,
+                                             sClass.ToJSClass(),
+                                             aOptions,
+                                             aPrincipal,
+                                             aInitStandardClasses,
+                                             aReflector)) {
+              $*{failureCode}
+            }
+
+            // aReflector is a new global, so has a new realm.  Enter it
+            // before doing anything with it.
+            JSAutoRealm ar(aCx, aReflector);
+
+            if (!DefineProperties(aCx, aReflector, ${properties}, ${chromeProperties})) {
+              $*{failureCode}
+            }
+            $*{unforgeable}
+
+            $*{slots}
+
+            return true;
+            """,
+            assertions=AssertInheritanceChain(self.descriptor),
+            nativeType=self.descriptor.nativeType,
+            getProto=getProto,
+            properties=properties,
+            chromeProperties=chromeProperties,
+            failureCode=failureCode,
+            unforgeable=unforgeable,
+            slots=InitMemberSlots(self.descriptor, failureCode),
+        )
+
+
+class CGUpdateMemberSlotsMethod(CGAbstractStaticMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("JS::Handle<JSObject*>", "aWrapper"),
+            Argument(descriptor.nativeType + "*", "aObject"),
+        ]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, "UpdateMemberSlots", "bool", args
+        )
+
+    def definition_body(self):
+        body = "JS::Rooted<JS::Value> temp(aCx);\n" "JSJitGetterCallArgs args(&temp);\n"
+        for m in self.descriptor.interface.members:
+            if m.isAttr() and m.getExtendedAttribute("StoreInSlot"):
+                # Skip doing this for the "window" and "self" attributes on the
+                # Window interface, because those can't be gotten safely until
+                # we have hooked it up correctly to the outer window.  The
+                # window code handles doing the get itself.
+                if self.descriptor.interface.identifier.name == "Window" and (
+                    m.identifier.name == "window" or m.identifier.name == "self"
+                ):
+                    continue
+                body += fill(
+                    """
+
+                    static_assert(${slot} < JS::shadow::Object::MAX_FIXED_SLOTS,
+                                  "Not enough fixed slots to fit '${interface}.${member}.  Ion's visitGetDOMMemberV/visitGetDOMMemberT assume StoreInSlot things are all in fixed slots.");
+                    if (!get_${member}(aCx, aWrapper, aObject, args)) {
+                      return false;
+                    }
+                    // Getter handled setting our reserved slots
+                    """,
+                    slot=memberReservedSlot(m, self.descriptor),
+                    interface=self.descriptor.interface.identifier.name,
+                    member=m.identifier.name,
+                )
+
+        body += "\nreturn true;\n"
+        return body
+
+
+class CGClearCachedValueMethod(CGAbstractMethod):
+    def __init__(self, descriptor, member):
+        self.member = member
+        # If we're StoreInSlot, we'll need to call the getter
+        if member.getExtendedAttribute("StoreInSlot"):
+            args = [Argument("JSContext*", "aCx")]
+            returnType = "bool"
+        else:
+            args = []
+            returnType = "void"
+        args.append(Argument(descriptor.nativeType + "*", "aObject"))
+        name = MakeClearCachedValueNativeName(member)
+        CGAbstractMethod.__init__(self, descriptor, name, returnType, args)
+
+    def definition_body(self):
+        slotIndex = memberReservedSlot(self.member, self.descriptor)
+        if self.member.getExtendedAttribute("StoreInSlot"):
+            # We have to root things and save the old value in case
+            # regetting fails, so we can restore it.
+            declObj = "JS::Rooted<JSObject*> obj(aCx);\n"
+            noopRetval = " true"
+            saveMember = (
+                "JS::Rooted<JS::Value> oldValue(aCx, JS::GetReservedSlot(obj, %s));\n"
+                % slotIndex
+            )
+            regetMember = fill(
+                """
+                JS::Rooted<JS::Value> temp(aCx);
+                JSJitGetterCallArgs args(&temp);
+                JSAutoRealm ar(aCx, obj);
+                if (!get_${name}(aCx, obj, aObject, args)) {
+                  JS::SetReservedSlot(obj, ${slotIndex}, oldValue);
+                  return false;
+                }
+                return true;
+                """,
+                name=self.member.identifier.name,
+                slotIndex=slotIndex,
+            )
+        else:
+            declObj = "JSObject* obj;\n"
+            noopRetval = ""
+            saveMember = ""
+            regetMember = ""
+
+        if self.descriptor.wantsXrays:
+            clearXrayExpandoSlots = fill(
+                """
+                xpc::ClearXrayExpandoSlots(obj, ${xraySlotIndex});
+                """,
+                xraySlotIndex=memberXrayExpandoReservedSlot(
+                    self.member, self.descriptor
+                ),
+            )
+        else:
+            clearXrayExpandoSlots = ""
+
+        return fill(
+            """
+            $*{declObj}
+            obj = aObject->GetWrapper();
+            if (!obj) {
+              return${noopRetval};
+            }
+            $*{saveMember}
+            JS::SetReservedSlot(obj, ${slotIndex}, JS::UndefinedValue());
+            $*{clearXrayExpandoSlots}
+            $*{regetMember}
+            """,
+            declObj=declObj,
+            noopRetval=noopRetval,
+            saveMember=saveMember,
+            slotIndex=slotIndex,
+            clearXrayExpandoSlots=clearXrayExpandoSlots,
+            regetMember=regetMember,
+        )
+
+
+class CGCrossOriginProperties(CGThing):
+    def __init__(self, descriptor):
+        attrs = []
+        chromeOnlyAttrs = []
+        methods = []
+        chromeOnlyMethods = []
+        for m in descriptor.interface.members:
+            if m.isAttr() and (
+                m.getExtendedAttribute("CrossOriginReadable")
+                or m.getExtendedAttribute("CrossOriginWritable")
+            ):
+                if m.isStatic():
+                    raise TypeError(
+                        "Don't know how to deal with static method %s"
+                        % m.identifier.name
+                    )
+                if PropertyDefiner.getControllingCondition(
+                    m, descriptor
+                ).hasDisablers():
+                    raise TypeError(
+                        "Don't know how to deal with disabler for %s"
+                        % m.identifier.name
+                    )
+                if len(m.bindingAliases) > 0:
+                    raise TypeError(
+                        "Don't know how to deal with aliases for %s" % m.identifier.name
+                    )
+                if m.getExtendedAttribute("ChromeOnly") is not None:
+                    chromeOnlyAttrs.extend(AttrDefiner.attrData(m, overrideFlags="0"))
+                else:
+                    attrs.extend(AttrDefiner.attrData(m, overrideFlags="0"))
+            elif m.isMethod() and m.getExtendedAttribute("CrossOriginCallable"):
+                if m.isStatic():
+                    raise TypeError(
+                        "Don't know how to deal with static method %s"
+                        % m.identifier.name
+                    )
+                if PropertyDefiner.getControllingCondition(
+                    m, descriptor
+                ).hasDisablers():
+                    raise TypeError(
+                        "Don't know how to deal with disabler for %s"
+                        % m.identifier.name
+                    )
+                if len(m.aliases) > 0:
+                    raise TypeError(
+                        "Don't know how to deal with aliases for %s" % m.identifier.name
+                    )
+                if m.getExtendedAttribute("ChromeOnly") is not None:
+                    chromeOnlyMethods.append(
+                        MethodDefiner.methodData(
+                            m, descriptor, overrideFlags="JSPROP_READONLY"
+                        )
+                    )
+                else:
+                    methods.append(
+                        MethodDefiner.methodData(
+                            m, descriptor, overrideFlags="JSPROP_READONLY"
+                        )
+                    )
+
+        if len(attrs) > 0:
+            self.attributeSpecs, _ = PropertyDefiner.generatePrefableArrayValues(
+                attrs,
+                descriptor,
+                AttrDefiner.formatSpec,
+                "  JS_PS_END\n",
+                AttrDefiner.condition,
+                functools.partial(AttrDefiner.specData, crossOriginOnly=True),
+            )
+        else:
+            self.attributeSpecs = [" JS_PS_END\n"]
+        if len(methods) > 0:
+            self.methodSpecs, _ = PropertyDefiner.generatePrefableArrayValues(
+                methods,
+                descriptor,
+                MethodDefiner.formatSpec,
+                "  JS_FS_END\n",
+                MethodDefiner.condition,
+                MethodDefiner.specData,
+            )
+        else:
+            self.methodSpecs = ["  JS_FS_END\n"]
+
+        if len(chromeOnlyAttrs) > 0:
+            (
+                self.chromeOnlyAttributeSpecs,
+                _,
+            ) = PropertyDefiner.generatePrefableArrayValues(
+                chromeOnlyAttrs,
+                descriptor,
+                AttrDefiner.formatSpec,
+                "  JS_PS_END\n",
+                AttrDefiner.condition,
+                functools.partial(AttrDefiner.specData, crossOriginOnly=True),
+            )
+        else:
+            self.chromeOnlyAttributeSpecs = []
+        if len(chromeOnlyMethods) > 0:
+            self.chromeOnlyMethodSpecs, _ = PropertyDefiner.generatePrefableArrayValues(
+                chromeOnlyMethods,
+                descriptor,
+                MethodDefiner.formatSpec,
+                "  JS_FS_END\n",
+                MethodDefiner.condition,
+                MethodDefiner.specData,
+            )
+        else:
+            self.chromeOnlyMethodSpecs = []
+
+    def declare(self):
+        return dedent(
+            """
+            extern const CrossOriginProperties sCrossOriginProperties;
+            """
+        )
+
+    def define(self):
+        def defineChromeOnly(name, specs, specType):
+            if len(specs) == 0:
+                return ("", "nullptr")
+            name = "sChromeOnlyCrossOrigin" + name
+            define = fill(
+                """
+                static const ${specType} ${name}[] = {
+                  $*{specs}
+                };
+                """,
+                specType=specType,
+                name=name,
+                specs=",\n".join(specs),
+            )
+            return (define, name)
+
+        chromeOnlyAttributes = defineChromeOnly(
+            "Attributes", self.chromeOnlyAttributeSpecs, "JSPropertySpec"
+        )
+        chromeOnlyMethods = defineChromeOnly(
+            "Methods", self.chromeOnlyMethodSpecs, "JSFunctionSpec"
+        )
+        return fill(
+            """
+            static const JSPropertySpec sCrossOriginAttributes[] = {
+              $*{attributeSpecs}
+            };
+            static const JSFunctionSpec sCrossOriginMethods[] = {
+              $*{methodSpecs}
+            };
+            $*{chromeOnlyAttributeSpecs}
+            $*{chromeOnlyMethodSpecs}
+            const CrossOriginProperties sCrossOriginProperties = {
+              sCrossOriginAttributes,
+              sCrossOriginMethods,
+              ${chromeOnlyAttributes},
+              ${chromeOnlyMethods}
+            };
+            """,
+            attributeSpecs=",\n".join(self.attributeSpecs),
+            methodSpecs=",\n".join(self.methodSpecs),
+            chromeOnlyAttributeSpecs=chromeOnlyAttributes[0],
+            chromeOnlyMethodSpecs=chromeOnlyMethods[0],
+            chromeOnlyAttributes=chromeOnlyAttributes[1],
+            chromeOnlyMethods=chromeOnlyMethods[1],
+        )
+
+
+class CGCycleCollectionTraverseForOwningUnionMethod(CGAbstractMethod):
+    """
+    ImplCycleCollectionUnlink for owning union type.
+    """
+
+    def __init__(self, type):
+        self.type = type
+        args = [
+            Argument("nsCycleCollectionTraversalCallback&", "aCallback"),
+            Argument("%s&" % CGUnionStruct.unionTypeName(type, True), "aUnion"),
+            Argument("const char*", "aName"),
+            Argument("uint32_t", "aFlags", "0"),
+        ]
+        CGAbstractMethod.__init__(
+            self, None, "ImplCycleCollectionTraverse", "void", args
+        )
+
+    def deps(self):
+        return self.type.getDeps()
+
+    def definition_body(self):
+        memberNames = [
+            getUnionMemberName(t)
+            for t in self.type.flatMemberTypes
+            if idlTypeNeedsCycleCollection(t)
+        ]
+        assert memberNames
+
+        conditionTemplate = "aUnion.Is%s()"
+        functionCallTemplate = (
+            'ImplCycleCollectionTraverse(aCallback, aUnion.GetAs%s(), "m%s", aFlags);\n'
+        )
+
+        ifStaments = (
+            CGIfWrapper(CGGeneric(functionCallTemplate % (m, m)), conditionTemplate % m)
+            for m in memberNames
+        )
+
+        return CGElseChain(ifStaments).define()
+
+
+class CGCycleCollectionUnlinkForOwningUnionMethod(CGAbstractMethod):
+    """
+    ImplCycleCollectionUnlink for owning union type.
+    """
+
+    def __init__(self, type):
+        self.type = type
+        args = [Argument("%s&" % CGUnionStruct.unionTypeName(type, True), "aUnion")]
+        CGAbstractMethod.__init__(self, None, "ImplCycleCollectionUnlink", "void", args)
+
+    def deps(self):
+        return self.type.getDeps()
+
+    def definition_body(self):
+        return "aUnion.Uninit();\n"
+
+
+builtinNames = {
+    IDLType.Tags.bool: "bool",
+    IDLType.Tags.int8: "int8_t",
+    IDLType.Tags.int16: "int16_t",
+    IDLType.Tags.int32: "int32_t",
+    IDLType.Tags.int64: "int64_t",
+    IDLType.Tags.uint8: "uint8_t",
+    IDLType.Tags.uint16: "uint16_t",
+    IDLType.Tags.uint32: "uint32_t",
+    IDLType.Tags.uint64: "uint64_t",
+    IDLType.Tags.unrestricted_float: "float",
+    IDLType.Tags.float: "float",
+    IDLType.Tags.unrestricted_double: "double",
+    IDLType.Tags.double: "double",
+}
+
+numericSuffixes = {
+    IDLType.Tags.int8: "",
+    IDLType.Tags.uint8: "",
+    IDLType.Tags.int16: "",
+    IDLType.Tags.uint16: "",
+    IDLType.Tags.int32: "",
+    IDLType.Tags.uint32: "U",
+    IDLType.Tags.int64: "LL",
+    IDLType.Tags.uint64: "ULL",
+    IDLType.Tags.unrestricted_float: "F",
+    IDLType.Tags.float: "F",
+    IDLType.Tags.unrestricted_double: "",
+    IDLType.Tags.double: "",
+}
+
+
+def numericValue(t, v):
+    if t == IDLType.Tags.unrestricted_double or t == IDLType.Tags.unrestricted_float:
+        typeName = builtinNames[t]
+        if v == float("inf"):
+            return "mozilla::PositiveInfinity<%s>()" % typeName
+        if v == float("-inf"):
+            return "mozilla::NegativeInfinity<%s>()" % typeName
+        if math.isnan(v):
+            return "mozilla::UnspecifiedNaN<%s>()" % typeName
+    return "%s%s" % (v, numericSuffixes[t])
+
+
+class CastableObjectUnwrapper:
+    """
+    A class for unwrapping an object stored in a JS Value (or
+    MutableHandle<Value> or Handle<Value>) named by the "source" and
+    "mutableSource" arguments based on the passed-in descriptor and storing it
+    in a variable called by the name in the "target" argument.  The "source"
+    argument should be able to produce a Value or Handle<Value>; the
+    "mutableSource" argument should be able to produce a MutableHandle<Value>
+
+    codeOnFailure is the code to run if unwrapping fails.
+
+    If isCallbackReturnValue is "JSImpl" and our descriptor is also
+    JS-implemented, fall back to just creating the right object if what we
+    have isn't one already.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        source,
+        mutableSource,
+        target,
+        codeOnFailure,
+        exceptionCode=None,
+        isCallbackReturnValue=False,
+    ):
+        self.substitution = {
+            "type": descriptor.nativeType,
+            "protoID": "prototypes::id::" + descriptor.name,
+            "target": target,
+            "codeOnFailure": codeOnFailure,
+            "source": source,
+            "mutableSource": mutableSource,
+        }
+
+        if isCallbackReturnValue == "JSImpl" and descriptor.interface.isJSImplemented():
+            exceptionCode = exceptionCode or codeOnFailure
+            self.substitution["codeOnFailure"] = fill(
+                """
+                // Be careful to not wrap random DOM objects here, even if
+                // they're wrapped in opaque security wrappers for some reason.
+                // XXXbz Wish we could check for a JS-implemented object
+                // that already has a content reflection...
+                if (!IsDOMObject(js::UncheckedUnwrap(&${source}.toObject()))) {
+                  nsCOMPtr<nsIGlobalObject> contentGlobal;
+                  JS::Rooted<JSObject*> callback(cx, CallbackOrNull());
+                  if (!callback ||
+                      !GetContentGlobalForJSImplementedObject(cx, callback, getter_AddRefs(contentGlobal))) {
+                    $*{exceptionCode}
+                  }
+                  JS::Rooted<JSObject*> jsImplSourceObj(cx, &${source}.toObject());
+                  MOZ_RELEASE_ASSERT(!js::IsWrapper(jsImplSourceObj),
+                                     "Don't return JS implementations from other compartments");
+                  JS::Rooted<JSObject*> jsImplSourceGlobal(cx, JS::GetNonCCWObjectGlobal(jsImplSourceObj));
+                  ${target} = new ${type}(jsImplSourceObj, jsImplSourceGlobal, contentGlobal);
+                } else {
+                  $*{codeOnFailure}
+                }
+                """,
+                exceptionCode=exceptionCode,
+                **self.substitution,
+            )
+        else:
+            self.substitution["codeOnFailure"] = codeOnFailure
+
+    def __str__(self):
+        substitution = self.substitution.copy()
+        substitution["codeOnFailure"] %= {
+            "securityError": "rv == NS_ERROR_XPC_SECURITY_MANAGER_VETO"
+        }
+        return fill(
+            """
+            {
+              // Our JSContext should be in the right global to do unwrapping in.
+              nsresult rv = UnwrapObject<${protoID}, ${type}>(${mutableSource}, ${target}, cx);
+              if (NS_FAILED(rv)) {
+                $*{codeOnFailure}
+              }
+            }
+            """,
+            **substitution,
+        )
+
+
+class FailureFatalCastableObjectUnwrapper(CastableObjectUnwrapper):
+    """
+    As CastableObjectUnwrapper, but defaulting to throwing if unwrapping fails
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        source,
+        mutableSource,
+        target,
+        exceptionCode,
+        isCallbackReturnValue,
+        sourceDescription,
+    ):
+        CastableObjectUnwrapper.__init__(
+            self,
+            descriptor,
+            source,
+            mutableSource,
+            target,
+            'cx.ThrowErrorMessage<MSG_DOES_NOT_IMPLEMENT_INTERFACE>("%s", "%s");\n'
+            "%s"
+            % (sourceDescription, descriptor.interface.identifier.name, exceptionCode),
+            exceptionCode,
+            isCallbackReturnValue,
+        )
+
+
+def getCallbackConversionInfo(
+    type, idlObject, isMember, isCallbackReturnValue, isOptional
+):
+    """
+    Returns a tuple containing the declType, declArgs, and basic
+    conversion for the given callback type, with the given callback
+    idl object in the given context (isMember/isCallbackReturnValue/isOptional).
+    """
+    name = idlObject.identifier.name
+
+    # We can't use fast callbacks if isOptional because then we get an
+    # Optional<RootedCallback> thing, which is not transparent to consumers.
+    useFastCallback = (
+        (not isMember or isMember == "Union")
+        and not isCallbackReturnValue
+        and not isOptional
+    )
+    if useFastCallback:
+        name = "binding_detail::Fast%s" % name
+        rootArgs = ""
+        args = "&${val}.toObject(), JS::CurrentGlobalOrNull(cx)"
+    else:
+        rootArgs = dedent(
+            """
+            JS::Rooted<JSObject*> tempRoot(cx, &${val}.toObject());
+            JS::Rooted<JSObject*> tempGlobalRoot(cx, JS::CurrentGlobalOrNull(cx));
+            """
+        )
+        args = "cx, tempRoot, tempGlobalRoot, GetIncumbentGlobal()"
+
+    if type.nullable() or isCallbackReturnValue:
+        declType = CGGeneric("RefPtr<%s>" % name)
+    else:
+        declType = CGGeneric("OwningNonNull<%s>" % name)
+
+    if useFastCallback:
+        declType = CGTemplatedType("RootedCallback", declType)
+        declArgs = "cx"
+    else:
+        declArgs = None
+
+    conversion = fill(
+        """
+        { // scope for tempRoot and tempGlobalRoot if needed
+          $*{rootArgs}
+          $${declName} = new ${name}(${args});
+        }
+        """,
+        rootArgs=rootArgs,
+        name=name,
+        args=args,
+    )
+    return (declType, declArgs, conversion)
+
+
+class JSToNativeConversionInfo:
+    """
+    An object representing information about a JS-to-native conversion.
+    """
+
+    def __init__(
+        self,
+        template,
+        declType=None,
+        holderType=None,
+        dealWithOptional=False,
+        declArgs=None,
+        holderArgs=None,
+    ):
+        """
+        template: A string representing the conversion code.  This will have
+                  template substitution performed on it as follows:
+
+          ${val} is a handle to the JS::Value in question
+          ${maybeMutableVal} May be a mutable handle to the JS::Value in
+                             question. This is only OK to use if ${val} is
+                             known to not be undefined.
+          ${holderName} replaced by the holder's name, if any
+          ${declName} replaced by the declaration's name
+          ${haveValue} replaced by an expression that evaluates to a boolean
+                       for whether we have a JS::Value.  Only used when
+                       defaultValue is not None or when True is passed for
+                       checkForValue to instantiateJSToNativeConversion.
+                       This expression may not be already-parenthesized, so if
+                       you use it with && or || make sure to put parens
+                       around it.
+          ${passedToJSImpl} replaced by an expression that evaluates to a boolean
+                            for whether this value is being passed to a JS-
+                            implemented interface.
+
+        declType: A CGThing representing the native C++ type we're converting
+                  to.  This is allowed to be None if the conversion code is
+                  supposed to be used as-is.
+
+        holderType: A CGThing representing the type of a "holder" which will
+                    hold a possible reference to the C++ thing whose type we
+                    returned in declType, or  None if no such holder is needed.
+
+        dealWithOptional: A boolean indicating whether the caller has to do
+                          optional-argument handling.  This should only be set
+                          to true if the JS-to-native conversion is being done
+                          for an optional argument or dictionary member with no
+                          default value and if the returned template expects
+                          both declType and holderType to be wrapped in
+                          Optional<>, with ${declName} and ${holderName}
+                          adjusted to point to the Value() of the Optional, and
+                          Construct() calls to be made on the Optional<>s as
+                          needed.
+
+        declArgs: If not None, the arguments to pass to the ${declName}
+                  constructor.  These will have template substitution performed
+                  on them so you can use things like ${val}.  This is a
+                  single string, not a list of strings.
+
+        holderArgs: If not None, the arguments to pass to the ${holderName}
+                    constructor.  These will have template substitution
+                    performed on them so you can use things like ${val}.
+                    This is a single string, not a list of strings.
+
+        ${declName} must be in scope before the code from 'template' is entered.
+
+        If holderType is not None then ${holderName} must be in scope before
+        the code from 'template' is entered.
+        """
+        assert isinstance(template, str)
+        assert declType is None or isinstance(declType, CGThing)
+        assert holderType is None or isinstance(holderType, CGThing)
+        self.template = template
+        self.declType = declType
+        self.holderType = holderType
+        self.dealWithOptional = dealWithOptional
+        self.declArgs = declArgs
+        self.holderArgs = holderArgs
+
+
+def getHandleDefault(defaultValue):
+    tag = defaultValue.type.tag()
+    if tag in numericSuffixes:
+        # Some numeric literals require a suffix to compile without warnings
+        return numericValue(tag, defaultValue.value)
+    assert tag == IDLType.Tags.bool
+    return toStringBool(defaultValue.value)
+
+
+def handleDefaultStringValue(defaultValue, method):
+    """
+    Returns a string which ends up calling 'method' with a (char_t*, length)
+    pair that sets this string default value.  This string is suitable for
+    passing as the second argument of handleDefault.
+    """
+    assert (
+        defaultValue.type.isDOMString()
+        or defaultValue.type.isUSVString()
+        or defaultValue.type.isUTF8String()
+        or defaultValue.type.isByteString()
+    )
+    # There shouldn't be any non-ASCII or embedded nulls in here; if
+    # it ever sneaks in we will need to think about how to properly
+    # represent that in the C++.
+    assert all(ord(c) < 128 and ord(c) > 0 for c in defaultValue.value)
+    if defaultValue.type.isByteString() or defaultValue.type.isUTF8String():
+        prefix = ""
+    else:
+        prefix = "u"
+    return fill(
+        """
+        ${method}(${prefix}"${value}");
+        """,
+        method=method,
+        prefix=prefix,
+        value=defaultValue.value,
+    )
+
+
+def recordKeyType(recordType):
+    assert recordType.keyType.isString()
+    if recordType.keyType.isByteString() or recordType.keyType.isUTF8String():
+        return "nsCString"
+    return "nsString"
+
+
+def recordKeyDeclType(recordType):
+    return CGGeneric(recordKeyType(recordType))
+
+
+def initializerForType(type):
+    """
+    Get the right initializer for the given type for a data location where we
+    plan to then initialize it from a JS::Value.  Some types need to always be
+    initialized even before we start the JS::Value-to-IDL-value conversion.
+
+    Returns a string or None if no initialization is needed.
+    """
+    if type.isObject():
+        return "nullptr"
+    # We could probably return CGDictionary.getNonInitializingCtorArg() for the
+    # dictionary case, but code outside DictionaryBase subclasses can't use
+    # that, so we can't do it across the board.
+    return None
+
+
+# If this function is modified, modify CGNativeMember.getArg and
+# CGNativeMember.getRetvalInfo accordingly.  The latter cares about the decltype
+# and holdertype we end up using, because it needs to be able to return the code
+# that will convert those to the actual return value of the callback function.
+def getJSToNativeConversionInfo(
+    type,
+    descriptorProvider,
+    failureCode=None,
+    isDefinitelyObject=False,
+    isMember=False,
+    isOptional=False,
+    invalidEnumValueFatal=True,
+    defaultValue=None,
+    isNullOrUndefined=False,
+    isKnownMissing=False,
+    exceptionCode=None,
+    lenientFloatCode=None,
+    allowTreatNonCallableAsNull=False,
+    isCallbackReturnValue=False,
+    sourceDescription="value",
+    nestingLevel="",
+):
+    """
+    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
+    testing whether we can convert the argument to the desired type.  That
+    means that failures to convert due to the JS value being the wrong type of
+    value need to use failureCode instead of throwing exceptions.  Failures to
+    convert that are due to JS exceptions (from toString or valueOf methods) or
+    out of memory conditions need to throw exceptions no matter what
+    failureCode is.  However what actually happens when throwing an exception
+    can be controlled by exceptionCode.  The only requirement on that is that
+    exceptionCode must end up doing a return, and every return from this
+    function must happen via exceptionCode if exceptionCode is not None.
+
+    If isDefinitelyObject is True, that means we have a value and the value
+    tests true for isObject(), so we have no need to recheck that.
+
+    If isNullOrUndefined is True, that means we have a value and the value
+    tests true for isNullOrUndefined(), so we have no need to recheck that.
+
+    If isKnownMissing is True, that means that we are known-missing, and for
+    cases when we have a default value we only need to output the default value.
+
+    if isMember is not False, we're being converted from a property of some JS
+    object, not from an actual method argument, so we can't rely on our jsval
+    being rooted or outliving us in any way.  Callers can pass "Dictionary",
+    "Variadic", "Sequence", "Union", or "OwningUnion" to indicate that the conversion
+    is for something that is a dictionary member, a variadic argument, a sequence,
+    an union, or an owning union respectively.
+    XXX Once we swtich *Rooter to Rooted* for Record and Sequence type entirely,
+        we could remove "Union" from isMember.
+
+    If isOptional is true, then we are doing conversion of an optional
+    argument with no default value.
+
+    invalidEnumValueFatal controls whether an invalid enum value conversion
+    attempt will throw (if true) or simply return without doing anything (if
+    false).
+
+    If defaultValue is not None, it's the IDL default value for this conversion
+
+    If isEnforceRange is true, we're converting an integer and throwing if the
+    value is out of range.
+
+    If isClamp is true, we're converting an integer and clamping if the
+    value is out of range.
+
+    If isAllowShared is false, we're converting a buffer source and throwing if
+    it is a SharedArrayBuffer or backed by a SharedArrayBuffer.
+
+    If lenientFloatCode is not None, it should be used in cases when
+    we're a non-finite float that's not unrestricted.
+
+    If allowTreatNonCallableAsNull is true, then [TreatNonCallableAsNull] and
+    [LegacyTreatNonObjectAsNull] extended attributes on nullable callback functions
+    will be honored.
+
+    If isCallbackReturnValue is "JSImpl" or "Callback", then the declType may be
+    adjusted to make it easier to return from a callback.  Since that type is
+    never directly observable by any consumers of the callback code, this is OK.
+    Furthermore, if isCallbackReturnValue is "JSImpl", that affects the behavior
+    of the FailureFatalCastableObjectUnwrapper conversion; this is used for
+    implementing auto-wrapping of JS-implemented return values from a
+    JS-implemented interface.
+
+    sourceDescription is a description of what this JS value represents, to be
+    used in error reporting.  Callers should assume that it might get placed in
+    the middle of a sentence.  If it ends up at the beginning of a sentence, its
+    first character will be automatically uppercased.
+
+    The return value from this function is a JSToNativeConversionInfo.
+    """
+    # If we have a defaultValue then we're not actually optional for
+    # purposes of what we need to be declared as.
+    assert defaultValue is None or not isOptional
+
+    # Also, we should not have a defaultValue if we know we're an object
+    assert not isDefinitelyObject or defaultValue is None
+
+    # And we can't both be an object and be null or undefined
+    assert not isDefinitelyObject or not isNullOrUndefined
+
+    isClamp = type.hasClamp()
+    isEnforceRange = type.hasEnforceRange()
+    isAllowShared = type.hasAllowShared()
+
+    # If exceptionCode is not set, we'll just rethrow the exception we got.
+    # Note that we can't just set failureCode to exceptionCode, because setting
+    # failureCode will prevent pending exceptions from being set in cases when
+    # they really should be!
+    if exceptionCode is None:
+        exceptionCode = "return false;\n"
+
+    # Unfortunately, .capitalize() on a string will lowercase things inside the
+    # string, which we do not want.
+    def firstCap(string):
+        return string[0].upper() + string[1:]
+
+    # Helper functions for dealing with failures due to the JS value being the
+    # wrong type of value
+    def onFailureNotAnObject(failureCode):
+        return CGGeneric(
+            failureCode
+            or (
+                'cx.ThrowErrorMessage<MSG_NOT_OBJECT>("%s");\n'
+                "%s" % (firstCap(sourceDescription), exceptionCode)
+            )
+        )
+
+    def onFailureBadType(failureCode, typeName):
+        return CGGeneric(
+            failureCode
+            or (
+                'cx.ThrowErrorMessage<MSG_DOES_NOT_IMPLEMENT_INTERFACE>("%s", "%s");\n'
+                "%s" % (firstCap(sourceDescription), typeName, exceptionCode)
+            )
+        )
+
+    # It's a failure in the committed-to conversion, not a failure to match up
+    # to a type, so we don't want to use failureCode in here. We want to just
+    # throw an exception unconditionally.
+    def onFailureIsShared():
+        return CGGeneric(
+            'cx.ThrowErrorMessage<MSG_TYPEDARRAY_IS_SHARED>("%s");\n'
+            "%s" % (firstCap(sourceDescription), exceptionCode)
+        )
+
+    def onFailureIsLarge():
+        return CGGeneric(
+            'cx.ThrowErrorMessage<MSG_TYPEDARRAY_IS_LARGE>("%s");\n'
+            "%s" % (firstCap(sourceDescription), exceptionCode)
+        )
+
+    def onFailureNotCallable(failureCode):
+        return CGGeneric(
+            failureCode
+            or (
+                'cx.ThrowErrorMessage<MSG_NOT_CALLABLE>("%s");\n'
+                "%s" % (firstCap(sourceDescription), exceptionCode)
+            )
+        )
+
+    # A helper function for handling default values.  Takes a template
+    # body and the C++ code to set the default value and wraps the
+    # given template body in handling for the default value.
+    def handleDefault(template, setDefault):
+        if defaultValue is None:
+            return template
+        if isKnownMissing:
+            return fill(
+                """
+                {
+                  // scope for any temporaries our default value setting needs.
+                  $*{setDefault}
+                }
+                """,
+                setDefault=setDefault,
+            )
+        return fill(
+            """
+            if ($${haveValue}) {
+              $*{templateBody}
+            } else {
+              $*{setDefault}
+            }
+            """,
+            templateBody=template,
+            setDefault=setDefault,
+        )
+
+    # A helper function for wrapping up the template body for
+    # possibly-nullable objecty stuff
+    def wrapObjectTemplate(templateBody, type, codeToSetNull, failureCode=None):
+        if isNullOrUndefined and type.nullable():
+            # Just ignore templateBody and set ourselves to null.
+            # Note that we don't have to worry about default values
+            # here either, since we already examined this value.
+            return codeToSetNull
+
+        if not isDefinitelyObject:
+            # Handle the non-object cases by wrapping up the whole
+            # thing in an if cascade.
+            if type.nullable():
+                elifLine = "} else if (${val}.isNullOrUndefined()) {\n"
+                elifBody = codeToSetNull
+            else:
+                elifLine = ""
+                elifBody = ""
+
+            # Note that $${val} below expands to ${val}. This string is
+            # used as a template later, and val will be filled in then.
+            templateBody = fill(
+                """
+                if ($${val}.isObject()) {
+                  $*{templateBody}
+                $*{elifLine}
+                  $*{elifBody}
+                } else {
+                  $*{failureBody}
+                }
+                """,
+                templateBody=templateBody,
+                elifLine=elifLine,
+                elifBody=elifBody,
+                failureBody=onFailureNotAnObject(failureCode).define(),
+            )
+
+            if isinstance(defaultValue, IDLNullValue):
+                assert type.nullable()  # Parser should enforce this
+                templateBody = handleDefault(templateBody, codeToSetNull)
+            elif isinstance(defaultValue, IDLEmptySequenceValue):
+                # Our caller will handle it
+                pass
+            else:
+                assert defaultValue is None
+
+        return templateBody
+
+    # A helper function for converting things that look like a JSObject*.
+    def handleJSObjectType(
+        type, isMember, failureCode, exceptionCode, sourceDescription
+    ):
+        if not isMember or isMember == "Union":
+            if isOptional:
+                # We have a specialization of Optional that will use a
+                # Rooted for the storage here.
+                declType = CGGeneric("JS::Handle<JSObject*>")
+            else:
+                declType = CGGeneric("JS::Rooted<JSObject*>")
+            declArgs = "cx"
+        else:
+            assert isMember in (
+                "Sequence",
+                "Variadic",
+                "Dictionary",
+                "OwningUnion",
+                "Record",
+            )
+            # We'll get traced by the sequence or dictionary or union tracer
+            declType = CGGeneric("JSObject*")
+            declArgs = None
+        templateBody = "${declName} = &${val}.toObject();\n"
+
+        # For JS-implemented APIs, we refuse to allow passing objects that the
+        # API consumer does not subsume. The extra parens around
+        # ($${passedToJSImpl}) suppress unreachable code warnings when
+        # $${passedToJSImpl} is the literal `false`.  But Apple is shipping a
+        # buggy clang (clang 3.9) in Xcode 8.3, so there even the parens are not
+        # enough.  So we manually disable some warnings in clang.
+        if (
+            not isinstance(descriptorProvider, Descriptor)
+            or descriptorProvider.interface.isJSImplemented()
+        ):
+            templateBody = (
+                fill(
+                    """
+                #ifdef __clang__
+                #pragma clang diagnostic push
+                #pragma clang diagnostic ignored "-Wunreachable-code"
+                #pragma clang diagnostic ignored "-Wunreachable-code-return"
+                #endif // __clang__
+                if (($${passedToJSImpl}) && !CallerSubsumes($${val})) {
+                  cx.ThrowErrorMessage<MSG_PERMISSION_DENIED_TO_PASS_ARG>("${sourceDescription}");
+                  $*{exceptionCode}
+                }
+                #ifdef __clang__
+                #pragma clang diagnostic pop
+                #endif // __clang__
+                """,
+                    sourceDescription=sourceDescription,
+                    exceptionCode=exceptionCode,
+                )
+                + templateBody
+            )
+
+        setToNullCode = "${declName} = nullptr;\n"
+        template = wrapObjectTemplate(templateBody, type, setToNullCode, failureCode)
+        return JSToNativeConversionInfo(
+            template, declType=declType, dealWithOptional=isOptional, declArgs=declArgs
+        )
+
+    def incrementNestingLevel():
+        if nestingLevel == "":
+            return 1
+        return nestingLevel + 1
+
+    assert not (isEnforceRange and isClamp)  # These are mutually exclusive
+
+    if type.isSequence() or type.isObservableArray():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        if failureCode is None:
+            notSequence = (
+                'cx.ThrowErrorMessage<MSG_CONVERSION_ERROR>("%s", "%s");\n'
+                "%s"
+                % (
+                    firstCap(sourceDescription),
+                    "sequence" if type.isSequence() else "observable array",
+                    exceptionCode,
+                )
+            )
+        else:
+            notSequence = failureCode
+
+        nullable = type.nullable()
+        # Be very careful not to change "type": we need it later
+        if nullable:
+            elementType = type.inner.inner
+        else:
+            elementType = type.inner
+
+        # We want to use auto arrays if we can, but we have to be careful with
+        # reallocation behavior for arrays.  In particular, if we use auto
+        # arrays for sequences and have a sequence of elements which are
+        # themselves sequences or have sequences as members, we have a problem.
+        # In that case, resizing the outermost AutoTArray to the right size
+        # will memmove its elements, but AutoTArrays are not memmovable and
+        # hence will end up with pointers to bogus memory, which is bad.  To
+        # deal with this, we typically map WebIDL sequences to our Sequence
+        # type, which is in fact memmovable.  The one exception is when we're
+        # passing in a sequence directly as an argument without any sort of
+        # optional or nullable complexity going on.  In that situation, we can
+        # use an AutoSequence instead.  We have to keep using Sequence in the
+        # nullable and optional cases because we don't want to leak the
+        # AutoSequence type to consumers, which would be unavoidable with
+        # Nullable<AutoSequence> or Optional<AutoSequence>.
+        if (
+            (isMember and isMember != "Union")
+            or isOptional
+            or nullable
+            or isCallbackReturnValue
+        ):
+            sequenceClass = "Sequence"
+        else:
+            sequenceClass = "binding_detail::AutoSequence"
+
+        # XXXbz we can't include the index in the sourceDescription, because
+        # we don't really have a way to pass one in dynamically at runtime...
+        elementInfo = getJSToNativeConversionInfo(
+            elementType,
+            descriptorProvider,
+            isMember="Sequence",
+            exceptionCode=exceptionCode,
+            lenientFloatCode=lenientFloatCode,
+            isCallbackReturnValue=isCallbackReturnValue,
+            sourceDescription="element of %s" % sourceDescription,
+            nestingLevel=incrementNestingLevel(),
+        )
+        if elementInfo.dealWithOptional:
+            raise TypeError("Shouldn't have optional things in sequences")
+        if elementInfo.holderType is not None:
+            raise TypeError("Shouldn't need holders for sequences")
+
+        typeName = CGTemplatedType(sequenceClass, elementInfo.declType)
+        sequenceType = typeName.define()
+
+        if isMember == "Union" and typeNeedsRooting(type):
+            assert not nullable
+            typeName = CGTemplatedType(
+                "binding_detail::RootedAutoSequence", elementInfo.declType
+            )
+        elif nullable:
+            typeName = CGTemplatedType("Nullable", typeName)
+
+        if nullable:
+            arrayRef = "${declName}.SetValue()"
+        else:
+            arrayRef = "${declName}"
+
+        elementConversion = string.Template(elementInfo.template).substitute(
+            {
+                "val": "temp" + str(nestingLevel),
+                "maybeMutableVal": "&temp" + str(nestingLevel),
+                "declName": "slot" + str(nestingLevel),
+                # We only need holderName here to handle isExternal()
+                # interfaces, which use an internal holder for the
+                # conversion even when forceOwningType ends up true.
+                "holderName": "tempHolder" + str(nestingLevel),
+                "passedToJSImpl": "${passedToJSImpl}",
+            }
+        )
+
+        elementInitializer = initializerForType(elementType)
+        if elementInitializer is None:
+            elementInitializer = ""
+        else:
+            elementInitializer = elementInitializer + ", "
+
+        # NOTE: Keep this in sync with variadic conversions as needed
+        templateBody = fill(
+            """
+            JS::ForOfIterator iter${nestingLevel}(cx);
+            if (!iter${nestingLevel}.init($${val}, JS::ForOfIterator::AllowNonIterable)) {
+              $*{exceptionCode}
+            }
+            if (!iter${nestingLevel}.valueIsIterable()) {
+              $*{notSequence}
+            }
+            ${sequenceType} &arr${nestingLevel} = ${arrayRef};
+            JS::Rooted<JS::Value> temp${nestingLevel}(cx);
+            while (true) {
+              bool done${nestingLevel};
+              if (!iter${nestingLevel}.next(&temp${nestingLevel}, &done${nestingLevel})) {
+                $*{exceptionCode}
+              }
+              if (done${nestingLevel}) {
+                break;
+              }
+              ${elementType}* slotPtr${nestingLevel} = arr${nestingLevel}.AppendElement(${elementInitializer}mozilla::fallible);
+              if (!slotPtr${nestingLevel}) {
+                JS_ReportOutOfMemory(cx);
+                $*{exceptionCode}
+              }
+              ${elementType}& slot${nestingLevel} = *slotPtr${nestingLevel};
+              $*{elementConversion}
+            }
+            """,
+            exceptionCode=exceptionCode,
+            notSequence=notSequence,
+            sequenceType=sequenceType,
+            arrayRef=arrayRef,
+            elementType=elementInfo.declType.define(),
+            elementConversion=elementConversion,
+            elementInitializer=elementInitializer,
+            nestingLevel=str(nestingLevel),
+        )
+
+        templateBody = wrapObjectTemplate(
+            templateBody, type, "${declName}.SetNull();\n", notSequence
+        )
+        if isinstance(defaultValue, IDLEmptySequenceValue):
+            if type.nullable():
+                codeToSetEmpty = "${declName}.SetValue();\n"
+            else:
+                codeToSetEmpty = (
+                    "/* ${declName} array is already empty; nothing to do */\n"
+                )
+            templateBody = handleDefault(templateBody, codeToSetEmpty)
+
+        declArgs = None
+        holderType = None
+        holderArgs = None
+        # Sequence arguments that might contain traceable things need
+        # to get traced
+        if typeNeedsRooting(elementType):
+            if not isMember:
+                holderType = CGTemplatedType("SequenceRooter", elementInfo.declType)
+                # If our sequence is nullable, this will set the Nullable to be
+                # not-null, but that's ok because we make an explicit SetNull() call
+                # on it as needed if our JS value is actually null.
+                holderArgs = "cx, &%s" % arrayRef
+            elif isMember == "Union":
+                declArgs = "cx"
+
+        return JSToNativeConversionInfo(
+            templateBody,
+            declType=typeName,
+            declArgs=declArgs,
+            holderType=holderType,
+            dealWithOptional=isOptional,
+            holderArgs=holderArgs,
+        )
+
+    if type.isRecord():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+        if failureCode is None:
+            notRecord = 'cx.ThrowErrorMessage<MSG_NOT_OBJECT>("%s");\n' "%s" % (
+                firstCap(sourceDescription),
+                exceptionCode,
+            )
+        else:
+            notRecord = failureCode
+
+        nullable = type.nullable()
+        # Be very careful not to change "type": we need it later
+        if nullable:
+            recordType = type.inner
+        else:
+            recordType = type
+        valueType = recordType.inner
+
+        valueInfo = getJSToNativeConversionInfo(
+            valueType,
+            descriptorProvider,
+            isMember="Record",
+            exceptionCode=exceptionCode,
+            lenientFloatCode=lenientFloatCode,
+            isCallbackReturnValue=isCallbackReturnValue,
+            sourceDescription="value in %s" % sourceDescription,
+            nestingLevel=incrementNestingLevel(),
+        )
+        if valueInfo.dealWithOptional:
+            raise TypeError("Shouldn't have optional things in record")
+        if valueInfo.holderType is not None:
+            raise TypeError("Shouldn't need holders for record")
+
+        declType = CGTemplatedType(
+            "Record", [recordKeyDeclType(recordType), valueInfo.declType]
+        )
+        typeName = declType.define()
+
+        if isMember == "Union" and typeNeedsRooting(type):
+            assert not nullable
+            declType = CGTemplatedType(
+                "RootedRecord", [recordKeyDeclType(recordType), valueInfo.declType]
+            )
+        elif nullable:
+            declType = CGTemplatedType("Nullable", declType)
+
+        if nullable:
+            recordRef = "${declName}.SetValue()"
+        else:
+            recordRef = "${declName}"
+
+        valueConversion = string.Template(valueInfo.template).substitute(
+            {
+                "val": "temp",
+                "maybeMutableVal": "&temp",
+                "declName": "slot",
+                # We only need holderName here to handle isExternal()
+                # interfaces, which use an internal holder for the
+                # conversion even when forceOwningType ends up true.
+                "holderName": "tempHolder",
+                "passedToJSImpl": "${passedToJSImpl}",
+            }
+        )
+
+        keyType = recordKeyType(recordType)
+        if recordType.keyType.isJSString():
+            raise TypeError(
+                "Have do deal with JSString record type, but don't know how"
+            )
+        if recordType.keyType.isByteString() or recordType.keyType.isUTF8String():
+            hashKeyType = "nsCStringHashKey"
+            if recordType.keyType.isByteString():
+                keyConversionFunction = "ConvertJSValueToByteString"
+            else:
+                keyConversionFunction = "ConvertJSValueToString"
+
+        else:
+            hashKeyType = "nsStringHashKey"
+            if recordType.keyType.isDOMString():
+                keyConversionFunction = "ConvertJSValueToString"
+            else:
+                assert recordType.keyType.isUSVString()
+                keyConversionFunction = "ConvertJSValueToUSVString"
+
+        templateBody = fill(
+            """
+            auto& recordEntries = ${recordRef}.Entries();
+
+            JS::Rooted<JSObject*> recordObj(cx, &$${val}.toObject());
+            JS::RootedVector<jsid> ids(cx);
+            if (!js::GetPropertyKeys(cx, recordObj,
+                                     JSITER_OWNONLY | JSITER_HIDDEN | JSITER_SYMBOLS, &ids)) {
+              $*{exceptionCode}
+            }
+            if (!recordEntries.SetCapacity(ids.length(), mozilla::fallible)) {
+              JS_ReportOutOfMemory(cx);
+              $*{exceptionCode}
+            }
+            JS::Rooted<JS::Value> propNameValue(cx);
+            JS::Rooted<JS::Value> temp(cx);
+            JS::Rooted<jsid> curId(cx);
+            JS::Rooted<JS::Value> idVal(cx);
+            // Use a hashset to keep track of ids seen, to avoid
+            // introducing nasty O(N^2) behavior scanning for them all the
+            // time.  Ideally we'd use a data structure with O(1) lookup
+            // _and_ ordering for the MozMap, but we don't have one lying
+            // around.
+            nsTHashtable<${hashKeyType}> idsSeen;
+            for (size_t i = 0; i < ids.length(); ++i) {
+              curId = ids[i];
+
+              JS::Rooted<mozilla::Maybe<JS::PropertyDescriptor>> desc(cx);
+              if (!JS_GetOwnPropertyDescriptorById(cx, recordObj, curId,
+                                                   &desc)) {
+                $*{exceptionCode}
+              }
+
+              if (desc.isNothing() || !desc->enumerable()) {
+                continue;
+              }
+
+              idVal = js::IdToValue(curId);
+              ${keyType} propName;
+              // This will just throw if idVal is a Symbol, like the spec says
+              // to do.
+              if (!${keyConversionFunction}(cx, idVal, "key of ${sourceDescription}", propName)) {
+                $*{exceptionCode}
+              }
+
+              if (!JS_GetPropertyById(cx, recordObj, curId, &temp)) {
+                $*{exceptionCode}
+              }
+
+              ${typeName}::EntryType* entry;
+              if (!idsSeen.EnsureInserted(propName)) {
+                // Find the existing entry.
+                auto idx = recordEntries.IndexOf(propName);
+                MOZ_ASSERT(idx != recordEntries.NoIndex,
+                           "Why is it not found?");
+                // Now blow it away to make it look like it was just added
+                // to the array, because it's not obvious that it's
+                // safe to write to its already-initialized mValue via our
+                // normal codegen conversions.  For example, the value
+                // could be a union and this would change its type, but
+                // codegen assumes we won't do that.
+                entry = recordEntries.ReconstructElementAt(idx);
+              } else {
+                // Safe to do an infallible append here, because we did a
+                // SetCapacity above to the right capacity.
+                entry = recordEntries.AppendElement();
+              }
+              entry->mKey = propName;
+              ${valueType}& slot = entry->mValue;
+              $*{valueConversion}
+            }
+            """,
+            exceptionCode=exceptionCode,
+            recordRef=recordRef,
+            hashKeyType=hashKeyType,
+            keyType=keyType,
+            keyConversionFunction=keyConversionFunction,
+            sourceDescription=sourceDescription,
+            typeName=typeName,
+            valueType=valueInfo.declType.define(),
+            valueConversion=valueConversion,
+        )
+
+        templateBody = wrapObjectTemplate(
+            templateBody, type, "${declName}.SetNull();\n", notRecord
+        )
+
+        declArgs = None
+        holderType = None
+        holderArgs = None
+        # record arguments that might contain traceable things need
+        # to get traced
+        if not isMember and isCallbackReturnValue:
+            # Go ahead and just convert directly into our actual return value
+            declType = CGWrapper(declType, post="&")
+            declArgs = "aRetVal"
+        elif typeNeedsRooting(valueType):
+            if not isMember:
+                holderType = CGTemplatedType(
+                    "RecordRooter", [recordKeyDeclType(recordType), valueInfo.declType]
+                )
+                # If our record is nullable, this will set the Nullable to be
+                # not-null, but that's ok because we make an explicit SetNull() call
+                # on it as needed if our JS value is actually null.
+                holderArgs = "cx, &%s" % recordRef
+            elif isMember == "Union":
+                declArgs = "cx"
+
+        return JSToNativeConversionInfo(
+            templateBody,
+            declType=declType,
+            declArgs=declArgs,
+            holderType=holderType,
+            dealWithOptional=isOptional,
+            holderArgs=holderArgs,
+        )
+
+    if type.isUnion():
+        nullable = type.nullable()
+        if nullable:
+            type = type.inner
+
+        isOwningUnion = (isMember and isMember != "Union") or isCallbackReturnValue
+        unionArgumentObj = "${declName}"
+        if nullable:
+            if isOptional and not isOwningUnion:
+                unionArgumentObj += ".Value()"
+            # If we're owning, we're a Nullable, which hasn't been told it has
+            # a value.  Otherwise we're an already-constructed Maybe.
+            unionArgumentObj += ".SetValue()"
+
+        templateBody = CGIfWrapper(
+            CGGeneric(exceptionCode),
+            '!%s.Init(cx, ${val}, "%s", ${passedToJSImpl})'
+            % (unionArgumentObj, firstCap(sourceDescription)),
+        )
+
+        if type.hasNullableType:
+            assert not nullable
+            # Make sure to handle a null default value here
+            if defaultValue and isinstance(defaultValue, IDLNullValue):
+                assert defaultValue.type == type
+                templateBody = CGIfElseWrapper(
+                    "!(${haveValue})",
+                    CGGeneric("%s.SetNull();\n" % unionArgumentObj),
+                    templateBody,
+                )
+
+        typeName = CGUnionStruct.unionTypeDecl(type, isOwningUnion)
+        argumentTypeName = typeName + "Argument"
+        if nullable:
+            typeName = "Nullable<" + typeName + " >"
+
+        declType = CGGeneric(typeName)
+        if isOwningUnion:
+            holderType = None
+        else:
+            holderType = CGGeneric(argumentTypeName)
+            if nullable:
+                holderType = CGTemplatedType("Maybe", holderType)
+
+        # If we're isOptional and not nullable the normal optional handling will
+        # handle lazy construction of our holder.  If we're nullable and not
+        # owning we do it all by hand because we do not want our holder
+        # constructed if we're null.  But if we're owning we don't have a
+        # holder anyway, so we can do the normal Optional codepath.
+        declLoc = "${declName}"
+        constructDecl = None
+        if nullable:
+            if isOptional and not isOwningUnion:
+                declType = CGTemplatedType("Optional", declType)
+                constructDecl = CGGeneric("${declName}.Construct();\n")
+                declLoc = "${declName}.Value()"
+
+        if not isMember and isCallbackReturnValue:
+            declType = CGWrapper(declType, post="&")
+            declArgs = "aRetVal"
+        else:
+            declArgs = None
+
+        if (
+            defaultValue
+            and not isinstance(defaultValue, IDLNullValue)
+            and not isinstance(defaultValue, IDLDefaultDictionaryValue)
+        ):
+            tag = defaultValue.type.tag()
+
+            if tag in numericSuffixes or tag is IDLType.Tags.bool:
+                defaultStr = getHandleDefault(defaultValue)
+                # Make sure we actually construct the thing inside the nullable.
+                value = declLoc + (".SetValue()" if nullable else "")
+                name = getUnionMemberName(defaultValue.type)
+                default = CGGeneric(
+                    "%s.RawSetAs%s() = %s;\n" % (value, name, defaultStr)
+                )
+            elif isinstance(defaultValue, IDLEmptySequenceValue):
+                name = getUnionMemberName(defaultValue.type)
+                # Make sure we actually construct the thing inside the nullable.
+                value = declLoc + (".SetValue()" if nullable else "")
+                if not isOwningUnion and typeNeedsRooting(defaultValue.type):
+                    ctorArgs = "cx"
+                else:
+                    ctorArgs = ""
+                # It's enough to set us to the right type; that will
+                # create an empty array, which is all we need here.
+                default = CGGeneric("%s.RawSetAs%s(%s);\n" % (value, name, ctorArgs))
+            elif defaultValue.type.isEnum():
+                name = getUnionMemberName(defaultValue.type)
+                # Make sure we actually construct the thing inside the nullable.
+                value = declLoc + (".SetValue()" if nullable else "")
+                default = CGGeneric(
+                    "%s.RawSetAs%s() = %s::%s;\n"
+                    % (
+                        value,
+                        name,
+                        defaultValue.type.inner.identifier.name,
+                        getEnumValueName(defaultValue.value),
+                    )
+                )
+            else:
+                default = CGGeneric(
+                    handleDefaultStringValue(
+                        defaultValue, "%s.SetStringLiteral" % unionArgumentObj
+                    )
+                )
+
+            templateBody = CGIfElseWrapper("!(${haveValue})", default, templateBody)
+
+        if nullable:
+            assert not type.hasNullableType
+            if defaultValue:
+                if isinstance(defaultValue, IDLNullValue):
+                    extraConditionForNull = "!(${haveValue}) || "
+                else:
+                    extraConditionForNull = "(${haveValue}) && "
+            else:
+                extraConditionForNull = ""
+
+            hasUndefinedType = any(t.isUndefined() for t in type.flatMemberTypes)
+            assert not hasUndefinedType or defaultValue is None
+
+            nullTest = (
+                "${val}.isNull()" if hasUndefinedType else "${val}.isNullOrUndefined()"
+            )
+            templateBody = CGIfElseWrapper(
+                extraConditionForNull + nullTest,
+                CGGeneric("%s.SetNull();\n" % declLoc),
+                templateBody,
+            )
+        elif (
+            not type.hasNullableType
+            and defaultValue
+            and isinstance(defaultValue, IDLDefaultDictionaryValue)
+        ):
+            assert type.hasDictionaryType()
+            assert defaultValue.type.isDictionary()
+            if not isOwningUnion and typeNeedsRooting(defaultValue.type):
+                ctorArgs = "cx"
+            else:
+                ctorArgs = ""
+            initDictionaryWithNull = CGIfWrapper(
+                CGGeneric("return false;\n"),
+                (
+                    '!%s.RawSetAs%s(%s).Init(cx, JS::NullHandleValue, "Member of %s")'
+                    % (
+                        declLoc,
+                        getUnionMemberName(defaultValue.type),
+                        ctorArgs,
+                        type.prettyName(),
+                    )
+                ),
+            )
+            templateBody = CGIfElseWrapper(
+                "!(${haveValue})", initDictionaryWithNull, templateBody
+            )
+
+        templateBody = CGList([constructDecl, templateBody])
+
+        return JSToNativeConversionInfo(
+            templateBody.define(),
+            declType=declType,
+            declArgs=declArgs,
+            dealWithOptional=isOptional and (not nullable or isOwningUnion),
+        )
+
+    if type.isPromise():
+        assert not type.nullable()
+        assert defaultValue is None
+
+        # We always have to hold a strong ref to Promise here, because
+        # Promise::resolve returns an addrefed thing.
+        argIsPointer = isCallbackReturnValue
+        if argIsPointer:
+            declType = CGGeneric("RefPtr<Promise>")
+        else:
+            declType = CGGeneric("OwningNonNull<Promise>")
+
+        # Per spec, what we're supposed to do is take the original
+        # Promise.resolve and call it with the original Promise as this
+        # value to make a Promise out of whatever value we actually have
+        # here.  The question is which global we should use.  There are
+        # several cases to consider:
+        #
+        # 1) Normal call to API with a Promise argument.  This is a case the
+        #    spec covers, and we should be using the current Realm's
+        #    Promise.  That means the current compartment.
+        # 2) Call to API with a Promise argument over Xrays.  In practice,
+        #    this sort of thing seems to be used for giving an API
+        #    implementation a way to wait for conclusion of an asyc
+        #    operation, _not_ to expose the Promise to content code.  So we
+        #    probably want to allow callers to use such an API in a
+        #    "natural" way, by passing chrome-side promises; indeed, that
+        #    may be all that the caller has to represent their async
+        #    operation.  That means we really need to do the
+        #    Promise.resolve() in the caller (chrome) compartment: if we do
+        #    it in the content compartment, we will try to call .then() on
+        #    the chrome promise while in the content compartment, which will
+        #    throw and we'll just get a rejected Promise.  Note that this is
+        #    also the reason why a caller who has a chrome Promise
+        #    representing an async operation can't itself convert it to a
+        #    content-side Promise (at least not without some serious
+        #    gyrations).
+        # 3) Promise return value from a callback or callback interface.
+        #    Per spec, this should use the Realm of the callback object.  In
+        #    our case, that's the compartment of the underlying callback,
+        #    not the current compartment (which may be the compartment of
+        #    some cross-compartment wrapper around said callback).
+        # 4) Return value from a JS-implemented interface.  In this case we
+        #    have a problem.  Our current compartment is the compartment of
+        #    the JS implementation.  But if the JS implementation returned
+        #    a page-side Promise (which is a totally sane thing to do, and
+        #    in fact the right thing to do given that this return value is
+        #    going right to content script) then we don't want to
+        #    Promise.resolve with our current compartment Promise, because
+        #    that will wrap it up in a chrome-side Promise, which is
+        #    decidedly _not_ what's desired here.  So in that case we
+        #    should really unwrap the return value and use the global of
+        #    the result.  CheckedUnwrapStatic should be good enough for that;
+        #    if it fails, then we're failing unwrap while in a
+        #    system-privileged compartment, so presumably we have a dead
+        #    object wrapper.  Just error out.  Do NOT fall back to using
+        #    the current compartment instead: that will return a
+        #    system-privileged rejected (because getting .then inside
+        #    resolve() failed) Promise to the caller, which they won't be
+        #    able to touch.  That's not helpful.  If we error out, on the
+        #    other hand, they will get a content-side rejected promise.
+        #    Same thing if the value returned is not even an object.
+        if isCallbackReturnValue == "JSImpl":
+            # Case 4 above.  Note that globalObj defaults to the current
+            # compartment global.  Note that we don't use $*{exceptionCode}
+            # here because that will try to aRv.Throw(NS_ERROR_UNEXPECTED)
+            # which we don't really want here.
+            assert exceptionCode == "aRv.Throw(NS_ERROR_UNEXPECTED);\nreturn nullptr;\n"
+            getPromiseGlobal = fill(
+                """
+                if (!$${val}.isObject()) {
+                  aRv.ThrowTypeError<MSG_NOT_OBJECT>("${sourceDescription}");
+                  return nullptr;
+                }
+                JSObject* unwrappedVal = js::CheckedUnwrapStatic(&$${val}.toObject());
+                if (!unwrappedVal) {
+                  // A slight lie, but not much of one, for a dead object wrapper.
+                  aRv.ThrowTypeError<MSG_NOT_OBJECT>("${sourceDescription}");
+                  return nullptr;
+                }
+                globalObj = JS::GetNonCCWObjectGlobal(unwrappedVal);
+                """,
+                sourceDescription=sourceDescription,
+            )
+        elif isCallbackReturnValue == "Callback":
+            getPromiseGlobal = dedent(
+                """
+                // We basically want our entry global here.  Play it safe
+                // and use GetEntryGlobal() to get it, with whatever
+                // principal-clamping it ends up doing.
+                globalObj = GetEntryGlobal()->GetGlobalJSObject();
+                """
+            )
+        else:
+            getPromiseGlobal = dedent(
+                """
+                globalObj = JS::CurrentGlobalOrNull(cx);
+                """
+            )
+
+        templateBody = fill(
+            """
+            { // Scope for our GlobalObject, FastErrorResult, JSAutoRealm,
+              // etc.
+
+              JS::Rooted<JSObject*> globalObj(cx);
+              $*{getPromiseGlobal}
+              JSAutoRealm ar(cx, globalObj);
+              GlobalObject promiseGlobal(cx, globalObj);
+              if (promiseGlobal.Failed()) {
+                $*{exceptionCode}
+              }
+
+              JS::Rooted<JS::Value> valueToResolve(cx, $${val});
+              if (!JS_WrapValue(cx, &valueToResolve)) {
+                $*{exceptionCode}
+              }
+              binding_detail::FastErrorResult promiseRv;
+              nsCOMPtr<nsIGlobalObject> global =
+                do_QueryInterface(promiseGlobal.GetAsSupports());
+              if (!global) {
+                promiseRv.Throw(NS_ERROR_UNEXPECTED);
+                MOZ_ALWAYS_TRUE(promiseRv.MaybeSetPendingException(cx));
+                $*{exceptionCode}
+              }
+              $${declName} = Promise::Resolve(global, cx, valueToResolve,
+                                              promiseRv);
+              if (promiseRv.MaybeSetPendingException(cx)) {
+                $*{exceptionCode}
+              }
+            }
+            """,
+            getPromiseGlobal=getPromiseGlobal,
+            exceptionCode=exceptionCode,
+        )
+
+        return JSToNativeConversionInfo(
+            templateBody, declType=declType, dealWithOptional=isOptional
+        )
+
+    if type.isGeckoInterface():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        descriptor = descriptorProvider.getDescriptor(
+            type.unroll().inner.identifier.name
+        )
+
+        assert descriptor.nativeType != "JSObject"
+
+        if descriptor.interface.isCallback():
+            (declType, declArgs, conversion) = getCallbackConversionInfo(
+                type, descriptor.interface, isMember, isCallbackReturnValue, isOptional
+            )
+            template = wrapObjectTemplate(
+                conversion, type, "${declName} = nullptr;\n", failureCode
+            )
+            return JSToNativeConversionInfo(
+                template,
+                declType=declType,
+                declArgs=declArgs,
+                dealWithOptional=isOptional,
+            )
+
+        if descriptor.interface.identifier.name == "WindowProxy":
+            declType = CGGeneric("mozilla::dom::WindowProxyHolder")
+            if type.nullable():
+                declType = CGTemplatedType("Nullable", declType)
+                windowProxyHolderRef = "${declName}.SetValue()"
+            else:
+                windowProxyHolderRef = "${declName}"
+
+            failureCode = onFailureBadType(
+                failureCode, descriptor.interface.identifier.name
+            ).define()
+            templateBody = fill(
+                """
+                JS::Rooted<JSObject*> source(cx, &$${val}.toObject());
+                if (NS_FAILED(UnwrapWindowProxyArg(cx, source, ${windowProxyHolderRef}))) {
+                    $*{onFailure}
+                }
+                """,
+                windowProxyHolderRef=windowProxyHolderRef,
+                onFailure=failureCode,
+            )
+            templateBody = wrapObjectTemplate(
+                templateBody, type, "${declName}.SetNull();\n", failureCode
+            )
+            return JSToNativeConversionInfo(
+                templateBody, declType=declType, dealWithOptional=isOptional
+            )
+
+        # This is an interface that we implement as a concrete class
+        # or an XPCOM interface.
+
+        # Allow null pointers for nullable types and old-binding classes, and
+        # use an RefPtr or raw pointer for callback return values to make
+        # them easier to return.
+        argIsPointer = (
+            type.nullable() or type.unroll().inner.isExternal() or isCallbackReturnValue
+        )
+
+        # Sequence and dictionary members, as well as owning unions (which can
+        # appear here as return values in JS-implemented interfaces) have to
+        # hold a strong ref to the thing being passed down.  Those all set
+        # isMember.
+        #
+        # Also, callback return values always end up addrefing anyway, so there
+        # is no point trying to avoid it here and it makes other things simpler
+        # since we can assume the return value is a strong ref.
+        assert not descriptor.interface.isCallback()
+        forceOwningType = (isMember and isMember != "Union") or isCallbackReturnValue
+
+        typeName = descriptor.nativeType
+        typePtr = typeName + "*"
+
+        # Compute a few things:
+        #  - declType is the type we want to return as the first element of our
+        #    tuple.
+        #  - holderType is the type we want to return as the third element
+        #    of our tuple.
+
+        # Set up some sensible defaults for these things insofar as we can.
+        holderType = None
+        if argIsPointer:
+            if forceOwningType:
+                declType = "RefPtr<" + typeName + ">"
+            else:
+                declType = typePtr
+        else:
+            if forceOwningType:
+                declType = "OwningNonNull<" + typeName + ">"
+            else:
+                declType = "NonNull<" + typeName + ">"
+
+        templateBody = ""
+        if forceOwningType:
+            templateBody += fill(
+                """
+                static_assert(IsRefcounted<${typeName}>::value, "We can only store refcounted classes.");
+                """,
+                typeName=typeName,
+            )
+
+        if not descriptor.interface.isExternal():
+            if failureCode is not None:
+                templateBody += str(
+                    CastableObjectUnwrapper(
+                        descriptor,
+                        "${val}",
+                        "${maybeMutableVal}",
+                        "${declName}",
+                        failureCode,
+                    )
+                )
+            else:
+                templateBody += str(
+                    FailureFatalCastableObjectUnwrapper(
+                        descriptor,
+                        "${val}",
+                        "${maybeMutableVal}",
+                        "${declName}",
+                        exceptionCode,
+                        isCallbackReturnValue,
+                        firstCap(sourceDescription),
+                    )
+                )
+        else:
+            # External interface.  We always have a holder for these, because we
+            # don't actually know whether we have to addref when unwrapping or not.
+            # So we just pass an getter_AddRefs(RefPtr) to XPConnect and if we'll
+            # need a release it'll put a non-null pointer in there.
+            if forceOwningType:
+                # Don't return a holderType in this case; our declName
+                # will just own stuff.
+                templateBody += "RefPtr<" + typeName + "> ${holderName};\n"
+            else:
+                holderType = "RefPtr<" + typeName + ">"
+            templateBody += (
+                "JS::Rooted<JSObject*> source(cx, &${val}.toObject());\n"
+                + "if (NS_FAILED(UnwrapArg<"
+                + typeName
+                + ">(cx, source, getter_AddRefs(${holderName})))) {\n"
+            )
+            templateBody += CGIndenter(
+                onFailureBadType(failureCode, descriptor.interface.identifier.name)
+            ).define()
+            templateBody += "}\n" "MOZ_ASSERT(${holderName});\n"
+
+            # And store our value in ${declName}
+            templateBody += "${declName} = ${holderName};\n"
+
+        # Just pass failureCode, not onFailureBadType, here, so we'll report
+        # the thing as not an object as opposed to not implementing whatever
+        # our interface is.
+        templateBody = wrapObjectTemplate(
+            templateBody, type, "${declName} = nullptr;\n", failureCode
+        )
+
+        declType = CGGeneric(declType)
+        if holderType is not None:
+            holderType = CGGeneric(holderType)
+        return JSToNativeConversionInfo(
+            templateBody,
+            declType=declType,
+            holderType=holderType,
+            dealWithOptional=isOptional,
+        )
+
+    if type.isSpiderMonkeyInterface():
+        assert not isEnforceRange and not isClamp
+        name = type.unroll().name  # unroll() because it may be nullable
+        interfaceType = CGGeneric(name)
+        declType = interfaceType
+        if type.nullable():
+            declType = CGTemplatedType("Nullable", declType)
+            objRef = "${declName}.SetValue()"
+        else:
+            objRef = "${declName}"
+
+        # Again, this is a bit strange since we are actually building a
+        # template string here. ${objRef} and $*{badType} below are filled in
+        # right now; $${val} expands to ${val}, to be filled in later.
+        template = fill(
+            """
+            if (!${objRef}.Init(&$${val}.toObject())) {
+              $*{badType}
+            }
+            """,
+            objRef=objRef,
+            badType=onFailureBadType(failureCode, type.name).define(),
+        )
+        if type.isBufferSource():
+            if type.isArrayBuffer():
+                isSharedMethod = "JS::IsSharedArrayBufferObject"
+                isLargeMethod = "JS::IsLargeArrayBufferMaybeShared"
+            else:
+                assert type.isArrayBufferView() or type.isTypedArray()
+                isSharedMethod = "JS::IsArrayBufferViewShared"
+                isLargeMethod = "JS::IsLargeArrayBufferView"
+            if not isAllowShared:
+                template += fill(
+                    """
+                    if (${isSharedMethod}(${objRef}.Obj())) {
+                      $*{badType}
+                    }
+                    """,
+                    isSharedMethod=isSharedMethod,
+                    objRef=objRef,
+                    badType=onFailureIsShared().define(),
+                )
+            # For now reject large (> 2 GB) ArrayBuffers and ArrayBufferViews.
+            # Supporting this will require changing dom::TypedArray and
+            # consumers.
+            template += fill(
+                """
+                if (${isLargeMethod}(${objRef}.Obj())) {
+                  $*{badType}
+                }
+                """,
+                isLargeMethod=isLargeMethod,
+                objRef=objRef,
+                badType=onFailureIsLarge().define(),
+            )
+        template = wrapObjectTemplate(
+            template, type, "${declName}.SetNull();\n", failureCode
+        )
+        if not isMember or isMember == "Union":
+            # This is a bit annoying.  In a union we don't want to have a
+            # holder, since unions don't support that.  But if we're optional we
+            # want to have a holder, so that the callee doesn't see
+            # Optional<RootedSpiderMonkeyInterface<InterfaceType>>.  So do a
+            # holder if we're optional and use a RootedSpiderMonkeyInterface
+            # otherwise.
+            if isOptional:
+                holderType = CGTemplatedType(
+                    "SpiderMonkeyInterfaceRooter", interfaceType
+                )
+                # If our SpiderMonkey interface is nullable, this will set the
+                # Nullable to be not-null, but that's ok because we make an
+                # explicit SetNull() call on it as needed if our JS value is
+                # actually null.  XXXbz Because "Maybe" takes const refs for
+                # constructor arguments, we can't pass a reference here; have
+                # to pass a pointer.
+                holderArgs = "cx, &%s" % objRef
+                declArgs = None
+            else:
+                holderType = None
+                holderArgs = None
+                declType = CGTemplatedType("RootedSpiderMonkeyInterface", declType)
+                declArgs = "cx"
+        else:
+            holderType = None
+            holderArgs = None
+            declArgs = None
+        return JSToNativeConversionInfo(
+            template,
+            declType=declType,
+            holderType=holderType,
+            dealWithOptional=isOptional,
+            declArgs=declArgs,
+            holderArgs=holderArgs,
+        )
+
+    if type.isJSString():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+        if type.nullable():
+            raise TypeError("Nullable JSString not supported")
+
+        declArgs = "cx"
+        if isMember:
+            raise TypeError("JSString not supported as member")
+        else:
+            declType = "JS::Rooted<JSString*>"
+
+        if isOptional:
+            raise TypeError("JSString not supported as optional")
+        templateBody = fill(
+            """
+                if (!($${declName} = ConvertJSValueToJSString(cx, $${val}))) {
+                  $*{exceptionCode}
+                }
+                """,
+            exceptionCode=exceptionCode,
+        )
+
+        if defaultValue is not None:
+            assert not isinstance(defaultValue, IDLNullValue)
+            defaultCode = fill(
+                """
+                static const char data[] = { ${data} };
+                $${declName} = JS_NewStringCopyN(cx, data, ArrayLength(data) - 1);
+                if (!$${declName}) {
+                    $*{exceptionCode}
+                }
+                """,
+                data=", ".join(
+                    ["'" + char + "'" for char in defaultValue.value] + ["0"]
+                ),
+                exceptionCode=exceptionCode,
+            )
+
+            templateBody = handleDefault(templateBody, defaultCode)
+        return JSToNativeConversionInfo(
+            templateBody, declType=CGGeneric(declType), declArgs=declArgs
+        )
+
+    if type.isDOMString() or type.isUSVString() or type.isUTF8String():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        treatAs = {
+            "Default": "eStringify",
+            "EmptyString": "eEmpty",
+            "Null": "eNull",
+        }
+        if type.nullable():
+            # For nullable strings null becomes a null string.
+            treatNullAs = "Null"
+            # For nullable strings undefined also becomes a null string.
+            undefinedBehavior = "eNull"
+        else:
+            undefinedBehavior = "eStringify"
+            if type.legacyNullToEmptyString:
+                treatNullAs = "EmptyString"
+            else:
+                treatNullAs = "Default"
+        nullBehavior = treatAs[treatNullAs]
+
+        def getConversionCode(varName):
+            normalizeCode = ""
+            if type.isUSVString():
+                normalizeCode = fill(
+                    """
+                    if (!NormalizeUSVString(${var})) {
+                      JS_ReportOutOfMemory(cx);
+                      $*{exceptionCode}
+                    }
+                    """,
+                    var=varName,
+                    exceptionCode=exceptionCode,
+                )
+
+            conversionCode = fill(
+                """
+                if (!ConvertJSValueToString(cx, $${val}, ${nullBehavior}, ${undefinedBehavior}, ${varName})) {
+                  $*{exceptionCode}
+                }
+                $*{normalizeCode}
+                """,
+                nullBehavior=nullBehavior,
+                undefinedBehavior=undefinedBehavior,
+                varName=varName,
+                exceptionCode=exceptionCode,
+                normalizeCode=normalizeCode,
+            )
+
+            if defaultValue is None:
+                return conversionCode
+
+            if isinstance(defaultValue, IDLNullValue):
+                assert type.nullable()
+                defaultCode = "%s.SetIsVoid(true);\n" % varName
+            else:
+                defaultCode = handleDefaultStringValue(
+                    defaultValue, "%s.AssignLiteral" % varName
+                )
+            return handleDefault(conversionCode, defaultCode)
+
+        if isMember and isMember != "Union":
+            # Convert directly into the ns[C]String member we have.
+            if type.isUTF8String():
+                declType = "nsCString"
+            else:
+                declType = "nsString"
+            return JSToNativeConversionInfo(
+                getConversionCode("${declName}"),
+                declType=CGGeneric(declType),
+                dealWithOptional=isOptional,
+            )
+
+        if isOptional:
+            if type.isUTF8String():
+                declType = "Optional<nsACString>"
+                holderType = CGGeneric("binding_detail::FakeString<char>")
+            else:
+                declType = "Optional<nsAString>"
+                holderType = CGGeneric("binding_detail::FakeString<char16_t>")
+            conversionCode = "%s" "${declName} = &${holderName};\n" % getConversionCode(
+                "${holderName}"
+            )
+        else:
+            if type.isUTF8String():
+                declType = "binding_detail::FakeString<char>"
+            else:
+                declType = "binding_detail::FakeString<char16_t>"
+            holderType = None
+            conversionCode = getConversionCode("${declName}")
+
+        # No need to deal with optional here; we handled it already
+        return JSToNativeConversionInfo(
+            conversionCode, declType=CGGeneric(declType), holderType=holderType
+        )
+
+    if type.isByteString():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        nullable = toStringBool(type.nullable())
+
+        conversionCode = fill(
+            """
+            if (!ConvertJSValueToByteString(cx, $${val}, ${nullable}, "${sourceDescription}", $${declName})) {
+              $*{exceptionCode}
+            }
+            """,
+            nullable=nullable,
+            sourceDescription=sourceDescription,
+            exceptionCode=exceptionCode,
+        )
+
+        if defaultValue is not None:
+            if isinstance(defaultValue, IDLNullValue):
+                assert type.nullable()
+                defaultCode = "${declName}.SetIsVoid(true);\n"
+            else:
+                defaultCode = handleDefaultStringValue(
+                    defaultValue, "${declName}.AssignLiteral"
+                )
+            conversionCode = handleDefault(conversionCode, defaultCode)
+
+        return JSToNativeConversionInfo(
+            conversionCode, declType=CGGeneric("nsCString"), dealWithOptional=isOptional
+        )
+
+    if type.isEnum():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        enumName = type.unroll().inner.identifier.name
+        declType = CGGeneric(enumName)
+        if type.nullable():
+            declType = CGTemplatedType("Nullable", declType)
+            declType = declType.define()
+            enumLoc = "${declName}.SetValue()"
+        else:
+            enumLoc = "${declName}"
+            declType = declType.define()
+
+        if invalidEnumValueFatal:
+            handleInvalidEnumValueCode = "MOZ_ASSERT(index >= 0);\n"
+        else:
+            # invalidEnumValueFatal is false only for attributes.  So we won't
+            # have a non-default exceptionCode here unless attribute "arg
+            # conversion" code starts passing in an exceptionCode.  At which
+            # point we'll need to figure out what that even means.
+            assert exceptionCode == "return false;\n"
+            handleInvalidEnumValueCode = dedent(
+                """
+                if (index < 0) {
+                  return true;
+                }
+                """
+            )
+
+        template = fill(
+            """
+            {
+              int index;
+              if (!FindEnumStringIndex<${invalidEnumValueFatal}>(cx, $${val}, ${values}, "${enumtype}", "${sourceDescription}", &index)) {
+                $*{exceptionCode}
+              }
+              $*{handleInvalidEnumValueCode}
+              ${enumLoc} = static_cast<${enumtype}>(index);
+            }
+            """,
+            enumtype=enumName,
+            values=enumName + "Values::" + ENUM_ENTRY_VARIABLE_NAME,
+            invalidEnumValueFatal=toStringBool(invalidEnumValueFatal),
+            handleInvalidEnumValueCode=handleInvalidEnumValueCode,
+            exceptionCode=exceptionCode,
+            enumLoc=enumLoc,
+            sourceDescription=sourceDescription,
+        )
+
+        setNull = "${declName}.SetNull();\n"
+
+        if type.nullable():
+            template = CGIfElseWrapper(
+                "${val}.isNullOrUndefined()", CGGeneric(setNull), CGGeneric(template)
+            ).define()
+
+        if defaultValue is not None:
+            if isinstance(defaultValue, IDLNullValue):
+                assert type.nullable()
+                template = handleDefault(template, setNull)
+            else:
+                assert defaultValue.type.tag() == IDLType.Tags.domstring
+                template = handleDefault(
+                    template,
+                    (
+                        "%s = %s::%s;\n"
+                        % (enumLoc, enumName, getEnumValueName(defaultValue.value))
+                    ),
+                )
+        return JSToNativeConversionInfo(
+            template, declType=CGGeneric(declType), dealWithOptional=isOptional
+        )
+
+    if type.isCallback():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+        assert not type.treatNonCallableAsNull() or type.nullable()
+        assert not type.treatNonObjectAsNull() or type.nullable()
+        assert not type.treatNonObjectAsNull() or not type.treatNonCallableAsNull()
+
+        callback = type.unroll().callback
+        name = callback.identifier.name
+        (declType, declArgs, conversion) = getCallbackConversionInfo(
+            type, callback, isMember, isCallbackReturnValue, isOptional
+        )
+
+        if allowTreatNonCallableAsNull and type.treatNonCallableAsNull():
+            haveCallable = "JS::IsCallable(&${val}.toObject())"
+            if not isDefinitelyObject:
+                haveCallable = "${val}.isObject() && " + haveCallable
+            if defaultValue is not None:
+                assert isinstance(defaultValue, IDLNullValue)
+                haveCallable = "(${haveValue}) && " + haveCallable
+            template = (
+                ("if (%s) {\n" % haveCallable) + conversion + "} else {\n"
+                "  ${declName} = nullptr;\n"
+                "}\n"
+            )
+        elif allowTreatNonCallableAsNull and type.treatNonObjectAsNull():
+            if not isDefinitelyObject:
+                haveObject = "${val}.isObject()"
+                if defaultValue is not None:
+                    assert isinstance(defaultValue, IDLNullValue)
+                    haveObject = "(${haveValue}) && " + haveObject
+                template = CGIfElseWrapper(
+                    haveObject,
+                    CGGeneric(conversion),
+                    CGGeneric("${declName} = nullptr;\n"),
+                ).define()
+            else:
+                template = conversion
+        else:
+            template = wrapObjectTemplate(
+                "if (JS::IsCallable(&${val}.toObject())) {\n"
+                + conversion
+                + "} else {\n"
+                + indent(onFailureNotCallable(failureCode).define())
+                + "}\n",
+                type,
+                "${declName} = nullptr;\n",
+                failureCode,
+            )
+        return JSToNativeConversionInfo(
+            template, declType=declType, declArgs=declArgs, dealWithOptional=isOptional
+        )
+
+    if type.isAny():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+
+        declArgs = None
+        if isMember in ("Variadic", "Sequence", "Dictionary", "Record"):
+            # Rooting is handled by the sequence and dictionary tracers.
+            declType = "JS::Value"
+        else:
+            assert not isMember
+            declType = "JS::Rooted<JS::Value>"
+            declArgs = "cx"
+
+        assert not isOptional
+        templateBody = "${declName} = ${val};\n"
+
+        # For JS-implemented APIs, we refuse to allow passing objects that the
+        # API consumer does not subsume. The extra parens around
+        # ($${passedToJSImpl}) suppress unreachable code warnings when
+        # $${passedToJSImpl} is the literal `false`.  But Apple is shipping a
+        # buggy clang (clang 3.9) in Xcode 8.3, so there even the parens are not
+        # enough.  So we manually disable some warnings in clang.
+        if (
+            not isinstance(descriptorProvider, Descriptor)
+            or descriptorProvider.interface.isJSImplemented()
+        ):
+            templateBody = (
+                fill(
+                    """
+                #ifdef __clang__
+                #pragma clang diagnostic push
+                #pragma clang diagnostic ignored "-Wunreachable-code"
+                #pragma clang diagnostic ignored "-Wunreachable-code-return"
+                #endif // __clang__
+                if (($${passedToJSImpl}) && !CallerSubsumes($${val})) {
+                  cx.ThrowErrorMessage<MSG_PERMISSION_DENIED_TO_PASS_ARG>("${sourceDescription}");
+                  $*{exceptionCode}
+                }
+                #ifdef __clang__
+                #pragma clang diagnostic pop
+                #endif // __clang__
+                """,
+                    sourceDescription=sourceDescription,
+                    exceptionCode=exceptionCode,
+                )
+                + templateBody
+            )
+
+        # We may not have a default value if we're being converted for
+        # a setter, say.
+        if defaultValue:
+            if isinstance(defaultValue, IDLNullValue):
+                defaultHandling = "${declName} = JS::NullValue();\n"
+            else:
+                assert isinstance(defaultValue, IDLUndefinedValue)
+                defaultHandling = "${declName} = JS::UndefinedValue();\n"
+            templateBody = handleDefault(templateBody, defaultHandling)
+        return JSToNativeConversionInfo(
+            templateBody, declType=CGGeneric(declType), declArgs=declArgs
+        )
+
+    if type.isObject():
+        assert not isEnforceRange and not isClamp and not isAllowShared
+        return handleJSObjectType(
+            type, isMember, failureCode, exceptionCode, sourceDescription
+        )
+
+    if type.isDictionary():
+        # There are no nullable dictionary-typed arguments or dictionary-typed
+        # dictionary members.
+        assert (
+            not type.nullable()
+            or isCallbackReturnValue
+            or (isMember and isMember != "Dictionary")
+        )
+        # All optional dictionary-typed arguments always have default values,
+        # but dictionary-typed dictionary members can be optional.
+        assert not isOptional or isMember == "Dictionary"
+        # In the callback return value case we never have to worry
+        # about a default value; we always have a value.
+        assert not isCallbackReturnValue or defaultValue is None
+
+        typeName = CGDictionary.makeDictionaryName(type.unroll().inner)
+        if (not isMember or isMember == "Union") and not isCallbackReturnValue:
+            # Since we're not a member and not nullable or optional, no one will
+            # see our real type, so we can do the fast version of the dictionary
+            # that doesn't pre-initialize members.
+            typeName = "binding_detail::Fast" + typeName
+
+        declType = CGGeneric(typeName)
+
+        # We do manual default value handling here, because we actually do want
+        # a jsval, and we only handle the default-dictionary case (which we map
+        # into initialization with the JS value `null`) anyway
+        # NOTE: if isNullOrUndefined or isDefinitelyObject are true,
+        # we know we have a value, so we don't have to worry about the
+        # default value.
+        if (
+            not isNullOrUndefined
+            and not isDefinitelyObject
+            and defaultValue is not None
+        ):
+            assert isinstance(defaultValue, IDLDefaultDictionaryValue)
+            # Initializing from JS null does the right thing to give
+            # us a default-initialized dictionary.
+            val = "(${haveValue}) ? ${val} : JS::NullHandleValue"
+        else:
+            val = "${val}"
+
+        dictLoc = "${declName}"
+        if type.nullable():
+            dictLoc += ".SetValue()"
+
+        if type.unroll().inner.needsConversionFromJS:
+            args = "cx, %s, " % val
+        else:
+            # We can end up in this case if a dictionary that does not need
+            # conversion from JS has a dictionary-typed member with a default
+            # value of {}.
+            args = ""
+        conversionCode = fill(
+            """
+            if (!${dictLoc}.Init(${args}"${desc}", $${passedToJSImpl})) {
+              $*{exceptionCode}
+            }
+            """,
+            dictLoc=dictLoc,
+            args=args,
+            desc=firstCap(sourceDescription),
+            exceptionCode=exceptionCode,
+        )
+
+        if failureCode is not None:
+            # This means we're part of an overload or union conversion, and
+            # should simply skip stuff if our value is not convertible to
+            # dictionary, instead of trying and throwing.  If we're either
+            # isDefinitelyObject or isNullOrUndefined then we're convertible to
+            # dictionary and don't need to check here.
+            if isDefinitelyObject or isNullOrUndefined:
+                template = conversionCode
+            else:
+                template = fill(
+                    """
+                    if (!IsConvertibleToDictionary(${val})) {
+                      $*{failureCode}
+                    }
+                    $*{conversionCode}
+                    """,
+                    val=val,
+                    failureCode=failureCode,
+                    conversionCode=conversionCode,
+                )
+        else:
+            template = conversionCode
+
+        if type.nullable():
+            declType = CGTemplatedType("Nullable", declType)
+            template = CGIfElseWrapper(
+                "${val}.isNullOrUndefined()",
+                CGGeneric("${declName}.SetNull();\n"),
+                CGGeneric(template),
+            ).define()
+
+        # Dictionary arguments that might contain traceable things need to get
+        # traced
+        if (not isMember or isMember == "Union") and isCallbackReturnValue:
+            # Go ahead and just convert directly into our actual return value
+            declType = CGWrapper(declType, post="&")
+            declArgs = "aRetVal"
+        elif (not isMember or isMember == "Union") and typeNeedsRooting(type):
+            declType = CGTemplatedType("RootedDictionary", declType)
+            declArgs = "cx"
+        else:
+            declArgs = None
+
+        return JSToNativeConversionInfo(
+            template, declType=declType, declArgs=declArgs, dealWithOptional=isOptional
+        )
+
+    if type.isUndefined():
+        assert not isOptional
+        # This one only happens for return values, and its easy: Just
+        # ignore the jsval.
+        return JSToNativeConversionInfo("")
+
+    if not type.isPrimitive():
+        raise TypeError("Need conversion for argument type '%s'" % str(type))
+
+    typeName = builtinNames[type.tag()]
+
+    conversionBehavior = "eDefault"
+    if isEnforceRange:
+        assert type.isInteger()
+        conversionBehavior = "eEnforceRange"
+    elif isClamp:
+        assert type.isInteger()
+        conversionBehavior = "eClamp"
+
+    alwaysNull = False
+    if type.nullable():
+        declType = CGGeneric("Nullable<" + typeName + ">")
+        writeLoc = "${declName}.SetValue()"
+        readLoc = "${declName}.Value()"
+        nullCondition = "${val}.isNullOrUndefined()"
+        if defaultValue is not None and isinstance(defaultValue, IDLNullValue):
+            nullCondition = "!(${haveValue}) || " + nullCondition
+            if isKnownMissing:
+                alwaysNull = True
+                template = dedent(
+                    """
+                    ${declName}.SetNull();
+                    """
+                )
+        if not alwaysNull:
+            template = fill(
+                """
+                if (${nullCondition}) {
+                  $${declName}.SetNull();
+                } else if (!ValueToPrimitive<${typeName}, ${conversionBehavior}>(cx, $${val}, "${sourceDescription}", &${writeLoc})) {
+                  $*{exceptionCode}
+                }
+                """,
+                nullCondition=nullCondition,
+                typeName=typeName,
+                conversionBehavior=conversionBehavior,
+                sourceDescription=firstCap(sourceDescription),
+                writeLoc=writeLoc,
+                exceptionCode=exceptionCode,
+            )
+    else:
+        assert defaultValue is None or not isinstance(defaultValue, IDLNullValue)
+        writeLoc = "${declName}"
+        readLoc = writeLoc
+        template = fill(
+            """
+            if (!ValueToPrimitive<${typeName}, ${conversionBehavior}>(cx, $${val}, "${sourceDescription}", &${writeLoc})) {
+              $*{exceptionCode}
+            }
+            """,
+            typeName=typeName,
+            conversionBehavior=conversionBehavior,
+            sourceDescription=firstCap(sourceDescription),
+            writeLoc=writeLoc,
+            exceptionCode=exceptionCode,
+        )
+        declType = CGGeneric(typeName)
+
+    if type.isFloat() and not type.isUnrestricted() and not alwaysNull:
+        if lenientFloatCode is not None:
+            nonFiniteCode = lenientFloatCode
+        else:
+            nonFiniteCode = 'cx.ThrowErrorMessage<MSG_NOT_FINITE>("%s");\n' "%s" % (
+                firstCap(sourceDescription),
+                exceptionCode,
+            )
+
+        # We're appending to an if-block brace, so strip trailing whitespace
+        # and add an extra space before the else.
+        template = template.rstrip()
+        template += fill(
+            """
+             else if (!std::isfinite(${readLoc})) {
+              $*{nonFiniteCode}
+            }
+            """,
+            readLoc=readLoc,
+            nonFiniteCode=nonFiniteCode,
+        )
+
+    if (
+        defaultValue is not None
+        and
+        # We already handled IDLNullValue, so just deal with the other ones
+        not isinstance(defaultValue, IDLNullValue)
+    ):
+        tag = defaultValue.type.tag()
+        defaultStr = getHandleDefault(defaultValue)
+        template = handleDefault(template, "%s = %s;\n" % (writeLoc, defaultStr))
+
+    return JSToNativeConversionInfo(
+        template, declType=declType, dealWithOptional=isOptional
+    )
+
+
+def instantiateJSToNativeConversion(info, replacements, checkForValue=False):
+    """
+    Take a JSToNativeConversionInfo as returned by getJSToNativeConversionInfo
+    and a set of replacements as required by the strings in such an object, and
+    generate code to convert into stack C++ types.
+
+    If checkForValue is True, then the conversion will get wrapped in
+    a check for ${haveValue}.
+    """
+    templateBody, declType, holderType, dealWithOptional = (
+        info.template,
+        info.declType,
+        info.holderType,
+        info.dealWithOptional,
+    )
+
+    if dealWithOptional and not checkForValue:
+        raise TypeError("Have to deal with optional things, but don't know how")
+    if checkForValue and declType is None:
+        raise TypeError(
+            "Need to predeclare optional things, so they will be "
+            "outside the check for big enough arg count!"
+        )
+
+    # We can't precompute our holder constructor arguments, since
+    # those might depend on ${declName}, which we change below.  Just
+    # compute arguments at the point when we need them as we go.
+    def getArgsCGThing(args):
+        return CGGeneric(string.Template(args).substitute(replacements))
+
+    result = CGList([])
+    # Make a copy of "replacements" since we may be about to start modifying it
+    replacements = dict(replacements)
+    originalDeclName = replacements["declName"]
+    if declType is not None:
+        if dealWithOptional:
+            replacements["declName"] = "%s.Value()" % originalDeclName
+            declType = CGTemplatedType("Optional", declType)
+            declCtorArgs = None
+        elif info.declArgs is not None:
+            declCtorArgs = CGWrapper(getArgsCGThing(info.declArgs), pre="(", post=")")
+        else:
+            declCtorArgs = None
+        result.append(
+            CGList(
+                [
+                    declType,
+                    CGGeneric(" "),
+                    CGGeneric(originalDeclName),
+                    declCtorArgs,
+                    CGGeneric(";\n"),
+                ]
+            )
+        )
+
+    originalHolderName = replacements["holderName"]
+    if holderType is not None:
+        if dealWithOptional:
+            replacements["holderName"] = "%s.ref()" % originalHolderName
+            holderType = CGTemplatedType("Maybe", holderType)
+            holderCtorArgs = None
+        elif info.holderArgs is not None:
+            holderCtorArgs = CGWrapper(
+                getArgsCGThing(info.holderArgs), pre="(", post=")"
+            )
+        else:
+            holderCtorArgs = None
+        result.append(
+            CGList(
+                [
+                    holderType,
+                    CGGeneric(" "),
+                    CGGeneric(originalHolderName),
+                    holderCtorArgs,
+                    CGGeneric(";\n"),
+                ]
+            )
+        )
+
+    if "maybeMutableVal" not in replacements:
+        replacements["maybeMutableVal"] = replacements["val"]
+
+    conversion = CGGeneric(string.Template(templateBody).substitute(replacements))
+
+    if checkForValue:
+        if dealWithOptional:
+            declConstruct = CGIndenter(
+                CGGeneric(
+                    "%s.Construct(%s);\n"
+                    % (
+                        originalDeclName,
+                        getArgsCGThing(info.declArgs).define() if info.declArgs else "",
+                    )
+                )
+            )
+            if holderType is not None:
+                holderConstruct = CGIndenter(
+                    CGGeneric(
+                        "%s.emplace(%s);\n"
+                        % (
+                            originalHolderName,
+                            getArgsCGThing(info.holderArgs).define()
+                            if info.holderArgs
+                            else "",
+                        )
+                    )
+                )
+            else:
+                holderConstruct = None
+        else:
+            declConstruct = None
+            holderConstruct = None
+
+        conversion = CGList(
+            [
+                CGGeneric(
+                    string.Template("if (${haveValue}) {\n").substitute(replacements)
+                ),
+                declConstruct,
+                holderConstruct,
+                CGIndenter(conversion),
+                CGGeneric("}\n"),
+            ]
+        )
+
+    result.append(conversion)
+    return result
+
+
+def convertConstIDLValueToJSVal(value):
+    if isinstance(value, IDLNullValue):
+        return "JS::NullValue()"
+    if isinstance(value, IDLUndefinedValue):
+        return "JS::UndefinedValue()"
+    tag = value.type.tag()
+    if tag in [
+        IDLType.Tags.int8,
+        IDLType.Tags.uint8,
+        IDLType.Tags.int16,
+        IDLType.Tags.uint16,
+        IDLType.Tags.int32,
+    ]:
+        return "JS::Int32Value(%s)" % (value.value)
+    if tag == IDLType.Tags.uint32:
+        return "JS::NumberValue(%sU)" % (value.value)
+    if tag in [IDLType.Tags.int64, IDLType.Tags.uint64]:
+        return "JS::CanonicalizedDoubleValue(%s)" % numericValue(tag, value.value)
+    if tag == IDLType.Tags.bool:
+        return "JS::BooleanValue(%s)" % (toStringBool(value.value))
+    if tag in [IDLType.Tags.float, IDLType.Tags.double]:
+        return "JS::CanonicalizedDoubleValue(%s)" % (value.value)
+    raise TypeError("Const value of unhandled type: %s" % value.type)
+
+
+class CGArgumentConverter(CGThing):
+    """
+    A class that takes an IDL argument object and its index in the
+    argument list and generates code to unwrap the argument to the
+    right native type.
+
+    argDescription is a description of the argument for error-reporting
+    purposes.  Callers should assume that it might get placed in the middle of a
+    sentence.  If it ends up at the beginning of a sentence, its first character
+    will be automatically uppercased.
+    """
+
+    def __init__(
+        self,
+        argument,
+        index,
+        descriptorProvider,
+        argDescription,
+        member,
+        invalidEnumValueFatal=True,
+        lenientFloatCode=None,
+    ):
+        CGThing.__init__(self)
+        self.argument = argument
+        self.argDescription = argDescription
+        assert not argument.defaultValue or argument.optional
+
+        replacer = {"index": index, "argc": "args.length()"}
+        self.replacementVariables = {
+            "declName": "arg%d" % index,
+            "holderName": ("arg%d" % index) + "_holder",
+            "obj": "obj",
+            "passedToJSImpl": toStringBool(
+                isJSImplementedDescriptor(descriptorProvider)
+            ),
+        }
+        # If we have a method generated by the maplike/setlike portion of an
+        # interface, arguments can possibly be undefined, but will need to be
+        # converted to the key/value type of the backing object. In this case,
+        # use .get() instead of direct access to the argument. This won't
+        # matter for iterable since generated functions for those interface
+        # don't take arguments.
+        if member.isMethod() and member.isMaplikeOrSetlikeOrIterableMethod():
+            self.replacementVariables["val"] = string.Template(
+                "args.get(${index})"
+            ).substitute(replacer)
+            self.replacementVariables["maybeMutableVal"] = string.Template(
+                "args[${index}]"
+            ).substitute(replacer)
+        else:
+            self.replacementVariables["val"] = string.Template(
+                "args[${index}]"
+            ).substitute(replacer)
+        haveValueCheck = string.Template("args.hasDefined(${index})").substitute(
+            replacer
+        )
+        self.replacementVariables["haveValue"] = haveValueCheck
+        self.descriptorProvider = descriptorProvider
+        if self.argument.canHaveMissingValue():
+            self.argcAndIndex = replacer
+        else:
+            self.argcAndIndex = None
+        self.invalidEnumValueFatal = invalidEnumValueFatal
+        self.lenientFloatCode = lenientFloatCode
+
+    def define(self):
+        typeConversion = getJSToNativeConversionInfo(
+            self.argument.type,
+            self.descriptorProvider,
+            isOptional=(self.argcAndIndex is not None and not self.argument.variadic),
+            invalidEnumValueFatal=self.invalidEnumValueFatal,
+            defaultValue=self.argument.defaultValue,
+            lenientFloatCode=self.lenientFloatCode,
+            isMember="Variadic" if self.argument.variadic else False,
+            allowTreatNonCallableAsNull=self.argument.allowTreatNonCallableAsNull(),
+            sourceDescription=self.argDescription,
+        )
+
+        if not self.argument.variadic:
+            return instantiateJSToNativeConversion(
+                typeConversion, self.replacementVariables, self.argcAndIndex is not None
+            ).define()
+
+        # Variadic arguments get turned into a sequence.
+        if typeConversion.dealWithOptional:
+            raise TypeError("Shouldn't have optional things in variadics")
+        if typeConversion.holderType is not None:
+            raise TypeError("Shouldn't need holders for variadics")
+
+        replacer = dict(self.argcAndIndex, **self.replacementVariables)
+        replacer["seqType"] = CGTemplatedType(
+            "AutoSequence", typeConversion.declType
+        ).define()
+        if typeNeedsRooting(self.argument.type):
+            rooterDecl = (
+                "SequenceRooter<%s> ${holderName}(cx, &${declName});\n"
+                % typeConversion.declType.define()
+            )
+        else:
+            rooterDecl = ""
+        replacer["elemType"] = typeConversion.declType.define()
+
+        replacer["elementInitializer"] = initializerForType(self.argument.type) or ""
+
+        # NOTE: Keep this in sync with sequence conversions as needed
+        variadicConversion = string.Template(
+            "${seqType} ${declName};\n"
+            + rooterDecl
+            + dedent(
+                """
+                if (${argc} > ${index}) {
+                  if (!${declName}.SetCapacity(${argc} - ${index}, mozilla::fallible)) {
+                    JS_ReportOutOfMemory(cx);
+                    return false;
+                  }
+                  for (uint32_t variadicArg = ${index}; variadicArg < ${argc}; ++variadicArg) {
+                    // OK to do infallible append here, since we ensured capacity already.
+                    ${elemType}& slot = *${declName}.AppendElement(${elementInitializer});
+                """
+            )
+        ).substitute(replacer)
+
+        val = string.Template("args[variadicArg]").substitute(replacer)
+        variadicConversion += indent(
+            string.Template(typeConversion.template).substitute(
+                {
+                    "val": val,
+                    "maybeMutableVal": val,
+                    "declName": "slot",
+                    # We only need holderName here to handle isExternal()
+                    # interfaces, which use an internal holder for the
+                    # conversion even when forceOwningType ends up true.
+                    "holderName": "tempHolder",
+                    # Use the same ${obj} as for the variadic arg itself
+                    "obj": replacer["obj"],
+                    "passedToJSImpl": toStringBool(
+                        isJSImplementedDescriptor(self.descriptorProvider)
+                    ),
+                }
+            ),
+            4,
+        )
+
+        variadicConversion += "  }\n" "}\n"
+        return variadicConversion
+
+
+def getMaybeWrapValueFuncForType(type):
+    if type.isJSString():
+        return "MaybeWrapStringValue"
+    # Callbacks might actually be DOM objects; nothing prevents a page from
+    # doing that.
+    if type.isCallback() or type.isCallbackInterface() or type.isObject():
+        if type.nullable():
+            return "MaybeWrapObjectOrNullValue"
+        return "MaybeWrapObjectValue"
+    # SpiderMonkey interfaces are never DOM objects.  Neither are sequences or
+    # dictionaries, since those are always plain JS objects.
+    if type.isSpiderMonkeyInterface() or type.isDictionary() or type.isSequence():
+        if type.nullable():
+            return "MaybeWrapNonDOMObjectOrNullValue"
+        return "MaybeWrapNonDOMObjectValue"
+    if type.isAny():
+        return "MaybeWrapValue"
+
+    # For other types, just go ahead an fall back on MaybeWrapValue for now:
+    # it's always safe to do, and shouldn't be particularly slow for any of
+    # them
+    return "MaybeWrapValue"
+
+
+sequenceWrapLevel = 0
+recordWrapLevel = 0
+
+
+def getWrapTemplateForType(
+    type,
+    descriptorProvider,
+    result,
+    successCode,
+    returnsNewObject,
+    exceptionCode,
+    spiderMonkeyInterfacesAreStructs,
+    isConstructorRetval=False,
+):
+    """
+    Reflect a C++ value stored in "result", of IDL type "type" into JS.  The
+    "successCode" is the code to run once we have successfully done the
+    conversion and must guarantee that execution of the conversion template
+    stops once the successCode has executed (e.g. by doing a 'return', or by
+    doing a 'break' if the entire conversion template is inside a block that
+    the 'break' will exit).
+
+    If spiderMonkeyInterfacesAreStructs is true, then if the type is a
+    SpiderMonkey interface, "result" is one of the
+    dom::SpiderMonkeyInterfaceObjectStorage subclasses, not a JSObject*.
+
+    The resulting string should be used with string.Template.  It
+    needs the following keys when substituting:
+
+      jsvalHandle: something that can be passed to methods taking a
+                   JS::MutableHandle<JS::Value>.  This can be a
+                   JS::MutableHandle<JS::Value> or a JS::Rooted<JS::Value>*.
+      jsvalRef: something that can have .address() called on it to get a
+                JS::Value* and .set() called on it to set it to a JS::Value.
+                This can be a JS::MutableHandle<JS::Value> or a
+                JS::Rooted<JS::Value>.
+      obj: a JS::Handle<JSObject*>.
+
+    Returns (templateString, infallibility of conversion template)
+    """
+    if successCode is None:
+        successCode = "return true;\n"
+
+    def setUndefined():
+        return _setValue("", setter="setUndefined")
+
+    def setNull():
+        return _setValue("", setter="setNull")
+
+    def setInt32(value):
+        return _setValue(value, setter="setInt32")
+
+    def setString(value):
+        return _setValue(value, wrapAsType=type, setter="setString")
+
+    def setObject(value, wrapAsType=None):
+        return _setValue(value, wrapAsType=wrapAsType, setter="setObject")
+
+    def setObjectOrNull(value, wrapAsType=None):
+        return _setValue(value, wrapAsType=wrapAsType, setter="setObjectOrNull")
+
+    def setUint32(value):
+        return _setValue(value, setter="setNumber")
+
+    def setDouble(value):
+        return _setValue("JS_NumberValue(%s)" % value)
+
+    def setBoolean(value):
+        return _setValue(value, setter="setBoolean")
+
+    def _setValue(value, wrapAsType=None, setter="set"):
+        """
+        Returns the code to set the jsval to value.
+
+        If wrapAsType is not None, then will wrap the resulting value using the
+        function that getMaybeWrapValueFuncForType(wrapAsType) returns.
+        Otherwise, no wrapping will be done.
+        """
+        if wrapAsType is None:
+            tail = successCode
+        else:
+            tail = fill(
+                """
+                if (!${maybeWrap}(cx, $${jsvalHandle})) {
+                  $*{exceptionCode}
+                }
+                $*{successCode}
+                """,
+                maybeWrap=getMaybeWrapValueFuncForType(wrapAsType),
+                exceptionCode=exceptionCode,
+                successCode=successCode,
+            )
+        return ("${jsvalRef}.%s(%s);\n" % (setter, value)) + tail
+
+    def wrapAndSetPtr(wrapCall, failureCode=None):
+        """
+        Returns the code to set the jsval by calling "wrapCall". "failureCode"
+        is the code to run if calling "wrapCall" fails
+        """
+        if failureCode is None:
+            failureCode = exceptionCode
+        return fill(
+            """
+            if (!${wrapCall}) {
+              $*{failureCode}
+            }
+            $*{successCode}
+            """,
+            wrapCall=wrapCall,
+            failureCode=failureCode,
+            successCode=successCode,
+        )
+
+    if type is None or type.isUndefined():
+        return (setUndefined(), True)
+
+    if (type.isSequence() or type.isRecord()) and type.nullable():
+        # These are both wrapped in Nullable<>
+        recTemplate, recInfall = getWrapTemplateForType(
+            type.inner,
+            descriptorProvider,
+            "%s.Value()" % result,
+            successCode,
+            returnsNewObject,
+            exceptionCode,
+            spiderMonkeyInterfacesAreStructs,
+        )
+        code = fill(
+            """
+
+            if (${result}.IsNull()) {
+              $*{setNull}
+            }
+            $*{recTemplate}
+            """,
+            result=result,
+            setNull=setNull(),
+            recTemplate=recTemplate,
+        )
+        return code, recInfall
+
+    if type.isSequence():
+        # Now do non-nullable sequences.  Our success code is just to break to
+        # where we set the element in the array.  Note that we bump the
+        # sequenceWrapLevel around this call so that nested sequence conversions
+        # will use different iteration variables.
+        global sequenceWrapLevel
+        index = "sequenceIdx%d" % sequenceWrapLevel
+        sequenceWrapLevel += 1
+        innerTemplate = wrapForType(
+            type.inner,
+            descriptorProvider,
+            {
+                "result": "%s[%s]" % (result, index),
+                "successCode": "break;\n",
+                "jsvalRef": "tmp",
+                "jsvalHandle": "&tmp",
+                "returnsNewObject": returnsNewObject,
+                "exceptionCode": exceptionCode,
+                "obj": "returnArray",
+                "spiderMonkeyInterfacesAreStructs": spiderMonkeyInterfacesAreStructs,
+            },
+        )
+        sequenceWrapLevel -= 1
+        code = fill(
+            """
+
+            uint32_t length = ${result}.Length();
+            JS::Rooted<JSObject*> returnArray(cx, JS::NewArrayObject(cx, length));
+            if (!returnArray) {
+              $*{exceptionCode}
+            }
+            // Scope for 'tmp'
+            {
+              JS::Rooted<JS::Value> tmp(cx);
+              for (uint32_t ${index} = 0; ${index} < length; ++${index}) {
+                // Control block to let us common up the JS_DefineElement calls when there
+                // are different ways to succeed at wrapping the object.
+                do {
+                  $*{innerTemplate}
+                } while (false);
+                if (!JS_DefineElement(cx, returnArray, ${index}, tmp,
+                                      JSPROP_ENUMERATE)) {
+                  $*{exceptionCode}
+                }
+              }
+            }
+            $*{set}
+            """,
+            result=result,
+            exceptionCode=exceptionCode,
+            index=index,
+            innerTemplate=innerTemplate,
+            set=setObject("*returnArray"),
+        )
+
+        return (code, False)
+
+    if type.isRecord():
+        # Now do non-nullable record.  Our success code is just to break to
+        # where we define the property on the object.  Note that we bump the
+        # recordWrapLevel around this call so that nested record conversions
+        # will use different temp value names.
+        global recordWrapLevel
+        valueName = "recordValue%d" % recordWrapLevel
+        recordWrapLevel += 1
+        innerTemplate = wrapForType(
+            type.inner,
+            descriptorProvider,
+            {
+                "result": valueName,
+                "successCode": "break;\n",
+                "jsvalRef": "tmp",
+                "jsvalHandle": "&tmp",
+                "returnsNewObject": returnsNewObject,
+                "exceptionCode": exceptionCode,
+                "obj": "returnObj",
+                "spiderMonkeyInterfacesAreStructs": spiderMonkeyInterfacesAreStructs,
+            },
+        )
+        recordWrapLevel -= 1
+        if type.keyType.isByteString():
+            # There is no length-taking JS_DefineProperty.  So to keep
+            # things sane with embedded nulls, we want to byte-inflate
+            # to an nsAString.  The only byte-inflation function we
+            # have around is AppendASCIItoUTF16, which luckily doesn't
+            # assert anything about the input being ASCII.
+            expandedKeyDecl = "NS_ConvertASCIItoUTF16 expandedKey(entry.mKey);\n"
+            keyName = "expandedKey"
+        elif type.keyType.isUTF8String():
+            # We do the same as above for utf8 strings. We could do better if
+            # we had a DefineProperty API that takes utf-8 property names.
+            expandedKeyDecl = "NS_ConvertUTF8toUTF16 expandedKey(entry.mKey);\n"
+            keyName = "expandedKey"
+        else:
+            expandedKeyDecl = ""
+            keyName = "entry.mKey"
+
+        code = fill(
+            """
+
+            JS::Rooted<JSObject*> returnObj(cx, JS_NewPlainObject(cx));
+            if (!returnObj) {
+              $*{exceptionCode}
+            }
+            // Scope for 'tmp'
+            {
+              JS::Rooted<JS::Value> tmp(cx);
+              for (auto& entry : ${result}.Entries()) {
+                auto& ${valueName} = entry.mValue;
+                // Control block to let us common up the JS_DefineUCProperty calls when there
+                // are different ways to succeed at wrapping the value.
+                do {
+                  $*{innerTemplate}
+                } while (false);
+                $*{expandedKeyDecl}
+                if (!JS_DefineUCProperty(cx, returnObj,
+                                         ${keyName}.BeginReading(),
+                                         ${keyName}.Length(), tmp,
+                                         JSPROP_ENUMERATE)) {
+                  $*{exceptionCode}
+                }
+              }
+            }
+            $*{set}
+            """,
+            result=result,
+            exceptionCode=exceptionCode,
+            valueName=valueName,
+            innerTemplate=innerTemplate,
+            expandedKeyDecl=expandedKeyDecl,
+            keyName=keyName,
+            set=setObject("*returnObj"),
+        )
+
+        return (code, False)
+
+    if type.isPromise():
+        assert not type.nullable()
+        # The use of ToJSValue here is a bit annoying because the Promise
+        # version is not inlined.  But we can't put an inline version in either
+        # ToJSValue.h or BindingUtils.h, because Promise.h includes ToJSValue.h
+        # and that includes BindingUtils.h, so we'd get an include loop if
+        # either of those headers included Promise.h.  And trying to write the
+        # conversion by hand here is pretty annoying because we have to handle
+        # the various RefPtr, rawptr, NonNull, etc cases, which ToJSValue will
+        # handle for us.  So just eat the cost of the function call.
+        return (wrapAndSetPtr("ToJSValue(cx, %s, ${jsvalHandle})" % result), False)
+
+    if type.isGeckoInterface() and not type.isCallbackInterface():
+        descriptor = descriptorProvider.getDescriptor(
+            type.unroll().inner.identifier.name
+        )
+        if type.nullable():
+            if descriptor.interface.identifier.name == "WindowProxy":
+                template, infal = getWrapTemplateForType(
+                    type.inner,
+                    descriptorProvider,
+                    "%s.Value()" % result,
+                    successCode,
+                    returnsNewObject,
+                    exceptionCode,
+                    spiderMonkeyInterfacesAreStructs,
+                )
+                return (
+                    "if (%s.IsNull()) {\n" % result
+                    + indent(setNull())
+                    + "}\n"
+                    + template,
+                    infal,
+                )
+
+            wrappingCode = "if (!%s) {\n" % (result) + indent(setNull()) + "}\n"
+        else:
+            wrappingCode = ""
+
+        if not descriptor.interface.isExternal():
+            if descriptor.wrapperCache:
+                wrapMethod = "GetOrCreateDOMReflector"
+                wrapArgs = "cx, %s, ${jsvalHandle}" % result
+            else:
+                wrapMethod = "WrapNewBindingNonWrapperCachedObject"
+                wrapArgs = "cx, ${obj}, %s, ${jsvalHandle}" % result
+            if isConstructorRetval:
+                wrapArgs += ", desiredProto"
+            wrap = "%s(%s)" % (wrapMethod, wrapArgs)
+            # Can only fail to wrap as a new-binding object if they already
+            # threw an exception.
+            failed = "MOZ_ASSERT(JS_IsExceptionPending(cx));\n" + exceptionCode
+        else:
+            if descriptor.notflattened:
+                getIID = "&NS_GET_IID(%s), " % descriptor.nativeType
+            else:
+                getIID = ""
+            wrap = "WrapObject(cx, %s, %s${jsvalHandle})" % (result, getIID)
+            failed = None
+
+        wrappingCode += wrapAndSetPtr(wrap, failed)
+        return (wrappingCode, False)
+
+    if type.isJSString():
+        return (setString(result), False)
+
+    if type.isDOMString() or type.isUSVString():
+        if type.nullable():
+            return (
+                wrapAndSetPtr("xpc::StringToJsval(cx, %s, ${jsvalHandle})" % result),
+                False,
+            )
+        else:
+            return (
+                wrapAndSetPtr(
+                    "xpc::NonVoidStringToJsval(cx, %s, ${jsvalHandle})" % result
+                ),
+                False,
+            )
+
+    if type.isByteString():
+        if type.nullable():
+            return (
+                wrapAndSetPtr("ByteStringToJsval(cx, %s, ${jsvalHandle})" % result),
+                False,
+            )
+        else:
+            return (
+                wrapAndSetPtr(
+                    "NonVoidByteStringToJsval(cx, %s, ${jsvalHandle})" % result
+                ),
+                False,
+            )
+
+    if type.isUTF8String():
+        if type.nullable():
+            return (
+                wrapAndSetPtr("UTF8StringToJsval(cx, %s, ${jsvalHandle})" % result),
+                False,
+            )
+        else:
+            return (
+                wrapAndSetPtr(
+                    "NonVoidUTF8StringToJsval(cx, %s, ${jsvalHandle})" % result
+                ),
+                False,
+            )
+
+    if type.isEnum():
+        if type.nullable():
+            resultLoc = "%s.Value()" % result
+        else:
+            resultLoc = result
+        conversion = fill(
+            """
+            if (!ToJSValue(cx, ${result}, $${jsvalHandle})) {
+              $*{exceptionCode}
+            }
+            $*{successCode}
+            """,
+            result=resultLoc,
+            exceptionCode=exceptionCode,
+            successCode=successCode,
+        )
+
+        if type.nullable():
+            conversion = CGIfElseWrapper(
+                "%s.IsNull()" % result, CGGeneric(setNull()), CGGeneric(conversion)
+            ).define()
+        return conversion, False
+
+    if type.isCallback() or type.isCallbackInterface():
+        # Callbacks can store null if we nuked the compartments their
+        # objects lived in.
+        wrapCode = setObjectOrNull(
+            "GetCallbackFromCallbackObject(cx, %(result)s)", wrapAsType=type
+        )
+        if type.nullable():
+            wrapCode = (
+                "if (%(result)s) {\n"
+                + indent(wrapCode)
+                + "} else {\n"
+                + indent(setNull())
+                + "}\n"
+            )
+        wrapCode = wrapCode % {"result": result}
+        return wrapCode, False
+
+    if type.isAny():
+        # See comments in GetOrCreateDOMReflector explaining why we need
+        # to wrap here.
+        # NB: _setValue(..., type-that-is-any) calls JS_WrapValue(), so is fallible
+        head = "JS::ExposeValueToActiveJS(%s);\n" % result
+        return (head + _setValue(result, wrapAsType=type), False)
+
+    if type.isObject() or (
+        type.isSpiderMonkeyInterface() and not spiderMonkeyInterfacesAreStructs
+    ):
+        # See comments in GetOrCreateDOMReflector explaining why we need
+        # to wrap here.
+        if type.nullable():
+            toValue = "%s"
+            setter = setObjectOrNull
+            head = """if (%s) {
+              JS::ExposeObjectToActiveJS(%s);
+            }
+            """ % (
+                result,
+                result,
+            )
+        else:
+            toValue = "*%s"
+            setter = setObject
+            head = "JS::ExposeObjectToActiveJS(%s);\n" % result
+        # NB: setObject{,OrNull}(..., some-object-type) calls JS_WrapValue(), so is fallible
+        return (head + setter(toValue % result, wrapAsType=type), False)
+
+    if type.isObservableArray():
+        # This first argument isn't used at all for now, the attribute getter
+        # for ObservableArray type are generated in getObservableArrayGetterBody
+        # instead.
+        return "", False
+
+    if not (
+        type.isUnion()
+        or type.isPrimitive()
+        or type.isDictionary()
+        or (type.isSpiderMonkeyInterface() and spiderMonkeyInterfacesAreStructs)
+    ):
+        raise TypeError("Need to learn to wrap %s" % type)
+
+    if type.nullable():
+        recTemplate, recInfal = getWrapTemplateForType(
+            type.inner,
+            descriptorProvider,
+            "%s.Value()" % result,
+            successCode,
+            returnsNewObject,
+            exceptionCode,
+            spiderMonkeyInterfacesAreStructs,
+        )
+        return (
+            "if (%s.IsNull()) {\n" % result + indent(setNull()) + "}\n" + recTemplate,
+            recInfal,
+        )
+
+    if type.isSpiderMonkeyInterface():
+        assert spiderMonkeyInterfacesAreStructs
+        # See comments in GetOrCreateDOMReflector explaining why we need
+        # to wrap here.
+        # NB: setObject(..., some-object-type) calls JS_WrapValue(), so is fallible
+        return (setObject("*%s.Obj()" % result, wrapAsType=type), False)
+
+    if type.isUnion():
+        return (wrapAndSetPtr("%s.ToJSVal(cx, ${obj}, ${jsvalHandle})" % result), False)
+
+    if type.isDictionary():
+        return (
+            wrapAndSetPtr("%s.ToObjectInternal(cx, ${jsvalHandle})" % result),
+            False,
+        )
+
+    tag = type.tag()
+
+    if tag in [
+        IDLType.Tags.int8,
+        IDLType.Tags.uint8,
+        IDLType.Tags.int16,
+        IDLType.Tags.uint16,
+        IDLType.Tags.int32,
+    ]:
+        return (setInt32("int32_t(%s)" % result), True)
+
+    elif tag in [
+        IDLType.Tags.int64,
+        IDLType.Tags.uint64,
+        IDLType.Tags.unrestricted_float,
+        IDLType.Tags.float,
+        IDLType.Tags.unrestricted_double,
+        IDLType.Tags.double,
+    ]:
+        # XXXbz will cast to double do the "even significand" thing that webidl
+        # calls for for 64-bit ints?  Do we care?
+        return (setDouble("double(%s)" % result), True)
+
+    elif tag == IDLType.Tags.uint32:
+        return (setUint32(result), True)
+
+    elif tag == IDLType.Tags.bool:
+        return (setBoolean(result), True)
+
+    else:
+        raise TypeError("Need to learn to wrap primitive: %s" % type)
+
+
+def wrapForType(type, descriptorProvider, templateValues):
+    """
+    Reflect a C++ value of IDL type "type" into JS.  TemplateValues is a dict
+    that should contain:
+
+      * 'jsvalRef': something that can have .address() called on it to get a
+                    JS::Value* and .set() called on it to set it to a JS::Value.
+                    This can be a JS::MutableHandle<JS::Value> or a
+                    JS::Rooted<JS::Value>.
+      * 'jsvalHandle': something that can be passed to methods taking a
+                       JS::MutableHandle<JS::Value>.  This can be a
+                       JS::MutableHandle<JS::Value> or a JS::Rooted<JS::Value>*.
+      * 'obj' (optional): the name of the variable that contains the JSObject to
+                          use as a scope when wrapping, if not supplied 'obj'
+                          will be used as the name
+      * 'result' (optional): the name of the variable in which the C++ value is
+                             stored, if not supplied 'result' will be used as
+                             the name
+      * 'successCode' (optional): the code to run once we have successfully
+                                  done the conversion, if not supplied 'return
+                                  true;' will be used as the code.  The
+                                  successCode must ensure that once it runs no
+                                  more of the conversion template will be
+                                  executed (e.g. by doing a 'return' or 'break'
+                                  as appropriate).
+      * 'returnsNewObject' (optional): If true, we're wrapping for the return
+                                       value of a [NewObject] method.  Assumed
+                                       false if not set.
+      * 'exceptionCode' (optional): Code to run when a JS exception is thrown.
+                                    The default is "return false;".  The code
+                                    passed here must return.
+      * 'isConstructorRetval' (optional): If true, we're wrapping a constructor
+                                          return value.
+    """
+    wrap = getWrapTemplateForType(
+        type,
+        descriptorProvider,
+        templateValues.get("result", "result"),
+        templateValues.get("successCode", None),
+        templateValues.get("returnsNewObject", False),
+        templateValues.get("exceptionCode", "return false;\n"),
+        templateValues.get("spiderMonkeyInterfacesAreStructs", False),
+        isConstructorRetval=templateValues.get("isConstructorRetval", False),
+    )[0]
+
+    defaultValues = {"obj": "obj"}
+    return string.Template(wrap).substitute(defaultValues, **templateValues)
+
+
+def infallibleForMember(member, type, descriptorProvider):
+    """
+    Determine the fallibility of changing a C++ value of IDL type "type" into
+    JS for the given attribute. Apart from returnsNewObject, all the defaults
+    are used, since the fallbility does not change based on the boolean values,
+    and the template will be discarded.
+
+    CURRENT ASSUMPTIONS:
+        We assume that successCode for wrapping up return values cannot contain
+        failure conditions.
+    """
+    return getWrapTemplateForType(
+        type,
+        descriptorProvider,
+        "result",
+        None,
+        memberReturnsNewObject(member),
+        "return false;\n",
+        False,
+    )[1]
+
+
+def leafTypeNeedsCx(type, retVal):
+    return (
+        type.isAny()
+        or type.isObject()
+        or type.isJSString()
+        or (retVal and type.isSpiderMonkeyInterface())
+    )
+
+
+def leafTypeNeedsScopeObject(type, retVal):
+    return retVal and type.isSpiderMonkeyInterface()
+
+
+def leafTypeNeedsRooting(type):
+    return leafTypeNeedsCx(type, False) or type.isSpiderMonkeyInterface()
+
+
+def typeNeedsRooting(type):
+    return typeMatchesLambda(type, lambda t: leafTypeNeedsRooting(t))
+
+
+def typeNeedsCx(type, retVal=False):
+    return typeMatchesLambda(type, lambda t: leafTypeNeedsCx(t, retVal))
+
+
+def typeNeedsScopeObject(type, retVal=False):
+    return typeMatchesLambda(type, lambda t: leafTypeNeedsScopeObject(t, retVal))
+
+
+def typeMatchesLambda(type, func):
+    if type is None:
+        return False
+    if type.nullable():
+        return typeMatchesLambda(type.inner, func)
+    if type.isSequence() or type.isRecord():
+        return typeMatchesLambda(type.inner, func)
+    if type.isUnion():
+        return any(typeMatchesLambda(t, func) for t in type.unroll().flatMemberTypes)
+    if type.isDictionary():
+        return dictionaryMatchesLambda(type.inner, func)
+    return func(type)
+
+
+def dictionaryMatchesLambda(dictionary, func):
+    return any(typeMatchesLambda(m.type, func) for m in dictionary.members) or (
+        dictionary.parent and dictionaryMatchesLambda(dictionary.parent, func)
+    )
+
+
+# Whenever this is modified, please update CGNativeMember.getRetvalInfo as
+# needed to keep the types compatible.
+def getRetvalDeclarationForType(returnType, descriptorProvider, isMember=False):
+    """
+    Returns a tuple containing five things:
+
+    1) A CGThing for the type of the return value, or None if there is no need
+       for a return value.
+
+    2) A value indicating the kind of ourparam to pass the value as.  Valid
+       options are None to not pass as an out param at all, "ref" (to pass a
+       reference as an out param), and "ptr" (to pass a pointer as an out
+       param).
+
+    3) A CGThing for a tracer for the return value, or None if no tracing is
+       needed.
+
+    4) An argument string to pass to the retval declaration
+       constructor or None if there are no arguments.
+
+    5) The name of a function that needs to be called with the return value
+       before using it, or None if no function needs to be called.
+    """
+    if returnType is None or returnType.isUndefined():
+        # Nothing to declare
+        return None, None, None, None, None
+    if returnType.isPrimitive() and returnType.tag() in builtinNames:
+        result = CGGeneric(builtinNames[returnType.tag()])
+        if returnType.nullable():
+            result = CGTemplatedType("Nullable", result)
+        return result, None, None, None, None
+    if returnType.isJSString():
+        if isMember:
+            raise TypeError("JSString not supported as return type member")
+        return CGGeneric("JS::Rooted<JSString*>"), "ptr", None, "cx", None
+    if returnType.isDOMString() or returnType.isUSVString():
+        if isMember:
+            return CGGeneric("nsString"), "ref", None, None, None
+        return CGGeneric("DOMString"), "ref", None, None, None
+    if returnType.isByteString() or returnType.isUTF8String():
+        if isMember:
+            return CGGeneric("nsCString"), "ref", None, None, None
+        return CGGeneric("nsAutoCString"), "ref", None, None, None
+    if returnType.isEnum():
+        result = CGGeneric(returnType.unroll().inner.identifier.name)
+        if returnType.nullable():
+            result = CGTemplatedType("Nullable", result)
+        return result, None, None, None, None
+    if returnType.isGeckoInterface() or returnType.isPromise():
+        if returnType.isGeckoInterface():
+            typeName = returnType.unroll().inner.identifier.name
+            if typeName == "WindowProxy":
+                result = CGGeneric("WindowProxyHolder")
+                if returnType.nullable():
+                    result = CGTemplatedType("Nullable", result)
+                return result, None, None, None, None
+
+            typeName = descriptorProvider.getDescriptor(typeName).nativeType
+        else:
+            typeName = "Promise"
+        if isMember:
+            conversion = None
+            result = CGGeneric("StrongPtrForMember<%s>" % typeName)
+        else:
+            conversion = CGGeneric("StrongOrRawPtr<%s>" % typeName)
+            result = CGGeneric("auto")
+        return result, None, None, None, conversion
+    if returnType.isCallback():
+        name = returnType.unroll().callback.identifier.name
+        return CGGeneric("RefPtr<%s>" % name), None, None, None, None
+    if returnType.isAny():
+        if isMember:
+            return CGGeneric("JS::Value"), None, None, None, None
+        return CGGeneric("JS::Rooted<JS::Value>"), "ptr", None, "cx", None
+    if returnType.isObject() or returnType.isSpiderMonkeyInterface():
+        if isMember:
+            return CGGeneric("JSObject*"), None, None, None, None
+        return CGGeneric("JS::Rooted<JSObject*>"), "ptr", None, "cx", None
+    if returnType.isSequence():
+        nullable = returnType.nullable()
+        if nullable:
+            returnType = returnType.inner
+        result, _, _, _, _ = getRetvalDeclarationForType(
+            returnType.inner, descriptorProvider, isMember="Sequence"
+        )
+        # While we have our inner type, set up our rooter, if needed
+        if not isMember and typeNeedsRooting(returnType):
+            rooter = CGGeneric(
+                "SequenceRooter<%s > resultRooter(cx, &result);\n" % result.define()
+            )
+        else:
+            rooter = None
+        result = CGTemplatedType("nsTArray", result)
+        if nullable:
+            result = CGTemplatedType("Nullable", result)
+        return result, "ref", rooter, None, None
+    if returnType.isRecord():
+        nullable = returnType.nullable()
+        if nullable:
+            returnType = returnType.inner
+        result, _, _, _, _ = getRetvalDeclarationForType(
+            returnType.inner, descriptorProvider, isMember="Record"
+        )
+        # While we have our inner type, set up our rooter, if needed
+        if not isMember and typeNeedsRooting(returnType):
+            rooter = CGGeneric(
+                "RecordRooter<%s> resultRooter(cx, &result);\n"
+                % ("nsString, " + result.define())
+            )
+        else:
+            rooter = None
+        result = CGTemplatedType("Record", [recordKeyDeclType(returnType), result])
+        if nullable:
+            result = CGTemplatedType("Nullable", result)
+        return result, "ref", rooter, None, None
+    if returnType.isDictionary():
+        nullable = returnType.nullable()
+        dictName = CGDictionary.makeDictionaryName(returnType.unroll().inner)
+        result = CGGeneric(dictName)
+        if not isMember and typeNeedsRooting(returnType):
+            if nullable:
+                result = CGTemplatedType("NullableRootedDictionary", result)
+            else:
+                result = CGTemplatedType("RootedDictionary", result)
+            resultArgs = "cx"
+        else:
+            if nullable:
+                result = CGTemplatedType("Nullable", result)
+            resultArgs = None
+        return result, "ref", None, resultArgs, None
+    if returnType.isUnion():
+        result = CGGeneric(CGUnionStruct.unionTypeName(returnType.unroll(), True))
+        if not isMember and typeNeedsRooting(returnType):
+            if returnType.nullable():
+                result = CGTemplatedType("NullableRootedUnion", result)
+            else:
+                result = CGTemplatedType("RootedUnion", result)
+            resultArgs = "cx"
+        else:
+            if returnType.nullable():
+                result = CGTemplatedType("Nullable", result)
+            resultArgs = None
+        return result, "ref", None, resultArgs, None
+    raise TypeError("Don't know how to declare return value for %s" % returnType)
+
+
+def needCx(returnType, arguments, extendedAttributes, considerTypes, static=False):
+    return (
+        not static
+        and considerTypes
+        and (
+            typeNeedsCx(returnType, True) or any(typeNeedsCx(a.type) for a in arguments)
+        )
+        or "implicitJSContext" in extendedAttributes
+    )
+
+
+def needScopeObject(
+    returnType, arguments, extendedAttributes, isWrapperCached, considerTypes, isMember
+):
+    """
+    isMember should be true if we're dealing with an attribute
+    annotated as [StoreInSlot].
+    """
+    return (
+        considerTypes
+        and not isWrapperCached
+        and (
+            (not isMember and typeNeedsScopeObject(returnType, True))
+            or any(typeNeedsScopeObject(a.type) for a in arguments)
+        )
+    )
+
+
+def callerTypeGetterForDescriptor(descriptor):
+    if descriptor.interface.isExposedInAnyWorker():
+        systemCallerGetter = "nsContentUtils::ThreadsafeIsSystemCaller"
+    else:
+        systemCallerGetter = "nsContentUtils::IsSystemCaller"
+    return "%s(cx) ? CallerType::System : CallerType::NonSystem" % systemCallerGetter
+
+
+class CGCallGenerator(CGThing):
+    """
+    A class to generate an actual call to a C++ object.  Assumes that the C++
+    object is stored in a variable whose name is given by the |object| argument.
+
+    needsCallerType is a boolean indicating whether the call should receive
+    a PrincipalType for the caller.
+
+    needsErrorResult is a boolean indicating whether the call should be
+    fallible and thus needs ErrorResult parameter.
+
+    resultVar: If the returnType is not void, then the result of the call is
+    stored in a C++ variable named by resultVar. The caller is responsible for
+    declaring the result variable. If the caller doesn't care about the result
+    value, resultVar can be omitted.
+
+    context: The context string to pass to MaybeSetPendingException.
+    """
+
+    def __init__(
+        self,
+        needsErrorResult,
+        needsCallerType,
+        isChromeOnly,
+        arguments,
+        argsPre,
+        returnType,
+        extendedAttributes,
+        descriptor,
+        nativeMethodName,
+        static,
+        object="self",
+        argsPost=[],
+        resultVar=None,
+        context="nullptr",
+    ):
+        CGThing.__init__(self)
+
+        (
+            result,
+            resultOutParam,
+            resultRooter,
+            resultArgs,
+            resultConversion,
+        ) = getRetvalDeclarationForType(returnType, descriptor)
+
+        args = CGList([CGGeneric(arg) for arg in argsPre], ", ")
+        for a, name in arguments:
+            arg = CGGeneric(name)
+
+            # Now constify the things that need it
+            def needsConst(a):
+                if a.type.isDictionary():
+                    return True
+                if a.type.isSequence():
+                    return True
+                if a.type.isRecord():
+                    return True
+                # isObject() types are always a JS::Rooted, whether
+                # nullable or not, and it turns out a const JS::Rooted
+                # is not very helpful at all (in particular, it won't
+                # even convert to a JS::Handle).
+                # XXX bz Well, why not???
+                if a.type.nullable() and not a.type.isObject():
+                    return True
+                if a.type.isString():
+                    return True
+                if a.canHaveMissingValue():
+                    # This will need an Optional or it's a variadic;
+                    # in both cases it should be const.
+                    return True
+                if a.type.isUnion():
+                    return True
+                if a.type.isSpiderMonkeyInterface():
+                    return True
+                return False
+
+            if needsConst(a):
+                arg = CGWrapper(arg, pre="Constify(", post=")")
+            # And convert NonNull<T> to T&
+            if (
+                (a.type.isGeckoInterface() or a.type.isCallback() or a.type.isPromise())
+                and not a.type.nullable()
+            ) or a.type.isDOMString():
+                arg = CGWrapper(arg, pre="NonNullHelper(", post=")")
+
+            # If it's a refcounted object, let the static analysis know it's
+            # alive for the duration of the call.
+            if a.type.isGeckoInterface() or a.type.isCallback():
+                arg = CGWrapper(arg, pre="MOZ_KnownLive(", post=")")
+
+            args.append(arg)
+
+        needResultDecl = False
+
+        # Build up our actual call
+        self.cgRoot = CGList([])
+
+        # Return values that go in outparams go here
+        if resultOutParam is not None:
+            if resultVar is None:
+                needResultDecl = True
+                resultVar = "result"
+            if resultOutParam == "ref":
+                args.append(CGGeneric(resultVar))
+            else:
+                assert resultOutParam == "ptr"
+                args.append(CGGeneric("&" + resultVar))
+
+        needsSubjectPrincipal = "needsSubjectPrincipal" in extendedAttributes
+        if needsSubjectPrincipal:
+            needsNonSystemPrincipal = (
+                "needsNonSystemSubjectPrincipal" in extendedAttributes
+            )
+            if needsNonSystemPrincipal:
+                checkPrincipal = dedent(
+                    """
+                    if (principal->IsSystemPrincipal()) {
+                      principal = nullptr;
+                    }
+                    """
+                )
+            else:
+                checkPrincipal = ""
+
+            getPrincipal = fill(
+                """
+                JS::Realm* realm = js::GetContextRealm(cx);
+                MOZ_ASSERT(realm);
+                JSPrincipals* principals = JS::GetRealmPrincipals(realm);
+                nsIPrincipal* principal = nsJSPrincipals::get(principals);
+                ${checkPrincipal}
+                """,
+                checkPrincipal=checkPrincipal,
+            )
+
+            if descriptor.interface.isExposedInAnyWorker():
+                self.cgRoot.append(
+                    CGGeneric(
+                        fill(
+                            """
+                    Maybe<nsIPrincipal*> subjectPrincipal;
+                    if (NS_IsMainThread()) {
+                      $*{getPrincipal}
+                      subjectPrincipal.emplace(principal);
+                    }
+                    """,
+                            getPrincipal=getPrincipal,
+                        )
+                    )
+                )
+                subjectPrincipalArg = "subjectPrincipal"
+            else:
+                if needsNonSystemPrincipal:
+                    principalType = "nsIPrincipal*"
+                    subjectPrincipalArg = "subjectPrincipal"
+                else:
+                    principalType = "NonNull<nsIPrincipal>"
+                    subjectPrincipalArg = "NonNullHelper(subjectPrincipal)"
+
+                self.cgRoot.append(
+                    CGGeneric(
+                        fill(
+                            """
+                    ${principalType} subjectPrincipal;
+                    {
+                      $*{getPrincipal}
+                      subjectPrincipal = principal;
+                    }
+                    """,
+                            principalType=principalType,
+                            getPrincipal=getPrincipal,
+                        )
+                    )
+                )
+
+            args.append(CGGeneric("MOZ_KnownLive(%s)" % subjectPrincipalArg))
+
+        if needsCallerType:
+            if isChromeOnly:
+                args.append(CGGeneric("SystemCallerGuarantee()"))
+            else:
+                args.append(CGGeneric(callerTypeGetterForDescriptor(descriptor)))
+
+        canOOM = "canOOM" in extendedAttributes
+        if needsErrorResult:
+            args.append(CGGeneric("rv"))
+        elif canOOM:
+            args.append(CGGeneric("OOMReporter::From(rv)"))
+        args.extend(CGGeneric(arg) for arg in argsPost)
+
+        call = CGGeneric(nativeMethodName)
+        if not static:
+            call = CGWrapper(call, pre="%s->" % object)
+        call = CGList([call, CGWrapper(args, pre="(", post=")")])
+        if returnType is None or returnType.isUndefined() or resultOutParam is not None:
+            assert resultConversion is None
+            call = CGList(
+                [
+                    CGWrapper(
+                        call,
+                        pre=(
+                            "// NOTE: This assert does NOT call the function.\n"
+                            "static_assert(std::is_void_v<decltype("
+                        ),
+                        post=')>, "Should be returning void here");',
+                    ),
+                    call,
+                ],
+                "\n",
+            )
+        elif resultConversion is not None:
+            call = CGList([resultConversion, CGWrapper(call, pre="(", post=")")])
+        if resultVar is None and result is not None:
+            needResultDecl = True
+            resultVar = "result"
+
+        if needResultDecl:
+            if resultArgs is not None:
+                resultArgsStr = "(%s)" % resultArgs
+            else:
+                resultArgsStr = ""
+            result = CGWrapper(result, post=(" %s%s" % (resultVar, resultArgsStr)))
+            if resultOutParam is None and resultArgs is None:
+                call = CGList([result, CGWrapper(call, pre="(", post=")")])
+            else:
+                self.cgRoot.append(CGWrapper(result, post=";\n"))
+                if resultOutParam is None:
+                    call = CGWrapper(call, pre=resultVar + " = ")
+            if resultRooter is not None:
+                self.cgRoot.append(resultRooter)
+        elif result is not None:
+            assert resultOutParam is None
+            call = CGWrapper(call, pre=resultVar + " = ")
+
+        call = CGWrapper(call, post=";\n")
+        self.cgRoot.append(call)
+
+        if needsErrorResult or canOOM:
+            self.cgRoot.prepend(CGGeneric("FastErrorResult rv;\n"))
+            self.cgRoot.append(
+                CGGeneric(
+                    fill(
+                        """
+                if (MOZ_UNLIKELY(rv.MaybeSetPendingException(cx, ${context}))) {
+                  return false;
+                }
+                """,
+                        context=context,
+                    )
+                )
+            )
+
+        self.cgRoot.append(CGGeneric("MOZ_ASSERT(!JS_IsExceptionPending(cx));\n"))
+
+    def define(self):
+        return self.cgRoot.define()
+
+
+def getUnionMemberName(type):
+    # Promises can't be in unions, because they're not distinguishable
+    # from anything else.
+    assert not type.isPromise()
+    if type.isGeckoInterface():
+        return type.inner.identifier.name
+    if type.isEnum():
+        return type.inner.identifier.name
+    return type.name
+
+
+# A counter for making sure that when we're wrapping up things in
+# nested sequences we don't use the same variable name to iterate over
+# different sequences.
+sequenceWrapLevel = 0
+recordWrapLevel = 0
+
+
+def wrapTypeIntoCurrentCompartment(type, value, isMember=True):
+    """
+    Take the thing named by "value" and if it contains "any",
+    "object", or spidermonkey-interface types inside return a CGThing
+    that will wrap them into the current compartment.
+    """
+    if type.isAny():
+        assert not type.nullable()
+        if isMember:
+            value = "JS::MutableHandle<JS::Value>::fromMarkedLocation(&%s)" % value
+        else:
+            value = "&" + value
+        return CGGeneric(
+            "if (!JS_WrapValue(cx, %s)) {\n" "  return false;\n" "}\n" % value
+        )
+
+    if type.isObject():
+        if isMember:
+            value = "JS::MutableHandle<JSObject*>::fromMarkedLocation(&%s)" % value
+        else:
+            value = "&" + value
+        return CGGeneric(
+            "if (!JS_WrapObject(cx, %s)) {\n" "  return false;\n" "}\n" % value
+        )
+
+    if type.isSpiderMonkeyInterface():
+        origValue = value
+        if type.nullable():
+            value = "%s.Value()" % value
+        wrapCode = CGGeneric(
+            "if (!%s.WrapIntoNewCompartment(cx)) {\n" "  return false;\n" "}\n" % value
+        )
+        if type.nullable():
+            wrapCode = CGIfWrapper(wrapCode, "!%s.IsNull()" % origValue)
+        return wrapCode
+
+    if type.isSequence():
+        origValue = value
+        origType = type
+        if type.nullable():
+            type = type.inner
+            value = "%s.Value()" % value
+        global sequenceWrapLevel
+        index = "indexName%d" % sequenceWrapLevel
+        sequenceWrapLevel += 1
+        wrapElement = wrapTypeIntoCurrentCompartment(
+            type.inner, "%s[%s]" % (value, index)
+        )
+        sequenceWrapLevel -= 1
+        if not wrapElement:
+            return None
+        wrapCode = CGWrapper(
+            CGIndenter(wrapElement),
+            pre=(
+                "for (uint32_t %s = 0; %s < %s.Length(); ++%s) {\n"
+                % (index, index, value, index)
+            ),
+            post="}\n",
+        )
+        if origType.nullable():
+            wrapCode = CGIfWrapper(wrapCode, "!%s.IsNull()" % origValue)
+        return wrapCode
+
+    if type.isRecord():
+        origType = type
+        if type.nullable():
+            type = type.inner
+            recordRef = "%s.Value()" % value
+        else:
+            recordRef = value
+        global recordWrapLevel
+        entryRef = "mapEntry%d" % recordWrapLevel
+        recordWrapLevel += 1
+        wrapElement = wrapTypeIntoCurrentCompartment(type.inner, "%s.mValue" % entryRef)
+        recordWrapLevel -= 1
+        if not wrapElement:
+            return None
+        wrapCode = CGWrapper(
+            CGIndenter(wrapElement),
+            pre=("for (auto& %s : %s.Entries()) {\n" % (entryRef, recordRef)),
+            post="}\n",
+        )
+        if origType.nullable():
+            wrapCode = CGIfWrapper(wrapCode, "!%s.IsNull()" % value)
+        return wrapCode
+
+    if type.isDictionary():
+        assert not type.nullable()
+        myDict = type.inner
+        memberWraps = []
+        while myDict:
+            for member in myDict.members:
+                memberWrap = wrapArgIntoCurrentCompartment(
+                    member,
+                    "%s.%s"
+                    % (value, CGDictionary.makeMemberName(member.identifier.name)),
+                )
+                if memberWrap:
+                    memberWraps.append(memberWrap)
+            myDict = myDict.parent
+        return CGList(memberWraps) if len(memberWraps) != 0 else None
+
+    if type.isUnion():
+        memberWraps = []
+        if type.nullable():
+            type = type.inner
+            value = "%s.Value()" % value
+        for member in type.flatMemberTypes:
+            memberName = getUnionMemberName(member)
+            memberWrap = wrapTypeIntoCurrentCompartment(
+                member, "%s.GetAs%s()" % (value, memberName)
+            )
+            if memberWrap:
+                memberWrap = CGIfWrapper(memberWrap, "%s.Is%s()" % (value, memberName))
+                memberWraps.append(memberWrap)
+        return CGList(memberWraps, "else ") if len(memberWraps) != 0 else None
+
+    if (
+        type.isUndefined()
+        or type.isString()
+        or type.isPrimitive()
+        or type.isEnum()
+        or type.isGeckoInterface()
+        or type.isCallback()
+        or type.isPromise()
+    ):
+        # All of these don't need wrapping.
+        return None
+
+    raise TypeError(
+        "Unknown type; we don't know how to wrap it in constructor "
+        "arguments: %s" % type
+    )
+
+
+def wrapArgIntoCurrentCompartment(arg, value, isMember=True):
+    """
+    As wrapTypeIntoCurrentCompartment but handles things being optional
+    """
+    origValue = value
+    isOptional = arg.canHaveMissingValue()
+    if isOptional:
+        value = value + ".Value()"
+    wrap = wrapTypeIntoCurrentCompartment(arg.type, value, isMember)
+    if wrap and isOptional:
+        wrap = CGIfWrapper(wrap, "%s.WasPassed()" % origValue)
+    return wrap
+
+
+def needsContainsHack(m):
+    return m.getExtendedAttribute("ReturnValueNeedsContainsHack")
+
+
+def needsCallerType(m):
+    return m.getExtendedAttribute("NeedsCallerType")
+
+
+class CGPerSignatureCall(CGThing):
+    """
+    This class handles the guts of generating code for a particular
+    call signature.  A call signature consists of four things:
+
+    1) A return type, which can be None to indicate that there is no
+       actual return value (e.g. this is an attribute setter) or an
+       IDLType if there's an IDL type involved (including |void|).
+    2) An argument list, which is allowed to be empty.
+    3) A name of a native method to call.
+    4) Whether or not this method is static. Note that this only controls how
+       the method is called (|self->nativeMethodName(...)| vs
+       |nativeMethodName(...)|).
+
+    We also need to know whether this is a method or a getter/setter
+    to do error reporting correctly.
+
+    The idlNode parameter can be either a method or an attr. We can query
+    |idlNode.identifier| in both cases, so we can be agnostic between the two.
+
+    dontSetSlot should be set to True if the value should not be cached in a
+    slot (even if the attribute is marked as StoreInSlot or Cached in the
+    WebIDL).
+    """
+
+    # XXXbz For now each entry in the argument list is either an
+    # IDLArgument or a FakeArgument, but longer-term we may want to
+    # have ways of flagging things like JSContext* or optional_argc in
+    # there.
+
+    def __init__(
+        self,
+        returnType,
+        arguments,
+        nativeMethodName,
+        static,
+        descriptor,
+        idlNode,
+        argConversionStartsAt=0,
+        getter=False,
+        setter=False,
+        isConstructor=False,
+        useCounterName=None,
+        resultVar=None,
+        objectName="obj",
+        dontSetSlot=False,
+        extendedAttributes=None,
+    ):
+        assert idlNode.isMethod() == (not getter and not setter)
+        assert idlNode.isAttr() == (getter or setter)
+        # Constructors are always static
+        assert not isConstructor or static
+
+        CGThing.__init__(self)
+        self.returnType = returnType
+        self.descriptor = descriptor
+        self.idlNode = idlNode
+        if extendedAttributes is None:
+            extendedAttributes = descriptor.getExtendedAttributes(
+                idlNode, getter=getter, setter=setter
+            )
+        self.extendedAttributes = extendedAttributes
+        self.arguments = arguments
+        self.argCount = len(arguments)
+        self.isConstructor = isConstructor
+        self.setSlot = (
+            not dontSetSlot and idlNode.isAttr() and idlNode.slotIndices is not None
+        )
+        cgThings = []
+
+        deprecated = idlNode.getExtendedAttribute("Deprecated") or (
+            idlNode.isStatic()
+            and descriptor.interface.getExtendedAttribute("Deprecated")
+        )
+        if deprecated:
+            cgThings.append(
+                CGGeneric(
+                    dedent(
+                        """
+                DeprecationWarning(cx, obj, DeprecatedOperations::e%s);
+                """
+                        % deprecated[0]
+                    )
+                )
+            )
+
+        lenientFloatCode = None
+        if idlNode.getExtendedAttribute("LenientFloat") is not None and (
+            setter or idlNode.isMethod()
+        ):
+            cgThings.append(
+                CGGeneric(
+                    dedent(
+                        """
+                bool foundNonFiniteFloat = false;
+                """
+                    )
+                )
+            )
+            lenientFloatCode = "foundNonFiniteFloat = true;\n"
+
+        argsPre = []
+        if idlNode.isStatic():
+            # If we're a constructor, "obj" may not be a function, so calling
+            # XrayAwareCalleeGlobal() on it is not safe.  Of course in the
+            # constructor case either "obj" is an Xray or we're already in the
+            # content compartment, not the Xray compartment, so just
+            # constructing the GlobalObject from "obj" is fine.
+            if isConstructor:
+                objForGlobalObject = "obj"
+            else:
+                objForGlobalObject = "xpc::XrayAwareCalleeGlobal(obj)"
+            cgThings.append(
+                CGGeneric(
+                    fill(
+                        """
+                GlobalObject global(cx, ${obj});
+                if (global.Failed()) {
+                  return false;
+                }
+
+                """,
+                        obj=objForGlobalObject,
+                    )
+                )
+            )
+            argsPre.append("global")
+
+        # For JS-implemented interfaces we do not want to base the
+        # needsCx decision on the types involved, just on our extended
+        # attributes. Also, JSContext is not needed for the static case
+        # since GlobalObject already contains the context.
+        needsCx = needCx(
+            returnType,
+            arguments,
+            self.extendedAttributes,
+            not descriptor.interface.isJSImplemented(),
+            static,
+        )
+        if needsCx:
+            argsPre.append("cx")
+
+        needsUnwrap = False
+        argsPost = []
+        runConstructorInCallerCompartment = descriptor.interface.getExtendedAttribute(
+            "RunConstructorInCallerCompartment"
+        )
+        if isConstructor and not runConstructorInCallerCompartment:
+            needsUnwrap = True
+            needsUnwrappedVar = False
+            unwrappedVar = "obj"
+            if descriptor.interface.isJSImplemented():
+                # We need the desired proto in our constructor, because the
+                # constructor will actually construct our reflector.
+                argsPost.append("desiredProto")
+        elif descriptor.interface.isJSImplemented():
+            if not idlNode.isStatic():
+                needsUnwrap = True
+                needsUnwrappedVar = True
+                argsPost.append(
+                    "(unwrappedObj ? js::GetNonCCWObjectRealm(*unwrappedObj) : js::GetContextRealm(cx))"
+                )
+        elif needScopeObject(
+            returnType,
+            arguments,
+            self.extendedAttributes,
+            descriptor.wrapperCache,
+            True,
+            idlNode.getExtendedAttribute("StoreInSlot"),
+        ):
+            # If we ever end up with APIs like this on cross-origin objects,
+            # figure out how the CheckedUnwrapDynamic bits should work.  Chances
+            # are, just calling it with "cx" is fine...  For now, though, just
+            # assert that it does not matter.
+            assert not descriptor.isMaybeCrossOriginObject()
+            # The scope object should always be from the relevant
+            # global.  Make sure to unwrap it as needed.
+            cgThings.append(
+                CGGeneric(
+                    dedent(
+                        """
+                JS::Rooted<JSObject*> unwrappedObj(cx, js::CheckedUnwrapStatic(obj));
+                // Caller should have ensured that "obj" can be unwrapped already.
+                MOZ_DIAGNOSTIC_ASSERT(unwrappedObj);
+                """
+                    )
+                )
+            )
+            argsPre.append("unwrappedObj")
+
+        if needsUnwrap and needsUnwrappedVar:
+            # We cannot assign into obj because it's a Handle, not a
+            # MutableHandle, so we need a separate Rooted.
+            cgThings.append(CGGeneric("Maybe<JS::Rooted<JSObject*> > unwrappedObj;\n"))
+            unwrappedVar = "unwrappedObj.ref()"
+
+        if idlNode.isMethod() and idlNode.isLegacycaller():
+            # If we can have legacycaller with identifier, we can't
+            # just use the idlNode to determine whether we're
+            # generating code for the legacycaller or not.
+            assert idlNode.isIdentifierLess()
+            # Pass in our thisVal
+            argsPre.append("args.thisv()")
+
+        if idlNode.isMethod():
+            argDescription = "argument %(index)d"
+        elif setter:
+            argDescription = "value being assigned"
+        else:
+            assert self.argCount == 0
+
+        if needsUnwrap:
+            # It's very important that we construct our unwrappedObj, if we need
+            # to do it, before we might start setting up Rooted things for our
+            # arguments, so that we don't violate the stack discipline Rooted
+            # depends on.
+            cgThings.append(
+                CGGeneric("bool objIsXray = xpc::WrapperFactory::IsXrayWrapper(obj);\n")
+            )
+            if needsUnwrappedVar:
+                cgThings.append(
+                    CGIfWrapper(
+                        CGGeneric("unwrappedObj.emplace(cx, obj);\n"), "objIsXray"
+                    )
+                )
+
+        for i in range(argConversionStartsAt, self.argCount):
+            cgThings.append(
+                CGArgumentConverter(
+                    arguments[i],
+                    i,
+                    self.descriptor,
+                    argDescription % {"index": i + 1},
+                    idlNode,
+                    invalidEnumValueFatal=not setter,
+                    lenientFloatCode=lenientFloatCode,
+                )
+            )
+
+        # Now that argument processing is done, enforce the LenientFloat stuff
+        if lenientFloatCode:
+            if setter:
+                foundNonFiniteFloatBehavior = "return true;\n"
+            else:
+                assert idlNode.isMethod()
+                foundNonFiniteFloatBehavior = dedent(
+                    """
+                    args.rval().setUndefined();
+                    return true;
+                    """
+                )
+            cgThings.append(
+                CGGeneric(
+                    fill(
+                        """
+                if (foundNonFiniteFloat) {
+                  $*{returnSteps}
+                }
+                """,
+                        returnSteps=foundNonFiniteFloatBehavior,
+                    )
+                )
+            )
+
+        if needsUnwrap:
+            # Something depends on having the unwrapped object, so unwrap it now.
+            xraySteps = []
+            # XXXkhuey we should be able to MOZ_ASSERT that ${obj} is
+            # not null.
+            xraySteps.append(
+                CGGeneric(
+                    fill(
+                        """
+                    // Since our object is an Xray, we can just CheckedUnwrapStatic:
+                    // we know Xrays have no dynamic unwrap behavior.
+                    ${obj} = js::CheckedUnwrapStatic(${obj});
+                    if (!${obj}) {
+                      return false;
+                    }
+                    """,
+                        obj=unwrappedVar,
+                    )
+                )
+            )
+            if isConstructor:
+                # If we're called via an xray, we need to enter the underlying
+                # object's compartment and then wrap up all of our arguments into
+                # that compartment as needed.  This is all happening after we've
+                # already done the conversions from JS values to WebIDL (C++)
+                # values, so we only need to worry about cases where there are 'any'
+                # or 'object' types, or other things that we represent as actual
+                # JSAPI types, present.  Effectively, we're emulating a
+                # CrossCompartmentWrapper, but working with the C++ types, not the
+                # original list of JS::Values.
+                cgThings.append(CGGeneric("Maybe<JSAutoRealm> ar;\n"))
+                xraySteps.append(CGGeneric("ar.emplace(cx, obj);\n"))
+                xraySteps.append(
+                    CGGeneric(
+                        dedent(
+                            """
+                    if (!JS_WrapObject(cx, &desiredProto)) {
+                      return false;
+                    }
+                    """
+                        )
+                    )
+                )
+                xraySteps.extend(
+                    wrapArgIntoCurrentCompartment(arg, argname, isMember=False)
+                    for arg, argname in self.getArguments()
+                )
+
+            cgThings.append(CGIfWrapper(CGList(xraySteps), "objIsXray"))
+
+        if idlNode.getExtendedAttribute("CEReactions") is not None and not getter:
+            cgThings.append(
+                CGGeneric(
+                    dedent(
+                        """
+                Maybe<AutoCEReaction> ceReaction;
+                DocGroup* docGroup = self->GetDocGroup();
+                if (docGroup) {
+                  ceReaction.emplace(docGroup->CustomElementReactionsStack(), cx);
+                }
+                """
+                    )
+                )
+            )
+
+        # If this is a method that was generated by a maplike/setlike
+        # interface, use the maplike/setlike generator to fill in the body.
+        # Otherwise, use CGCallGenerator to call the native method.
+        if idlNode.isMethod() and idlNode.isMaplikeOrSetlikeOrIterableMethod():
+            if (
+                idlNode.maplikeOrSetlikeOrIterable.isMaplike()
+                or idlNode.maplikeOrSetlikeOrIterable.isSetlike()
+            ):
+                cgThings.append(
+                    CGMaplikeOrSetlikeMethodGenerator(
+                        descriptor,
+                        idlNode.maplikeOrSetlikeOrIterable,
+                        idlNode.identifier.name,
+                    )
+                )
+            else:
+                cgThings.append(
+                    CGIterableMethodGenerator(
+                        descriptor,
+                        idlNode.identifier.name,
+                        self.getArgumentNames(),
+                    )
+                )
+        elif idlNode.isAttr() and idlNode.type.isObservableArray():
+            assert setter
+            cgThings.append(CGObservableArraySetterGenerator(descriptor, idlNode))
+        else:
+            context = GetLabelForErrorReporting(descriptor, idlNode, isConstructor)
+            if getter:
+                context = context + " getter"
+            elif setter:
+                context = context + " setter"
+            # Callee expects a quoted string for the context if
+            # there's a context.
+            context = '"%s"' % context
+
+            if idlNode.isMethod() and idlNode.getExtendedAttribute("WebExtensionStub"):
+                [
+                    nativeMethodName,
+                    argsPre,
+                    args,
+                ] = self.processWebExtensionStubAttribute(idlNode, cgThings)
+            else:
+                args = self.getArguments()
+
+            cgThings.append(
+                CGCallGenerator(
+                    self.needsErrorResult(),
+                    needsCallerType(idlNode),
+                    isChromeOnly(idlNode),
+                    args,
+                    argsPre,
+                    returnType,
+                    self.extendedAttributes,
+                    descriptor,
+                    nativeMethodName,
+                    static,
+                    # We know our "self" must be being kept alive; otherwise we have
+                    # a serious problem.  In common cases it's just an argument and
+                    # we're MOZ_CAN_RUN_SCRIPT, but in some cases it's on the stack
+                    # and being kept alive via references from JS.
+                    object="MOZ_KnownLive(self)",
+                    argsPost=argsPost,
+                    resultVar=resultVar,
+                    context=context,
+                )
+            )
+
+        if useCounterName:
+            # Generate a telemetry call for when [UseCounter] is used.
+            windowCode = fill(
+                """
+                SetUseCounter(obj, eUseCounter_${useCounterName});
+                """,
+                useCounterName=useCounterName,
+            )
+            workerCode = fill(
+                """
+                SetUseCounter(UseCounterWorker::${useCounterName});
+                """,
+                useCounterName=useCounterName,
+            )
+            code = ""
+            if idlNode.isExposedInWindow() and idlNode.isExposedInAnyWorker():
+                code += fill(
+                    """
+                    if (NS_IsMainThread()) {
+                      ${windowCode}
+                    } else {
+                      ${workerCode}
+                    }
+                    """,
+                    windowCode=windowCode,
+                    workerCode=workerCode,
+                )
+            elif idlNode.isExposedInWindow():
+                code += windowCode
+            elif idlNode.isExposedInAnyWorker():
+                code += workerCode
+
+            cgThings.append(CGGeneric(code))
+
+        self.cgRoot = CGList(cgThings)
+
+    def getArgumentNames(self):
+        return ["arg" + str(i) for i in range(len(self.arguments))]
+
+    def getArguments(self):
+        return list(zip(self.arguments, self.getArgumentNames()))
+
+    def processWebExtensionStubAttribute(self, idlNode, cgThings):
+        nativeMethodName = "CallWebExtMethod"
+        stubNameSuffix = idlNode.getExtendedAttribute("WebExtensionStub")
+        if isinstance(stubNameSuffix, list):
+            nativeMethodName += stubNameSuffix[0]
+
+        argsLength = len(self.getArguments())
+        singleVariadicArg = argsLength == 1 and self.getArguments()[0][0].variadic
+
+        # If the method signature does only include a single variadic arguments,
+        # then `arg0` is already a Sequence of JS values and we can pass that
+        # to the WebExtensions Stub method as is.
+        if singleVariadicArg:
+            argsPre = [
+                "cx",
+                'u"%s"_ns' % idlNode.identifier.name,
+                "Constify(%s)" % "arg0",
+            ]
+            args = []
+            return [nativeMethodName, argsPre, args]
+
+        argsPre = [
+            "cx",
+            'u"%s"_ns' % idlNode.identifier.name,
+            "Constify(%s)" % "args_sequence",
+        ]
+        args = []
+
+        # Determine the maximum number of elements of the js values sequence argument,
+        # skipping the last optional callback argument if any:
+        #
+        # if this WebExtensions API method does expect a last optional callback argument,
+        # then it is the callback parameter supported for chrome-compatibility
+        # reasons, and we want it as a separate argument passed to the WebExtension
+        # stub method and skip it from the js values sequence including all other
+        # arguments.
+        maxArgsSequenceLen = argsLength
+        if argsLength > 0:
+            lastArg = self.getArguments()[argsLength - 1]
+            isCallback = lastArg[0].type.tag() == IDLType.Tags.callback
+            if isCallback and lastArg[0].optional:
+                argsPre.append(
+                    "MOZ_KnownLive(NonNullHelper(Constify(%s)))" % lastArg[1]
+                )
+                maxArgsSequenceLen = argsLength - 1
+
+        cgThings.append(
+            CGGeneric(
+                dedent(
+                    fill(
+                        """
+            // Collecting all args js values into the single sequence argument
+            // passed to the webextensions stub method.
+            //
+            // NOTE: The stub method will receive the original non-normalized js values,
+            // but those arguments will still be normalized on the main thread by the
+            // WebExtensions API request handler using the same JSONSchema defnition
+            // used by the non-webIDL webextensions API bindings.
+            AutoSequence<JS::Value> args_sequence;
+            SequenceRooter<JS::Value> args_sequence_holder(cx, &args_sequence);
+
+            // maximum number of arguments expected by the WebExtensions API method
+            // excluding the last optional chrome-compatible callback argument (which
+            // is being passed to the stub method as a separate additional argument).
+            uint32_t maxArgsSequenceLen = ${maxArgsSequenceLen};
+
+            uint32_t sequenceArgsLen = args.length() <= maxArgsSequenceLen ?
+              args.length() : maxArgsSequenceLen;
+
+            if (sequenceArgsLen > 0) {
+              if (!args_sequence.SetCapacity(sequenceArgsLen, mozilla::fallible)) {
+                JS_ReportOutOfMemory(cx);
+                return false;
+              }
+              for (uint32_t argIdx = 0; argIdx < sequenceArgsLen; ++argIdx) {
+                // OK to do infallible append here, since we ensured capacity already.
+                JS::Value& slot = *args_sequence.AppendElement();
+                slot = args[argIdx];
+              }
+            }
+            """,
+                        maxArgsSequenceLen=maxArgsSequenceLen,
+                    )
+                )
+            )
+        )
+
+        return [nativeMethodName, argsPre, args]
+
+    def needsErrorResult(self):
+        return "needsErrorResult" in self.extendedAttributes
+
+    def wrap_return_value(self):
+        wrapCode = ""
+
+        returnsNewObject = memberReturnsNewObject(self.idlNode)
+        if returnsNewObject and (
+            self.returnType.isGeckoInterface() or self.returnType.isPromise()
+        ):
+            wrapCode += dedent(
+                """
+                static_assert(!std::is_pointer_v<decltype(result)>,
+                              "NewObject implies that we need to keep the object alive with a strong reference.");
+                """
+            )
+
+        if self.setSlot:
+            # For attributes in slots, we want to do some
+            # post-processing once we've wrapped them.
+            successCode = "break;\n"
+        else:
+            successCode = None
+
+        resultTemplateValues = {
+            "jsvalRef": "args.rval()",
+            "jsvalHandle": "args.rval()",
+            "returnsNewObject": returnsNewObject,
+            "isConstructorRetval": self.isConstructor,
+            "successCode": successCode,
+            # 'obj' in this dictionary is the thing whose compartment we are
+            # trying to do the to-JS conversion in.  We're going to put that
+            # thing in a variable named "conversionScope" if setSlot is true.
+            # Otherwise, just use "obj" for lack of anything better.
+            "obj": "conversionScope" if self.setSlot else "obj",
+        }
+
+        wrapCode += wrapForType(self.returnType, self.descriptor, resultTemplateValues)
+
+        if self.setSlot:
+            if self.idlNode.isStatic():
+                raise TypeError(
+                    "Attribute %s.%s is static, so we don't have a useful slot "
+                    "to cache it in, because we don't have support for that on "
+                    "interface objects.  See "
+                    "https://bugzilla.mozilla.org/show_bug.cgi?id=1363870"
+                    % (
+                        self.descriptor.interface.identifier.name,
+                        self.idlNode.identifier.name,
+                    )
+                )
+
+            # When using a slot on the Xray expando, we need to make sure that
+            # our initial conversion to a JS::Value is done in the caller
+            # compartment.  When using a slot on our reflector, we want to do
+            # the conversion in the compartment of that reflector (that is,
+            # slotStorage).  In both cases we want to make sure that we finally
+            # set up args.rval() to be in the caller compartment.  We also need
+            # to make sure that the conversion steps happen inside a do/while
+            # that they can break out of on success.
+            #
+            # Of course we always have to wrap the value into the slotStorage
+            # compartment before we store it in slotStorage.
+
+            # postConversionSteps are the steps that run while we're still in
+            # the compartment we do our conversion in but after we've finished
+            # the initial conversion into args.rval().
+            postConversionSteps = ""
+            if needsContainsHack(self.idlNode):
+                # Define a .contains on the object that has the same value as
+                # .includes; needed for backwards compat in extensions as we
+                # migrate some DOMStringLists to FrozenArray.
+                postConversionSteps += dedent(
+                    """
+                    if (args.rval().isObject() && nsContentUtils::ThreadsafeIsSystemCaller(cx)) {
+                      JS::Rooted<JSObject*> rvalObj(cx, &args.rval().toObject());
+                      JS::Rooted<JS::Value> includesVal(cx);
+                      if (!JS_GetProperty(cx, rvalObj, "includes", &includesVal) ||
+                          !JS_DefineProperty(cx, rvalObj, "contains", includesVal, JSPROP_ENUMERATE)) {
+                        return false;
+                      }
+                    }
+
+                    """
+                )
+            if self.idlNode.getExtendedAttribute("Frozen"):
+                assert (
+                    self.idlNode.type.isSequence() or self.idlNode.type.isDictionary()
+                )
+                freezeValue = CGGeneric(
+                    "JS::Rooted<JSObject*> rvalObj(cx, &args.rval().toObject());\n"
+                    "if (!JS_FreezeObject(cx, rvalObj)) {\n"
+                    "  return false;\n"
+                    "}\n"
+                )
+                if self.idlNode.type.nullable():
+                    freezeValue = CGIfWrapper(freezeValue, "args.rval().isObject()")
+                postConversionSteps += freezeValue.define()
+
+            # slotStorageSteps are steps that run once we have entered the
+            # slotStorage compartment.
+            slotStorageSteps = fill(
+                """
+                // Make a copy so that we don't do unnecessary wrapping on args.rval().
+                JS::Rooted<JS::Value> storedVal(cx, args.rval());
+                if (!${maybeWrap}(cx, &storedVal)) {
+                  return false;
+                }
+                JS::SetReservedSlot(slotStorage, slotIndex, storedVal);
+                """,
+                maybeWrap=getMaybeWrapValueFuncForType(self.idlNode.type),
+            )
+
+            checkForXray = mayUseXrayExpandoSlots(self.descriptor, self.idlNode)
+
+            # For the case of Cached attributes, go ahead and preserve our
+            # wrapper if needed.  We need to do this because otherwise the
+            # wrapper could get garbage-collected and the cached value would
+            # suddenly disappear, but the whole premise of cached values is that
+            # they never change without explicit action on someone's part.  We
+            # don't do this for StoreInSlot, since those get dealt with during
+            # wrapper setup, and failure would involve us trying to clear an
+            # already-preserved wrapper.
+            if (
+                self.idlNode.getExtendedAttribute("Cached")
+                and self.descriptor.wrapperCache
+            ):
+                preserveWrapper = dedent(
+                    """
+                    PreserveWrapper(self);
+                    """
+                )
+                if checkForXray:
+                    preserveWrapper = fill(
+                        """
+                        if (!isXray) {
+                          // In the Xray case we don't need to do this, because getting the
+                          // expando object already preserved our wrapper.
+                          $*{preserveWrapper}
+                        }
+                        """,
+                        preserveWrapper=preserveWrapper,
+                    )
+                slotStorageSteps += preserveWrapper
+
+            if checkForXray:
+                # In the Xray case we use the current global as conversion
+                # scope, as explained in the big compartment/conversion comment
+                # above.
+                conversionScope = "isXray ? JS::CurrentGlobalOrNull(cx) : slotStorage"
+            else:
+                conversionScope = "slotStorage"
+
+            wrapCode = fill(
+                """
+                {
+                  JS::Rooted<JSObject*> conversionScope(cx, ${conversionScope});
+                  JSAutoRealm ar(cx, conversionScope);
+                  do { // block we break out of when done wrapping
+                    $*{wrapCode}
+                  } while (false);
+                  $*{postConversionSteps}
+                }
+                { // And now store things in the realm of our slotStorage.
+                  JSAutoRealm ar(cx, slotStorage);
+                  $*{slotStorageSteps}
+                }
+                // And now make sure args.rval() is in the caller realm.
+                return ${maybeWrap}(cx, args.rval());
+                """,
+                conversionScope=conversionScope,
+                wrapCode=wrapCode,
+                postConversionSteps=postConversionSteps,
+                slotStorageSteps=slotStorageSteps,
+                maybeWrap=getMaybeWrapValueFuncForType(self.idlNode.type),
+            )
+        return wrapCode
+
+    def define(self):
+        return self.cgRoot.define() + self.wrap_return_value()
+
+
+class CGSwitch(CGList):
+    """
+    A class to generate code for a switch statement.
+
+    Takes three constructor arguments: an expression, a list of cases,
+    and an optional default.
+
+    Each case is a CGCase.  The default is a CGThing for the body of
+    the default case, if any.
+    """
+
+    def __init__(self, expression, cases, default=None):
+        CGList.__init__(self, [CGIndenter(c) for c in cases])
+        self.prepend(CGGeneric("switch (" + expression + ") {\n"))
+        if default is not None:
+            self.append(
+                CGIndenter(
+                    CGWrapper(CGIndenter(default), pre="default: {\n", post="}\n")
+                )
+            )
+
+        self.append(CGGeneric("}\n"))
+
+
+class CGCase(CGList):
+    """
+    A class to generate code for a case statement.
+
+    Takes three constructor arguments: an expression, a CGThing for
+    the body (allowed to be None if there is no body), and an optional
+    argument for whether add a break, add fallthrough annotation or add nothing
+    (defaulting to add a break).
+    """
+
+    ADD_BREAK = 0
+    ADD_FALLTHROUGH = 1
+    DONT_ADD_BREAK = 2
+
+    def __init__(self, expression, body, breakOrFallthrough=ADD_BREAK):
+        CGList.__init__(self, [])
+
+        assert (
+            breakOrFallthrough == CGCase.ADD_BREAK
+            or breakOrFallthrough == CGCase.ADD_FALLTHROUGH
+            or breakOrFallthrough == CGCase.DONT_ADD_BREAK
+        )
+
+        self.append(CGGeneric("case " + expression + ": {\n"))
+        bodyList = CGList([body])
+        if breakOrFallthrough == CGCase.ADD_FALLTHROUGH:
+            bodyList.append(CGGeneric("[[fallthrough]];\n"))
+        elif breakOrFallthrough == CGCase.ADD_BREAK:
+            bodyList.append(CGGeneric("break;\n"))
+        self.append(CGIndenter(bodyList))
+        self.append(CGGeneric("}\n"))
+
+
+class CGMethodCall(CGThing):
+    """
+    A class to generate selection of a method signature from a set of
+    signatures and generation of a call to that signature.
+    """
+
+    def __init__(
+        self, nativeMethodName, static, descriptor, method, isConstructor=False
+    ):
+        CGThing.__init__(self)
+
+        methodName = GetLabelForErrorReporting(descriptor, method, isConstructor)
+        argDesc = "argument %d"
+
+        if method.getExtendedAttribute("UseCounter"):
+            useCounterName = methodName.replace(".", "_").replace(" ", "_")
+        else:
+            useCounterName = None
+
+        if method.isStatic():
+            nativeType = descriptor.nativeType
+            staticTypeOverride = PropertyDefiner.getStringAttr(
+                method, "StaticClassOverride"
+            )
+            if staticTypeOverride:
+                nativeType = staticTypeOverride
+            nativeMethodName = "%s::%s" % (nativeType, nativeMethodName)
+
+        def requiredArgCount(signature):
+            arguments = signature[1]
+            if len(arguments) == 0:
+                return 0
+            requiredArgs = len(arguments)
+            while requiredArgs and arguments[requiredArgs - 1].optional:
+                requiredArgs -= 1
+            return requiredArgs
+
+        def getPerSignatureCall(signature, argConversionStartsAt=0):
+            return CGPerSignatureCall(
+                signature[0],
+                signature[1],
+                nativeMethodName,
+                static,
+                descriptor,
+                method,
+                argConversionStartsAt=argConversionStartsAt,
+                isConstructor=isConstructor,
+                useCounterName=useCounterName,
+            )
+
+        signatures = method.signatures()
+        if len(signatures) == 1:
+            # Special case: we can just do a per-signature method call
+            # here for our one signature and not worry about switching
+            # on anything.
+            signature = signatures[0]
+            self.cgRoot = CGList([getPerSignatureCall(signature)])
+            requiredArgs = requiredArgCount(signature)
+
+            # Skip required arguments check for maplike/setlike interfaces, as
+            # they can have arguments which are not passed, and are treated as
+            # if undefined had been explicitly passed.
+            if requiredArgs > 0 and not method.isMaplikeOrSetlikeOrIterableMethod():
+                code = fill(
+                    """
+                    if (!args.requireAtLeast(cx, "${methodName}", ${requiredArgs})) {
+                      return false;
+                    }
+                    """,
+                    requiredArgs=requiredArgs,
+                    methodName=methodName,
+                )
+                self.cgRoot.prepend(CGGeneric(code))
+            return
+
+        # Need to find the right overload
+        maxArgCount = method.maxArgCount
+        allowedArgCounts = method.allowedArgCounts
+
+        argCountCases = []
+        for argCountIdx, argCount in enumerate(allowedArgCounts):
+            possibleSignatures = method.signaturesForArgCount(argCount)
+
+            # Try to optimize away cases when the next argCount in the list
+            # will have the same code as us; if it does, we can fall through to
+            # that case.
+            if argCountIdx + 1 < len(allowedArgCounts):
+                nextPossibleSignatures = method.signaturesForArgCount(
+                    allowedArgCounts[argCountIdx + 1]
+                )
+            else:
+                nextPossibleSignatures = None
+            if possibleSignatures == nextPossibleSignatures:
+                # Same set of signatures means we better have the same
+                # distinguishing index.  So we can in fact just fall through to
+                # the next case here.
+                assert len(possibleSignatures) == 1 or (
+                    method.distinguishingIndexForArgCount(argCount)
+                    == method.distinguishingIndexForArgCount(
+                        allowedArgCounts[argCountIdx + 1]
+                    )
+                )
+                argCountCases.append(
+                    CGCase(str(argCount), None, CGCase.ADD_FALLTHROUGH)
+                )
+                continue
+
+            if len(possibleSignatures) == 1:
+                # easy case!
+                signature = possibleSignatures[0]
+                argCountCases.append(
+                    CGCase(str(argCount), getPerSignatureCall(signature))
+                )
+                continue
+
+            distinguishingIndex = method.distinguishingIndexForArgCount(argCount)
+
+            def distinguishingArgument(signature):
+                args = signature[1]
+                if distinguishingIndex < len(args):
+                    return args[distinguishingIndex]
+                assert args[-1].variadic
+                return args[-1]
+
+            def distinguishingType(signature):
+                return distinguishingArgument(signature).type
+
+            for sig in possibleSignatures:
+                # We should not have "any" args at distinguishingIndex,
+                # since we have multiple possible signatures remaining,
+                # but "any" is never distinguishable from anything else.
+                assert not distinguishingType(sig).isAny()
+                # We can't handle unions at the distinguishing index.
+                if distinguishingType(sig).isUnion():
+                    raise TypeError(
+                        "No support for unions as distinguishing "
+                        "arguments yet: %s" % distinguishingArgument(sig).location
+                    )
+                # We don't support variadics as the distinguishingArgument yet.
+                # If you want to add support, consider this case:
+                #
+                #   undefined(long... foo);
+                #   undefined(long bar, Int32Array baz);
+                #
+                # in which we have to convert argument 0 to long before picking
+                # an overload... but all the variadic stuff needs to go into a
+                # single array in case we pick that overload, so we have to have
+                # machinery for converting argument 0 to long and then either
+                # placing it in the variadic bit or not.  Or something.  We may
+                # be able to loosen this restriction if the variadic arg is in
+                # fact at distinguishingIndex, perhaps.  Would need to
+                # double-check.
+                if distinguishingArgument(sig).variadic:
+                    raise TypeError(
+                        "No support for variadics as distinguishing "
+                        "arguments yet: %s" % distinguishingArgument(sig).location
+                    )
+
+            # Convert all our arguments up to the distinguishing index.
+            # Doesn't matter which of the possible signatures we use, since
+            # they all have the same types up to that point; just use
+            # possibleSignatures[0]
+            caseBody = [
+                CGArgumentConverter(
+                    possibleSignatures[0][1][i],
+                    i,
+                    descriptor,
+                    argDesc % (i + 1),
+                    method,
+                )
+                for i in range(0, distinguishingIndex)
+            ]
+
+            # Select the right overload from our set.
+            distinguishingArg = "args[%d]" % distinguishingIndex
+
+            def tryCall(
+                signature, indent, isDefinitelyObject=False, isNullOrUndefined=False
+            ):
+                assert not isDefinitelyObject or not isNullOrUndefined
+                assert isDefinitelyObject or isNullOrUndefined
+                if isDefinitelyObject:
+                    failureCode = "break;\n"
+                else:
+                    failureCode = None
+                type = distinguishingType(signature)
+                # The argument at index distinguishingIndex can't possibly be
+                # unset here, because we've already checked that argc is large
+                # enough that we can examine this argument.  But note that we
+                # still want to claim that optional arguments are optional, in
+                # case undefined was passed in.
+                argIsOptional = distinguishingArgument(signature).canHaveMissingValue()
+                testCode = instantiateJSToNativeConversion(
+                    getJSToNativeConversionInfo(
+                        type,
+                        descriptor,
+                        failureCode=failureCode,
+                        isDefinitelyObject=isDefinitelyObject,
+                        isNullOrUndefined=isNullOrUndefined,
+                        isOptional=argIsOptional,
+                        sourceDescription=(argDesc % (distinguishingIndex + 1)),
+                    ),
+                    {
+                        "declName": "arg%d" % distinguishingIndex,
+                        "holderName": ("arg%d" % distinguishingIndex) + "_holder",
+                        "val": distinguishingArg,
+                        "obj": "obj",
+                        "haveValue": "args.hasDefined(%d)" % distinguishingIndex,
+                        "passedToJSImpl": toStringBool(
+                            isJSImplementedDescriptor(descriptor)
+                        ),
+                    },
+                    checkForValue=argIsOptional,
+                )
+                caseBody.append(CGIndenter(testCode, indent))
+
+                # If we got this far, we know we unwrapped to the right
+                # C++ type, so just do the call.  Start conversion with
+                # distinguishingIndex + 1, since we already converted
+                # distinguishingIndex.
+                caseBody.append(
+                    CGIndenter(
+                        getPerSignatureCall(signature, distinguishingIndex + 1), indent
+                    )
+                )
+
+            def hasConditionalConversion(type):
+                """
+                Return whether the argument conversion for this type will be
+                conditional on the type of incoming JS value.  For example, for
+                interface types the conversion is conditional on the incoming
+                value being isObject().
+
+                For the types for which this returns false, we do not have to
+                output extra isUndefined() or isNullOrUndefined() cases, because
+                null/undefined values will just fall through into our
+                unconditional conversion.
+                """
+                if type.isString() or type.isEnum():
+                    return False
+                if type.isBoolean():
+                    distinguishingTypes = (
+                        distinguishingType(s) for s in possibleSignatures
+                    )
+                    return any(
+                        t.isString() or t.isEnum() or t.isNumeric()
+                        for t in distinguishingTypes
+                    )
+                if type.isNumeric():
+                    distinguishingTypes = (
+                        distinguishingType(s) for s in possibleSignatures
+                    )
+                    return any(t.isString() or t.isEnum() for t in distinguishingTypes)
+                return True
+
+            def needsNullOrUndefinedCase(type):
+                """
+                Return true if the type needs a special isNullOrUndefined() case
+                """
+                return (
+                    type.nullable() and hasConditionalConversion(type)
+                ) or type.isDictionary()
+
+            # First check for undefined and optional distinguishing arguments
+            # and output a special branch for that case.  Note that we don't
+            # use distinguishingArgument here because we actualy want to
+            # exclude variadic arguments.  Also note that we skip this check if
+            # we plan to output a isNullOrUndefined() special case for this
+            # argument anyway, since that will subsume our isUndefined() check.
+            # This is safe, because there can be at most one nullable
+            # distinguishing argument, so if we're it we'll definitely get
+            # picked up by the nullable handling.  Also, we can skip this check
+            # if the argument has an unconditional conversion later on.
+            undefSigs = [
+                s
+                for s in possibleSignatures
+                if distinguishingIndex < len(s[1])
+                and s[1][distinguishingIndex].optional
+                and hasConditionalConversion(s[1][distinguishingIndex].type)
+                and not needsNullOrUndefinedCase(s[1][distinguishingIndex].type)
+            ]
+            # Can't have multiple signatures with an optional argument at the
+            # same index.
+            assert len(undefSigs) < 2
+            if len(undefSigs) > 0:
+                caseBody.append(
+                    CGGeneric("if (%s.isUndefined()) {\n" % distinguishingArg)
+                )
+                tryCall(undefSigs[0], 2, isNullOrUndefined=True)
+                caseBody.append(CGGeneric("}\n"))
+
+            # Next, check for null or undefined.  That means looking for
+            # nullable arguments at the distinguishing index and outputting a
+            # separate branch for them.  But if the nullable argument has an
+            # unconditional conversion, we don't need to do that.  The reason
+            # for that is that at most one argument at the distinguishing index
+            # is nullable (since two nullable arguments are not
+            # distinguishable), and null/undefined values will always fall
+            # through to the unconditional conversion we have, if any, since
+            # they will fail whatever the conditions on the input value are for
+            # our other conversions.
+            nullOrUndefSigs = [
+                s
+                for s in possibleSignatures
+                if needsNullOrUndefinedCase(distinguishingType(s))
+            ]
+            # Can't have multiple nullable types here
+            assert len(nullOrUndefSigs) < 2
+            if len(nullOrUndefSigs) > 0:
+                caseBody.append(
+                    CGGeneric("if (%s.isNullOrUndefined()) {\n" % distinguishingArg)
+                )
+                tryCall(nullOrUndefSigs[0], 2, isNullOrUndefined=True)
+                caseBody.append(CGGeneric("}\n"))
+
+            # Now check for distinguishingArg being various kinds of objects.
+            # The spec says to check for the following things in order:
+            # 1)  A platform object that's not a platform array object, being
+            #     passed to an interface or "object" arg.
+            # 2)  A callable object being passed to a callback or "object" arg.
+            # 3)  An iterable object being passed to a sequence arg.
+            # 4)  Any object being passed to a array or callback interface or
+            #     dictionary or "object" arg.
+
+            # First grab all the overloads that have a non-callback interface
+            # (which includes SpiderMonkey interfaces) at the distinguishing
+            # index.  We can also include the ones that have an "object" here,
+            # since if those are present no other object-typed argument will
+            # be.
+            objectSigs = [
+                s
+                for s in possibleSignatures
+                if (
+                    distinguishingType(s).isObject()
+                    or distinguishingType(s).isNonCallbackInterface()
+                )
+            ]
+
+            # And all the overloads that take callbacks
+            objectSigs.extend(
+                s for s in possibleSignatures if distinguishingType(s).isCallback()
+            )
+
+            # And all the overloads that take sequences
+            objectSigs.extend(
+                s for s in possibleSignatures if distinguishingType(s).isSequence()
+            )
+
+            # Now append all the overloads that take a dictionary or callback
+            # interface or record.  There should be only one of these!
+            genericObjectSigs = [
+                s
+                for s in possibleSignatures
+                if (
+                    distinguishingType(s).isDictionary()
+                    or distinguishingType(s).isRecord()
+                    or distinguishingType(s).isCallbackInterface()
+                )
+            ]
+            assert len(genericObjectSigs) <= 1
+            objectSigs.extend(genericObjectSigs)
+
+            # There might be more than one thing in objectSigs; we need to check
+            # which ones we unwrap to.
+            if len(objectSigs) > 0:
+                # Here it's enough to guard on our argument being an object.
+                # The code for unwrapping non-callback interfaces, spiderMonkey
+                # interfaces, and sequences will just bail out and move
+                # on to the next overload if the object fails to unwrap
+                # correctly, while "object" accepts any object anyway.  We
+                # could even not do the isObject() check up front here, but in
+                # cases where we have multiple object overloads it makes sense
+                # to do it only once instead of for each overload.  That will
+                # also allow the unwrapping test to skip having to do codegen
+                # for the null-or-undefined case, which we already handled
+                # above.
+                caseBody.append(CGGeneric("if (%s.isObject()) {\n" % distinguishingArg))
+                for sig in objectSigs:
+                    caseBody.append(CGIndenter(CGGeneric("do {\n")))
+                    # Indent by 4, since we need to indent further
+                    # than our "do" statement
+                    tryCall(sig, 4, isDefinitelyObject=True)
+                    caseBody.append(CGIndenter(CGGeneric("} while (false);\n")))
+
+                caseBody.append(CGGeneric("}\n"))
+
+            # Now we only have to consider booleans, numerics, and strings.  If
+            # we only have one of them, then we can just output it.  But if not,
+            # then we need to output some of the cases conditionally: if we have
+            # a string overload, then boolean and numeric are conditional, and
+            # if not then boolean is conditional if we have a numeric overload.
+            def findUniqueSignature(filterLambda):
+                sigs = [s for s in possibleSignatures if filterLambda(s)]
+                assert len(sigs) < 2
+                if len(sigs) > 0:
+                    return sigs[0]
+                return None
+
+            stringSignature = findUniqueSignature(
+                lambda s: (
+                    distinguishingType(s).isString() or distinguishingType(s).isEnum()
+                )
+            )
+            numericSignature = findUniqueSignature(
+                lambda s: distinguishingType(s).isNumeric()
+            )
+            booleanSignature = findUniqueSignature(
+                lambda s: distinguishingType(s).isBoolean()
+            )
+
+            if stringSignature or numericSignature:
+                booleanCondition = "%s.isBoolean()"
+            else:
+                booleanCondition = None
+
+            if stringSignature:
+                numericCondition = "%s.isNumber()"
+            else:
+                numericCondition = None
+
+            def addCase(sig, condition):
+                sigCode = getPerSignatureCall(sig, distinguishingIndex)
+                if condition:
+                    sigCode = CGIfWrapper(sigCode, condition % distinguishingArg)
+                caseBody.append(sigCode)
+
+            if booleanSignature:
+                addCase(booleanSignature, booleanCondition)
+            if numericSignature:
+                addCase(numericSignature, numericCondition)
+            if stringSignature:
+                addCase(stringSignature, None)
+
+            if not booleanSignature and not numericSignature and not stringSignature:
+                # Just throw; we have no idea what we're supposed to
+                # do with this.
+                caseBody.append(
+                    CGGeneric(
+                        'return cx.ThrowErrorMessage<MSG_OVERLOAD_RESOLUTION_FAILED>("%d", "%d");\n'
+                        % (distinguishingIndex + 1, argCount)
+                    )
+                )
+
+            argCountCases.append(CGCase(str(argCount), CGList(caseBody)))
+
+        overloadCGThings = []
+        overloadCGThings.append(
+            CGGeneric(
+                "unsigned argcount = std::min(args.length(), %du);\n" % maxArgCount
+            )
+        )
+        overloadCGThings.append(
+            CGSwitch(
+                "argcount",
+                argCountCases,
+                CGGeneric(
+                    dedent(
+                        """
+                         // Using nsPrintfCString here would require including that
+                         // header.  Let's not worry about it.
+                         nsAutoCString argCountStr;
+                         argCountStr.AppendPrintf("%u", args.length());
+                         return cx.ThrowErrorMessage<MSG_INVALID_OVERLOAD_ARGCOUNT>(argCountStr.get());
+                         """
+                    )
+                ),
+            )
+        )
+        overloadCGThings.append(
+            CGGeneric(
+                'MOZ_CRASH("We have an always-returning default case");\n'
+                "return false;\n"
+            )
+        )
+        self.cgRoot = CGList(overloadCGThings)
+
+    def define(self):
+        return self.cgRoot.define()
+
+
+class CGGetterCall(CGPerSignatureCall):
+    """
+    A class to generate a native object getter call for a particular IDL
+    getter.
+    """
+
+    def __init__(
+        self,
+        returnType,
+        nativeMethodName,
+        descriptor,
+        attr,
+        dontSetSlot=False,
+        extendedAttributes=None,
+    ):
+        if attr.getExtendedAttribute("UseCounter"):
+            useCounterName = "%s_%s_getter" % (
+                descriptor.interface.identifier.name,
+                attr.identifier.name,
+            )
+        else:
+            useCounterName = None
+        if attr.isStatic():
+            nativeMethodName = "%s::%s" % (descriptor.nativeType, nativeMethodName)
+        CGPerSignatureCall.__init__(
+            self,
+            returnType,
+            [],
+            nativeMethodName,
+            attr.isStatic(),
+            descriptor,
+            attr,
+            getter=True,
+            useCounterName=useCounterName,
+            dontSetSlot=dontSetSlot,
+            extendedAttributes=extendedAttributes,
+        )
+
+
+class FakeIdentifier:
+    def __init__(self, name):
+        self.name = name
+
+
+class FakeArgument:
+    """
+    A class that quacks like an IDLArgument.  This is used to make
+    setters look like method calls or for special operations.
+    """
+
+    def __init__(self, type, name="arg", allowTreatNonCallableAsNull=False):
+        self.type = type
+        self.optional = False
+        self.variadic = False
+        self.defaultValue = None
+        self._allowTreatNonCallableAsNull = allowTreatNonCallableAsNull
+
+        self.identifier = FakeIdentifier(name)
+
+    def allowTreatNonCallableAsNull(self):
+        return self._allowTreatNonCallableAsNull
+
+    def canHaveMissingValue(self):
+        return False
+
+
+class CGSetterCall(CGPerSignatureCall):
+    """
+    A class to generate a native object setter call for a particular IDL
+    setter.
+    """
+
+    def __init__(self, argType, nativeMethodName, descriptor, attr):
+        if attr.getExtendedAttribute("UseCounter"):
+            useCounterName = "%s_%s_setter" % (
+                descriptor.interface.identifier.name,
+                attr.identifier.name,
+            )
+        else:
+            useCounterName = None
+        if attr.isStatic():
+            nativeMethodName = "%s::%s" % (descriptor.nativeType, nativeMethodName)
+        CGPerSignatureCall.__init__(
+            self,
+            None,
+            [FakeArgument(argType, allowTreatNonCallableAsNull=True)],
+            nativeMethodName,
+            attr.isStatic(),
+            descriptor,
+            attr,
+            setter=True,
+            useCounterName=useCounterName,
+        )
+
+    def wrap_return_value(self):
+        attr = self.idlNode
+        clearSlot = ""
+        if self.descriptor.wrapperCache and attr.slotIndices is not None:
+            if attr.getExtendedAttribute("StoreInSlot"):
+                clearSlot = "%s(cx, self);\n" % MakeClearCachedValueNativeName(
+                    self.idlNode
+                )
+            elif attr.getExtendedAttribute("Cached"):
+                clearSlot = "%s(self);\n" % MakeClearCachedValueNativeName(self.idlNode)
+
+        # We have no return value
+        return "\n" "%s" "return true;\n" % clearSlot
+
+
+class CGAbstractBindingMethod(CGAbstractStaticMethod):
+    """
+    Common class to generate some of our class hooks.  This will generate the
+    function declaration, get a reference to the JS object for our binding
+    object (which might be an argument of the class hook or something we get
+    from a JS::CallArgs), and unwrap into the right C++ type. Subclasses are
+    expected to override the generate_code function to do the rest of the work.
+    This function should return a CGThing which is already properly indented.
+
+    getThisObj should be code for getting a JSObject* for the binding
+    object.  "" can be passed in if the binding object is already stored in
+    'obj'.
+
+    callArgs should be code for getting a JS::CallArgs into a variable
+    called 'args'.  This can be "" if there is already such a variable
+    around or if the body does not need a JS::CallArgs.
+
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        name,
+        args,
+        getThisObj,
+        callArgs="JS::CallArgs args = JS::CallArgsFromVp(argc, vp);\n",
+    ):
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", args, canRunScript=True
+        )
+
+        # This can't ever happen, because we only use this for class hooks.
+        self.unwrapFailureCode = fill(
+            """
+            MOZ_CRASH("Unexpected object in '${name}' hook");
+            return false;
+            """,
+            name=name,
+        )
+
+        if getThisObj == "":
+            self.getThisObj = None
+        else:
+            self.getThisObj = CGGeneric(
+                "JS::Rooted<JSObject*> obj(cx, %s);\n" % getThisObj
+            )
+        self.callArgs = callArgs
+
+    def definition_body(self):
+        body = self.callArgs
+        if self.getThisObj is not None:
+            body += self.getThisObj.define() + "\n"
+        body += "%s* self;\n" % self.descriptor.nativeType
+        body += dedent(
+            """
+            JS::Rooted<JS::Value> rootSelf(cx, JS::ObjectValue(*obj));
+            """
+        )
+
+        body += str(
+            CastableObjectUnwrapper(
+                self.descriptor, "rootSelf", "&rootSelf", "self", self.unwrapFailureCode
+            )
+        )
+
+        return body + self.generate_code().define()
+
+    def generate_code(self):
+        assert False  # Override me
+
+
+class CGAbstractStaticBindingMethod(CGAbstractStaticMethod):
+    """
+    Common class to generate the JSNatives for all our static methods, getters
+    and setters.  This will generate the function declaration and unwrap the
+    global object.  Subclasses are expected to override the generate_code
+    function to do the rest of the work.  This function should return a
+    CGThing which is already properly indented.
+    """
+
+    def __init__(self, descriptor, name):
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", JSNativeArguments(), canRunScript=True
+        )
+
+    def definition_body(self):
+        # Make sure that "obj" is in the same compartment as "cx", since we'll
+        # later use it to wrap return values.
+        unwrap = dedent(
+            """
+            JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+            JS::Rooted<JSObject*> obj(cx, &args.callee());
+
+            """
+        )
+        return unwrap + self.generate_code().define()
+
+    def generate_code(self):
+        assert False  # Override me
+
+
+def MakeNativeName(name):
+    return name[0].upper() + IDLToCIdentifier(name[1:])
+
+
+def GetWebExposedName(idlObject, descriptor):
+    if idlObject == descriptor.operations["Stringifier"]:
+        return "toString"
+    name = idlObject.identifier.name
+    if name == "__namedsetter":
+        return "named setter"
+    if name == "__namedgetter":
+        return "named getter"
+    if name == "__indexedsetter":
+        return "indexed setter"
+    if name == "__indexedgetter":
+        return "indexed getter"
+    if name == "__legacycaller":
+        return "legacy caller"
+    return name
+
+
+def GetConstructorNameForReporting(descriptor, ctor):
+    # Figure out the name of our constructor for reporting purposes.
+    # For unnamed webidl constructors, identifier.name is "constructor" but
+    # the name JS sees is the interface name; for legacy factory functions
+    # identifier.name is the actual name.
+    ctorName = ctor.identifier.name
+    if ctorName == "constructor":
+        return descriptor.interface.identifier.name
+    return ctorName
+
+
+def GetLabelForErrorReporting(descriptor, idlObject, isConstructor):
+    """
+    descriptor is the descriptor for the interface involved
+
+    idlObject is the method (regular or static), attribute (regular or
+    static), or constructor (named or not) involved.
+
+    isConstructor is true if idlObject is a constructor and false otherwise.
+    """
+    if isConstructor:
+        return "%s constructor" % GetConstructorNameForReporting(descriptor, idlObject)
+
+    namePrefix = descriptor.interface.identifier.name
+    name = GetWebExposedName(idlObject, descriptor)
+    if " " in name:
+        # It's got a space already, so just space-separate.
+        return "%s %s" % (namePrefix, name)
+
+    return "%s.%s" % (namePrefix, name)
+
+
+class CGSpecializedMethod(CGAbstractStaticMethod):
+    """
+    A class for generating the C++ code for a specialized method that the JIT
+    can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, method):
+        self.method = method
+        name = CppKeywords.checkMethodName(IDLToCIdentifier(method.identifier.name))
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("void*", "void_self"),
+            Argument("const JSJitMethodCallArgs&", "args"),
+        ]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", args, canRunScript=True
+        )
+
+    def definition_body(self):
+        nativeName = CGSpecializedMethod.makeNativeName(self.descriptor, self.method)
+        call = CGMethodCall(
+            nativeName, self.method.isStatic(), self.descriptor, self.method
+        ).define()
+        prefix = ""
+        if self.method.getExtendedAttribute("CrossOriginCallable"):
+            for signature in self.method.signatures():
+                # non-undefined signatures would require us to deal with remote proxies for the
+                # return value here.
+                if not signature[0].isUndefined():
+                    raise TypeError(
+                        "We don't support a method marked as CrossOriginCallable "
+                        "with non-undefined return type"
+                    )
+            prototypeID, _ = PrototypeIDAndDepth(self.descriptor)
+            prefix = fill(
+                """
+                // CrossOriginThisPolicy::UnwrapThisObject stores a ${nativeType}::RemoteProxy in void_self
+                // if obj is a proxy with a RemoteObjectProxy handler for the right type, or else it stores
+                // a ${nativeType}. If we get here from the JIT (without going through UnwrapThisObject) we
+                // know void_self contains a ${nativeType}; we don't have special cases in the JIT to deal
+                // with remote object proxies.
+                if (IsRemoteObjectProxy(obj, ${prototypeID})) {
+                    auto* self = static_cast<${nativeType}::RemoteProxy*>(void_self);
+                    $*{call}
+                }
+                """,
+                prototypeID=prototypeID,
+                nativeType=self.descriptor.nativeType,
+                call=call,
+            )
+        return prefix + fill(
+            """
+            auto* self = static_cast<${nativeType}*>(void_self);
+            $*{call}
+            """,
+            nativeType=self.descriptor.nativeType,
+            call=call,
+        )
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        method_name = self.method.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${method_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_METHOD) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            method_name=method_name,
+        )
+
+    @staticmethod
+    def should_have_method_description(descriptor, idlMethod):
+        """
+        Returns whether the given IDL method (static, non-static, constructor)
+        should have a method description declaration, for use in error
+        reporting.
+        """
+        # If a method has overloads, it needs a method description, because it
+        # can throw MSG_INVALID_OVERLOAD_ARGCOUNT at the very least.
+        if len(idlMethod.signatures()) != 1:
+            return True
+
+        # Methods with only one signature need a method description if one of
+        # their args needs it.
+        sig = idlMethod.signatures()[0]
+        args = sig[1]
+        return any(
+            idlTypeNeedsCallContext(
+                arg.type,
+                descriptor,
+                allowTreatNonCallableAsNull=arg.allowTreatNonCallableAsNull(),
+            )
+            for arg in args
+        )
+
+    @staticmethod
+    def error_reporting_label_helper(descriptor, idlMethod, isConstructor):
+        """
+        Returns the method description to use for error reporting for the given
+        IDL method.  Used to implement common error_reporting_label() functions
+        across different classes.
+        """
+        if not CGSpecializedMethod.should_have_method_description(
+            descriptor, idlMethod
+        ):
+            return None
+        return GetLabelForErrorReporting(descriptor, idlMethod, isConstructor)
+
+    def error_reporting_label(self):
+        return CGSpecializedMethod.error_reporting_label_helper(
+            self.descriptor, self.method, isConstructor=False
+        )
+
+    @staticmethod
+    def makeNativeName(descriptor, method):
+        if method.underlyingAttr:
+            return CGSpecializedGetter.makeNativeName(descriptor, method.underlyingAttr)
+        name = method.identifier.name
+        return MakeNativeName(descriptor.binaryNameFor(name, method.isStatic()))
+
+
+class CGMethodPromiseWrapper(CGAbstractStaticMethod):
+    """
+    A class for generating a wrapper around another method that will
+    convert exceptions to promises.
+    """
+
+    def __init__(self, descriptor, methodToWrap):
+        self.method = methodToWrap
+        name = self.makeName(methodToWrap.name)
+        args = list(methodToWrap.args)
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", args, canRunScript=True
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            bool ok = ${methodName}(${args});
+            if (ok) {
+              return true;
+            }
+            return ConvertExceptionToPromise(cx, args.rval());
+            """,
+            methodName=self.method.name,
+            args=", ".join(arg.name for arg in self.args),
+        )
+
+    @staticmethod
+    def makeName(methodName):
+        return methodName + "_promiseWrapper"
+
+
+class CGDefaultToJSONMethod(CGSpecializedMethod):
+    def __init__(self, descriptor, method):
+        assert method.isDefaultToJSON()
+        CGSpecializedMethod.__init__(self, descriptor, method)
+
+    def definition_body(self):
+        ret = fill(
+            """
+            auto* self = static_cast<${nativeType}*>(void_self);
+            JS::Rooted<JSObject*> result(cx, JS_NewPlainObject(cx));
+            if (!result) {
+              return false;
+            }
+            """,
+            nativeType=self.descriptor.nativeType,
+        )
+
+        jsonDescriptors = [self.descriptor]
+        interface = self.descriptor.interface.parent
+        while interface:
+            descriptor = self.descriptor.getDescriptor(interface.identifier.name)
+            if descriptor.hasDefaultToJSON:
+                jsonDescriptors.append(descriptor)
+            interface = interface.parent
+
+        # Iterate the array in reverse: oldest ancestor first
+        for descriptor in jsonDescriptors[::-1]:
+            ret += fill(
+                """
+                if (!${parentclass}::CollectJSONAttributes(cx, obj, MOZ_KnownLive(self), result)) {
+                  return false;
+                }
+                """,
+                parentclass=toBindingNamespace(descriptor.name),
+            )
+        ret += "args.rval().setObject(*result);\n" "return true;\n"
+        return ret
+
+
+class CGLegacyCallHook(CGAbstractBindingMethod):
+    """
+    Call hook for our object
+    """
+
+    def __init__(self, descriptor):
+        self._legacycaller = descriptor.operations["LegacyCaller"]
+        # Our "self" is actually the callee in this case, not the thisval.
+        CGAbstractBindingMethod.__init__(
+            self,
+            descriptor,
+            LEGACYCALLER_HOOK_NAME,
+            JSNativeArguments(),
+            getThisObj="&args.callee()",
+        )
+
+    def define(self):
+        if not self._legacycaller:
+            return ""
+        return CGAbstractBindingMethod.define(self)
+
+    def generate_code(self):
+        name = self._legacycaller.identifier.name
+        nativeName = MakeNativeName(self.descriptor.binaryNameFor(name, False))
+        return CGMethodCall(nativeName, False, self.descriptor, self._legacycaller)
+
+    def error_reporting_label(self):
+        # Should act like methods.
+        return CGSpecializedMethod.error_reporting_label_helper(
+            self.descriptor, self._legacycaller, isConstructor=False
+        )
+
+
+class CGResolveHook(CGAbstractClassHook):
+    """
+    Resolve hook for objects that have the NeedResolve extended attribute.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.interface.getExtendedAttribute("NeedResolve")
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("bool*", "resolvedp"),
+        ]
+        CGAbstractClassHook.__init__(self, descriptor, RESOLVE_HOOK_NAME, "bool", args)
+
+    def generate_code(self):
+        return dedent(
+            """
+            JS::Rooted<mozilla::Maybe<JS::PropertyDescriptor>> desc(cx);
+            if (!self->DoResolve(cx, obj, id, &desc)) {
+              return false;
+            }
+            if (desc.isNothing()) {
+              return true;
+            }
+            // If desc.value() is undefined, then the DoResolve call
+            // has already defined it on the object.  Don't try to also
+            // define it.
+            MOZ_ASSERT(desc->isDataDescriptor());
+            if (!desc->value().isUndefined()) {
+              JS::Rooted<JS::PropertyDescriptor> defineDesc(cx, *desc);
+              defineDesc.setResolving(true);
+              if (!JS_DefinePropertyById(cx, obj, id, defineDesc)) {
+                return false;
+              }
+            }
+            *resolvedp = true;
+            return true;
+            """
+        )
+
+    def definition_body(self):
+        if self.descriptor.isGlobal():
+            # Resolve standard classes
+            prefix = dedent(
+                """
+                if (!ResolveGlobal(cx, obj, id, resolvedp)) {
+                  return false;
+                }
+                if (*resolvedp) {
+                  return true;
+                }
+
+                """
+            )
+        else:
+            prefix = ""
+        return prefix + CGAbstractClassHook.definition_body(self)
+
+
+class CGMayResolveHook(CGAbstractStaticMethod):
+    """
+    Resolve hook for objects that have the NeedResolve extended attribute.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.interface.getExtendedAttribute("NeedResolve")
+
+        args = [
+            Argument("const JSAtomState&", "names"),
+            Argument("jsid", "id"),
+            Argument("JSObject*", "maybeObj"),
+        ]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, MAY_RESOLVE_HOOK_NAME, "bool", args
+        )
+
+    def definition_body(self):
+        if self.descriptor.isGlobal():
+            # Check whether this would resolve as a standard class.
+            prefix = dedent(
+                """
+                if (MayResolveGlobal(names, id, maybeObj)) {
+                  return true;
+                }
+
+                """
+            )
+        else:
+            prefix = ""
+        return prefix + "return %s::MayResolve(id);\n" % self.descriptor.nativeType
+
+
+class CGEnumerateHook(CGAbstractBindingMethod):
+    """
+    Enumerate hook for objects with custom hooks.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.interface.getExtendedAttribute("NeedResolve")
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("JS::MutableHandleVector<jsid>", "properties"),
+            Argument("bool", "enumerableOnly"),
+        ]
+        # Our "self" is actually the "obj" argument in this case, not the thisval.
+        CGAbstractBindingMethod.__init__(
+            self, descriptor, NEW_ENUMERATE_HOOK_NAME, args, getThisObj="", callArgs=""
+        )
+
+    def generate_code(self):
+        return CGGeneric(
+            dedent(
+                """
+            FastErrorResult rv;
+            self->GetOwnPropertyNames(cx, properties, enumerableOnly, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return false;
+            }
+            return true;
+            """
+            )
+        )
+
+    def definition_body(self):
+        if self.descriptor.isGlobal():
+            # Enumerate standard classes
+            prefix = dedent(
+                """
+                if (!EnumerateGlobal(cx, obj, properties, enumerableOnly)) {
+                  return false;
+                }
+
+                """
+            )
+        else:
+            prefix = ""
+        return prefix + CGAbstractBindingMethod.definition_body(self)
+
+
+class CppKeywords:
+    """
+    A class for checking if method names declared in webidl
+    are not in conflict with C++ keywords.
+    """
+
+    keywords = frozenset(
+        [
+            "alignas",
+            "alignof",
+            "and",
+            "and_eq",
+            "asm",
+            "assert",
+            "auto",
+            "bitand",
+            "bitor",
+            "bool",
+            "break",
+            "case",
+            "catch",
+            "char",
+            "char16_t",
+            "char32_t",
+            "class",
+            "compl",
+            "const",
+            "constexpr",
+            "const_cast",
+            "continue",
+            "decltype",
+            "default",
+            "delete",
+            "do",
+            "double",
+            "dynamic_cast",
+            "else",
+            "enum",
+            "explicit",
+            "export",
+            "extern",
+            "false",
+            "final",
+            "float",
+            "for",
+            "friend",
+            "goto",
+            "if",
+            "inline",
+            "int",
+            "long",
+            "mutable",
+            "namespace",
+            "new",
+            "noexcept",
+            "not",
+            "not_eq",
+            "nullptr",
+            "operator",
+            "or",
+            "or_eq",
+            "override",
+            "private",
+            "protected",
+            "public",
+            "register",
+            "reinterpret_cast",
+            "return",
+            "short",
+            "signed",
+            "sizeof",
+            "static",
+            "static_assert",
+            "static_cast",
+            "struct",
+            "switch",
+            "template",
+            "this",
+            "thread_local",
+            "throw",
+            "true",
+            "try",
+            "typedef",
+            "typeid",
+            "typename",
+            "union",
+            "unsigned",
+            "using",
+            "virtual",
+            "void",
+            "volatile",
+            "wchar_t",
+            "while",
+            "xor",
+            "xor_eq",
+        ]
+    )
+
+    @staticmethod
+    def checkMethodName(name):
+        # Double '_' because 'assert' and '_assert' cannot be used in MS2013 compiler.
+        # Bug 964892 and bug 963560.
+        if name in CppKeywords.keywords:
+            name = "_" + name + "_"
+        return name
+
+
+class CGStaticMethod(CGAbstractStaticBindingMethod):
+    """
+    A class for generating the C++ code for an IDL static method.
+    """
+
+    def __init__(self, descriptor, method):
+        self.method = method
+        name = CppKeywords.checkMethodName(IDLToCIdentifier(method.identifier.name))
+        CGAbstractStaticBindingMethod.__init__(self, descriptor, name)
+
+    def generate_code(self):
+        nativeName = CGSpecializedMethod.makeNativeName(self.descriptor, self.method)
+        return CGMethodCall(nativeName, True, self.descriptor, self.method)
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        method_name = self.method.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${method_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_METHOD) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            method_name=method_name,
+        )
+
+    def error_reporting_label(self):
+        return CGSpecializedMethod.error_reporting_label_helper(
+            self.descriptor, self.method, isConstructor=False
+        )
+
+
+class CGSpecializedGetter(CGAbstractStaticMethod):
+    """
+    A class for generating the code for a specialized attribute getter
+    that the JIT can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, attr):
+        self.attr = attr
+        name = "get_" + IDLToCIdentifier(attr.identifier.name)
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("void*", "void_self"),
+            Argument("JSJitGetterCallArgs", "args"),
+        ]
+        # StoreInSlot attributes have their getters called from Wrap().  We
+        # really hope they can't run script, and don't want to annotate Wrap()
+        # methods as doing that anyway, so let's not annotate them as
+        # MOZ_CAN_RUN_SCRIPT.
+        CGAbstractStaticMethod.__init__(
+            self,
+            descriptor,
+            name,
+            "bool",
+            args,
+            canRunScript=not attr.getExtendedAttribute("StoreInSlot"),
+        )
+
+    def definition_body(self):
+        prefix = fill(
+            """
+            auto* self = static_cast<${nativeType}*>(void_self);
+            """,
+            nativeType=self.descriptor.nativeType,
+        )
+
+        if self.attr.isMaplikeOrSetlikeAttr():
+            assert not self.attr.getExtendedAttribute("CrossOriginReadable")
+            # If the interface is maplike/setlike, there will be one getter
+            # method for the size property of the backing object. Due to having
+            # to unpack the backing object from the slot, this requires its own
+            # generator.
+            return prefix + getMaplikeOrSetlikeSizeGetterBody(
+                self.descriptor, self.attr
+            )
+
+        if self.attr.type.isObservableArray():
+            assert not self.attr.getExtendedAttribute("CrossOriginReadable")
+            # If the attribute is observableArray, due to having to unpack the
+            # backing object from the slot, this requires its own generator.
+            return prefix + getObservableArrayGetterBody(self.descriptor, self.attr)
+
+        nativeName = CGSpecializedGetter.makeNativeName(self.descriptor, self.attr)
+        type = self.attr.type
+        if self.attr.getExtendedAttribute("CrossOriginReadable"):
+            remoteType = type
+            extendedAttributes = self.descriptor.getExtendedAttributes(
+                self.attr, getter=True
+            )
+            if (
+                remoteType.isGeckoInterface()
+                and not remoteType.unroll().inner.isExternal()
+                and remoteType.unroll().inner.getExtendedAttribute("ChromeOnly") is None
+            ):
+                # We'll use a JSObject. It might make more sense to use remoteType's
+                # RemoteProxy, but it's not easy to construct a type for that from here.
+                remoteType = BuiltinTypes[IDLBuiltinType.Types.object]
+                if "needsErrorResult" not in extendedAttributes:
+                    extendedAttributes.append("needsErrorResult")
+            prototypeID, _ = PrototypeIDAndDepth(self.descriptor)
+            prefix = (
+                fill(
+                    """
+                if (IsRemoteObjectProxy(obj, ${prototypeID})) {
+                    ${nativeType}::RemoteProxy* self = static_cast<${nativeType}::RemoteProxy*>(void_self);
+                    $*{call}
+                }
+            """,
+                    prototypeID=prototypeID,
+                    nativeType=self.descriptor.nativeType,
+                    call=CGGetterCall(
+                        remoteType,
+                        nativeName,
+                        self.descriptor,
+                        self.attr,
+                        dontSetSlot=True,
+                        extendedAttributes=extendedAttributes,
+                    ).define(),
+                )
+                + prefix
+            )
+
+        if self.attr.slotIndices is not None:
+            # We're going to store this return value in a slot on some object,
+            # to cache it.  The question is, which object?  For dictionary and
+            # sequence return values, we want to use a slot on the Xray expando
+            # if we're called via Xrays, and a slot on our reflector otherwise.
+            # On the other hand, when dealing with some interfacce types
+            # (e.g. window.document) we want to avoid calling the getter more
+            # than once.  In the case of window.document, it's because the
+            # getter can start returning null, which would get hidden in the
+            # non-Xray case by the fact that it's [StoreOnSlot], so the cached
+            # version is always around.
+            #
+            # The upshot is that we use the reflector slot for any getter whose
+            # type is a gecko interface, whether we're called via Xrays or not.
+            # Since [Cached] and [StoreInSlot] cannot be used with "NewObject",
+            # we know that in the interface type case the returned object is
+            # wrappercached.  So creating Xrays to it is reasonable.
+            if mayUseXrayExpandoSlots(self.descriptor, self.attr):
+                prefix += fill(
+                    """
+                    // Have to either root across the getter call or reget after.
+                    bool isXray;
+                    JS::Rooted<JSObject*> slotStorage(cx, GetCachedSlotStorageObject(cx, obj, &isXray));
+                    if (!slotStorage) {
+                      return false;
+                    }
+                    const size_t slotIndex = isXray ? ${xraySlotIndex} : ${slotIndex};
+                    """,
+                    xraySlotIndex=memberXrayExpandoReservedSlot(
+                        self.attr, self.descriptor
+                    ),
+                    slotIndex=memberReservedSlot(self.attr, self.descriptor),
+                )
+            else:
+                prefix += fill(
+                    """
+                    // Have to either root across the getter call or reget after.
+                    JS::Rooted<JSObject*> slotStorage(cx, js::UncheckedUnwrap(obj, /* stopAtWindowProxy = */ false));
+                    MOZ_ASSERT(IsDOMObject(slotStorage));
+                    const size_t slotIndex = ${slotIndex};
+                    """,
+                    slotIndex=memberReservedSlot(self.attr, self.descriptor),
+                )
+
+            prefix += fill(
+                """
+                MOZ_ASSERT(JSCLASS_RESERVED_SLOTS(JS::GetClass(slotStorage)) > slotIndex);
+                {
+                  // Scope for cachedVal
+                  JS::Value cachedVal = JS::GetReservedSlot(slotStorage, slotIndex);
+                  if (!cachedVal.isUndefined()) {
+                    args.rval().set(cachedVal);
+                    // The cached value is in the compartment of slotStorage,
+                    // so wrap into the caller compartment as needed.
+                    return ${maybeWrap}(cx, args.rval());
+                  }
+                }
+
+                """,
+                maybeWrap=getMaybeWrapValueFuncForType(self.attr.type),
+            )
+
+        return (
+            prefix + CGGetterCall(type, nativeName, self.descriptor, self.attr).define()
+        )
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        attr_name = self.attr.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${attr_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_GETTER) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            attr_name=attr_name,
+        )
+
+    def error_reporting_label(self):
+        # Getters never need a BindingCallContext.
+        return None
+
+    @staticmethod
+    def makeNativeName(descriptor, attr):
+        name = attr.identifier.name
+        nativeName = MakeNativeName(descriptor.binaryNameFor(name, attr.isStatic()))
+        _, resultOutParam, _, _, _ = getRetvalDeclarationForType(attr.type, descriptor)
+        extendedAttrs = descriptor.getExtendedAttributes(attr, getter=True)
+        canFail = "needsErrorResult" in extendedAttrs or "canOOM" in extendedAttrs
+        if resultOutParam or attr.type.nullable() or canFail:
+            nativeName = "Get" + nativeName
+        return nativeName
+
+
+class CGGetterPromiseWrapper(CGAbstractStaticMethod):
+    """
+    A class for generating a wrapper around another getter that will
+    convert exceptions to promises.
+    """
+
+    def __init__(self, descriptor, getterToWrap):
+        self.getter = getterToWrap
+        name = self.makeName(getterToWrap.name)
+        args = list(getterToWrap.args)
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", args, canRunScript=True
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            bool ok = ${getterName}(${args});
+            if (ok) {
+              return true;
+            }
+            return ConvertExceptionToPromise(cx, args.rval());
+            """,
+            getterName=self.getter.name,
+            args=", ".join(arg.name for arg in self.args),
+        )
+
+    @staticmethod
+    def makeName(getterName):
+        return getterName + "_promiseWrapper"
+
+
+class CGStaticGetter(CGAbstractStaticBindingMethod):
+    """
+    A class for generating the C++ code for an IDL static attribute getter.
+    """
+
+    def __init__(self, descriptor, attr):
+        self.attr = attr
+        name = "get_" + IDLToCIdentifier(attr.identifier.name)
+        CGAbstractStaticBindingMethod.__init__(self, descriptor, name)
+
+    def generate_code(self):
+        nativeName = CGSpecializedGetter.makeNativeName(self.descriptor, self.attr)
+        return CGGetterCall(self.attr.type, nativeName, self.descriptor, self.attr)
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        attr_name = self.attr.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${attr_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_GETTER) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            attr_name=attr_name,
+        )
+
+    def error_reporting_label(self):
+        # Getters never need a BindingCallContext.
+        return None
+
+
+class CGSpecializedSetter(CGAbstractStaticMethod):
+    """
+    A class for generating the code for a specialized attribute setter
+    that the JIT can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, attr):
+        self.attr = attr
+        name = "set_" + IDLToCIdentifier(attr.identifier.name)
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("void*", "void_self"),
+            Argument("JSJitSetterCallArgs", "args"),
+        ]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, name, "bool", args, canRunScript=True
+        )
+
+    def definition_body(self):
+        nativeName = CGSpecializedSetter.makeNativeName(self.descriptor, self.attr)
+        type = self.attr.type
+        call = CGSetterCall(type, nativeName, self.descriptor, self.attr).define()
+        prefix = ""
+        if self.attr.getExtendedAttribute("CrossOriginWritable"):
+            if type.isGeckoInterface() and not type.unroll().inner.isExternal():
+                # a setter taking a Gecko interface would require us to deal with remote
+                # proxies for the value here.
+                raise TypeError(
+                    "We don't support the setter of %s marked as "
+                    "CrossOriginWritable because it takes a Gecko interface "
+                    "as the value",
+                    self.attr.identifier.name,
+                )
+            prototypeID, _ = PrototypeIDAndDepth(self.descriptor)
+            prefix = fill(
+                """
+                if (IsRemoteObjectProxy(obj, ${prototypeID})) {
+                    auto* self = static_cast<${nativeType}::RemoteProxy*>(void_self);
+                    $*{call}
+                }
+                """,
+                prototypeID=prototypeID,
+                nativeType=self.descriptor.nativeType,
+                call=call,
+            )
+
+        return prefix + fill(
+            """
+            auto* self = static_cast<${nativeType}*>(void_self);
+            $*{call}
+            """,
+            nativeType=self.descriptor.nativeType,
+            call=call,
+        )
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        attr_name = self.attr.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${attr_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_SETTER) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            attr_name=attr_name,
+        )
+
+    @staticmethod
+    def error_reporting_label_helper(descriptor, attr):
+        # Setters need a BindingCallContext if the type of the attribute needs
+        # one.
+        if not idlTypeNeedsCallContext(
+            attr.type, descriptor, allowTreatNonCallableAsNull=True
+        ):
+            return None
+        return (
+            GetLabelForErrorReporting(descriptor, attr, isConstructor=False) + " setter"
+        )
+
+    def error_reporting_label(self):
+        return CGSpecializedSetter.error_reporting_label_helper(
+            self.descriptor, self.attr
+        )
+
+    @staticmethod
+    def makeNativeName(descriptor, attr):
+        name = attr.identifier.name
+        return "Set" + MakeNativeName(descriptor.binaryNameFor(name, attr.isStatic()))
+
+
+class CGStaticSetter(CGAbstractStaticBindingMethod):
+    """
+    A class for generating the C++ code for an IDL static attribute setter.
+    """
+
+    def __init__(self, descriptor, attr):
+        self.attr = attr
+        name = "set_" + IDLToCIdentifier(attr.identifier.name)
+        CGAbstractStaticBindingMethod.__init__(self, descriptor, name)
+
+    def generate_code(self):
+        nativeName = CGSpecializedSetter.makeNativeName(self.descriptor, self.attr)
+        checkForArg = CGGeneric(
+            fill(
+                """
+            if (!args.requireAtLeast(cx, "${name} setter", 1)) {
+              return false;
+            }
+            """,
+                name=self.attr.identifier.name,
+            )
+        )
+        call = CGSetterCall(self.attr.type, nativeName, self.descriptor, self.attr)
+        return CGList([checkForArg, call])
+
+    def auto_profiler_label(self):
+        interface_name = self.descriptor.interface.identifier.name
+        attr_name = self.attr.identifier.name
+        return fill(
+            """
+            AUTO_PROFILER_LABEL_DYNAMIC_FAST(
+              "${interface_name}", "${attr_name}", DOM, cx,
+              uint32_t(js::ProfilingStackFrame::Flags::STRING_TEMPLATE_SETTER) |
+              uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS));
+            """,
+            interface_name=interface_name,
+            attr_name=attr_name,
+        )
+
+    def error_reporting_label(self):
+        return CGSpecializedSetter.error_reporting_label_helper(
+            self.descriptor, self.attr
+        )
+
+
+class CGSpecializedForwardingSetter(CGSpecializedSetter):
+    """
+    A class for generating the code for a specialized attribute setter with
+    PutForwards that the JIT can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, attr):
+        CGSpecializedSetter.__init__(self, descriptor, attr)
+
+    def definition_body(self):
+        attrName = self.attr.identifier.name
+        forwardToAttrName = self.attr.getExtendedAttribute("PutForwards")[0]
+        # JS_GetProperty and JS_SetProperty can only deal with ASCII
+        assert all(ord(c) < 128 for c in attrName)
+        assert all(ord(c) < 128 for c in forwardToAttrName)
+        return fill(
+            """
+            JS::Rooted<JS::Value> v(cx);
+            if (!JS_GetProperty(cx, obj, "${attr}", &v)) {
+              return false;
+            }
+
+            if (!v.isObject()) {
+              return cx.ThrowErrorMessage<MSG_NOT_OBJECT>("${interface}.${attr}");
+            }
+
+            JS::Rooted<JSObject*> targetObj(cx, &v.toObject());
+            return JS_SetProperty(cx, targetObj, "${forwardToAttrName}", args[0]);
+            """,
+            attr=attrName,
+            interface=self.descriptor.interface.identifier.name,
+            forwardToAttrName=forwardToAttrName,
+        )
+
+    def error_reporting_label(self):
+        # We always need to be able to throw.
+        return (
+            GetLabelForErrorReporting(self.descriptor, self.attr, isConstructor=False)
+            + " setter"
+        )
+
+
+class CGSpecializedReplaceableSetter(CGSpecializedSetter):
+    """
+    A class for generating the code for a specialized attribute setter with
+    Replaceable that the JIT can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, attr):
+        CGSpecializedSetter.__init__(self, descriptor, attr)
+
+    def definition_body(self):
+        attrName = self.attr.identifier.name
+        # JS_DefineProperty can only deal with ASCII
+        assert all(ord(c) < 128 for c in attrName)
+        return (
+            'return JS_DefineProperty(cx, obj, "%s", args[0], JSPROP_ENUMERATE);\n'
+            % attrName
+        )
+
+    def error_reporting_label(self):
+        # We never throw directly.
+        return None
+
+
+class CGSpecializedLenientSetter(CGSpecializedSetter):
+    """
+    A class for generating the code for a specialized attribute setter with
+    LenientSetter that the JIT can call with lower overhead.
+    """
+
+    def __init__(self, descriptor, attr):
+        CGSpecializedSetter.__init__(self, descriptor, attr)
+
+    def definition_body(self):
+        attrName = self.attr.identifier.name
+        # JS_DefineProperty can only deal with ASCII
+        assert all(ord(c) < 128 for c in attrName)
+        return dedent(
+            """
+            DeprecationWarning(cx, obj, DeprecatedOperations::eLenientSetter);
+            return true;
+            """
+        )
+
+    def error_reporting_label(self):
+        # We never throw; that's the whole point.
+        return None
+
+
+def memberReturnsNewObject(member):
+    return member.getExtendedAttribute("NewObject") is not None
+
+
+class CGMemberJITInfo(CGThing):
+    """
+    A class for generating the JITInfo for a property that points to
+    our specialized getter and setter.
+    """
+
+    def __init__(self, descriptor, member):
+        self.member = member
+        self.descriptor = descriptor
+
+    def declare(self):
+        return ""
+
+    def defineJitInfo(
+        self,
+        infoName,
+        opName,
+        opType,
+        infallible,
+        movable,
+        eliminatable,
+        aliasSet,
+        alwaysInSlot,
+        lazilyInSlot,
+        slotIndex,
+        returnTypes,
+        args,
+    ):
+        """
+        aliasSet is a JSJitInfo::AliasSet value, without the "JSJitInfo::" bit.
+
+        args is None if we don't want to output argTypes for some
+        reason (e.g. we have overloads or we're not a method) and
+        otherwise an iterable of the arguments for this method.
+        """
+        assert (
+            not movable or aliasSet != "AliasEverything"
+        )  # Can't move write-aliasing things
+        assert (
+            not alwaysInSlot or movable
+        )  # Things always in slots had better be movable
+        assert (
+            not eliminatable or aliasSet != "AliasEverything"
+        )  # Can't eliminate write-aliasing things
+        assert (
+            not alwaysInSlot or eliminatable
+        )  # Things always in slots had better be eliminatable
+
+        def jitInfoInitializer(isTypedMethod):
+            initializer = fill(
+                """
+                {
+                  { ${opName} },
+                  { prototypes::id::${name} },
+                  { PrototypeTraits<prototypes::id::${name}>::Depth },
+                  JSJitInfo::${opType},
+                  JSJitInfo::${aliasSet}, /* aliasSet.  Not relevant for setters. */
+                  ${returnType},  /* returnType.  Not relevant for setters. */
+                  ${isInfallible},  /* isInfallible. False in setters. */
+                  ${isMovable},  /* isMovable.  Not relevant for setters. */
+                  ${isEliminatable}, /* isEliminatable.  Not relevant for setters. */
+                  ${isAlwaysInSlot}, /* isAlwaysInSlot.  Only relevant for getters. */
+                  ${isLazilyCachedInSlot}, /* isLazilyCachedInSlot.  Only relevant for getters. */
+                  ${isTypedMethod},  /* isTypedMethod.  Only relevant for methods. */
+                  ${slotIndex}   /* Reserved slot index, if we're stored in a slot, else 0. */
+                }
+                """,
+                opName=opName,
+                name=self.descriptor.name,
+                opType=opType,
+                aliasSet=aliasSet,
+                returnType=functools.reduce(
+                    CGMemberJITInfo.getSingleReturnType, returnTypes, ""
+                ),
+                isInfallible=toStringBool(infallible),
+                isMovable=toStringBool(movable),
+                isEliminatable=toStringBool(eliminatable),
+                isAlwaysInSlot=toStringBool(alwaysInSlot),
+                isLazilyCachedInSlot=toStringBool(lazilyInSlot),
+                isTypedMethod=toStringBool(isTypedMethod),
+                slotIndex=slotIndex,
+            )
+            return initializer.rstrip()
+
+        slotAssert = fill(
+            """
+            static_assert(${slotIndex} <= JSJitInfo::maxSlotIndex, "We won't fit");
+            static_assert(${slotIndex} < ${classReservedSlots}, "There is no slot for us");
+            """,
+            slotIndex=slotIndex,
+            classReservedSlots=INSTANCE_RESERVED_SLOTS
+            + self.descriptor.interface.totalMembersInSlots,
+        )
+        if args is not None:
+            argTypes = "%s_argTypes" % infoName
+            args = [CGMemberJITInfo.getJSArgType(arg.type) for arg in args]
+            args.append("JSJitInfo::ArgTypeListEnd")
+            argTypesDecl = "static const JSJitInfo::ArgType %s[] = { %s };\n" % (
+                argTypes,
+                ", ".join(args),
+            )
+            return fill(
+                """
+                $*{argTypesDecl}
+                static const JSTypedMethodJitInfo ${infoName} = {
+                ${jitInfo},
+                  ${argTypes}
+                };
+                $*{slotAssert}
+                """,
+                argTypesDecl=argTypesDecl,
+                infoName=infoName,
+                jitInfo=indent(jitInfoInitializer(True)),
+                argTypes=argTypes,
+                slotAssert=slotAssert,
+            )
+
+        # Unexposed things are meant to be used from C++ directly, so we make
+        # their jitinfo non-static.  That way C++ can get at it.
+        if self.member.getExtendedAttribute("Unexposed"):
+            storageClass = "extern"
+        else:
+            storageClass = "static"
+
+        return fill(
+            """
+            ${storageClass} const JSJitInfo ${infoName} = ${jitInfo};
+            $*{slotAssert}
+            """,
+            storageClass=storageClass,
+            infoName=infoName,
+            jitInfo=jitInfoInitializer(False),
+            slotAssert=slotAssert,
+        )
+
+    def define(self):
+        if self.member.isAttr():
+            getterinfo = "%s_getterinfo" % IDLToCIdentifier(self.member.identifier.name)
+            name = IDLToCIdentifier(self.member.identifier.name)
+            if self.member.type.isPromise():
+                name = CGGetterPromiseWrapper.makeName(name)
+            getter = "get_%s" % name
+            extendedAttrs = self.descriptor.getExtendedAttributes(
+                self.member, getter=True
+            )
+            getterinfal = "needsErrorResult" not in extendedAttrs
+
+            # At this point getterinfal is true if our getter either can't throw
+            # at all, or can only throw OOM.  In both cases, it's safe to move,
+            # or dead-code-eliminate, the getter, because throwing OOM is not
+            # semantically meaningful, so code can't rely on it happening.  Note
+            # that this makes the behavior consistent for OOM thrown from the
+            # getter itself and OOM thrown from the to-JS conversion of the
+            # return value (see the "canOOM" and "infallibleForMember" checks
+            # below).
+            movable = self.mayBeMovable() and getterinfal
+            eliminatable = self.mayBeEliminatable() and getterinfal
+            aliasSet = self.aliasSet()
+
+            # Now we have to set getterinfal to whether we can _really_ ever
+            # throw, from the point of view of the JS engine.
+            getterinfal = (
+                getterinfal
+                and "canOOM" not in extendedAttrs
+                and infallibleForMember(self.member, self.member.type, self.descriptor)
+            )
+            isAlwaysInSlot = self.member.getExtendedAttribute("StoreInSlot")
+
+            if self.member.slotIndices is not None:
+                assert (
+                    isAlwaysInSlot
+                    or self.member.getExtendedAttribute("Cached")
+                    or self.member.type.isObservableArray()
+                )
+                isLazilyCachedInSlot = not isAlwaysInSlot
+                slotIndex = memberReservedSlot(self.member, self.descriptor)
+                # We'll statically assert that this is not too big in
+                # CGUpdateMemberSlotsMethod, in the case when
+                # isAlwaysInSlot is true.
+            else:
+                isLazilyCachedInSlot = False
+                slotIndex = "0"
+
+            result = self.defineJitInfo(
+                getterinfo,
+                getter,
+                "Getter",
+                getterinfal,
+                movable,
+                eliminatable,
+                aliasSet,
+                isAlwaysInSlot,
+                isLazilyCachedInSlot,
+                slotIndex,
+                [self.member.type],
+                None,
+            )
+            if (
+                not self.member.readonly
+                or self.member.getExtendedAttribute("PutForwards") is not None
+                or self.member.getExtendedAttribute("Replaceable") is not None
+                or self.member.getExtendedAttribute("LegacyLenientSetter") is not None
+            ):
+                setterinfo = "%s_setterinfo" % IDLToCIdentifier(
+                    self.member.identifier.name
+                )
+                # Actually a JSJitSetterOp, but JSJitGetterOp is first in the
+                # union.
+                setter = "(JSJitGetterOp)set_%s" % IDLToCIdentifier(
+                    self.member.identifier.name
+                )
+                # Setters are always fallible, since they have to do a typed unwrap.
+                result += self.defineJitInfo(
+                    setterinfo,
+                    setter,
+                    "Setter",
+                    False,
+                    False,
+                    False,
+                    "AliasEverything",
+                    False,
+                    False,
+                    "0",
+                    [BuiltinTypes[IDLBuiltinType.Types.undefined]],
+                    None,
+                )
+            return result
+        if self.member.isMethod():
+            methodinfo = "%s_methodinfo" % IDLToCIdentifier(self.member.identifier.name)
+            name = CppKeywords.checkMethodName(
+                IDLToCIdentifier(self.member.identifier.name)
+            )
+            if self.member.returnsPromise():
+                name = CGMethodPromiseWrapper.makeName(name)
+            # Actually a JSJitMethodOp, but JSJitGetterOp is first in the union.
+            method = "(JSJitGetterOp)%s" % name
+
+            # Methods are infallible if they are infallible, have no arguments
+            # to unwrap, and have a return type that's infallible to wrap up for
+            # return.
+            sigs = self.member.signatures()
+            if len(sigs) != 1:
+                # Don't handle overloading.  If there's more than one signature,
+                # one of them must take arguments.
+                methodInfal = False
+                args = None
+                movable = False
+                eliminatable = False
+            else:
+                sig = sigs[0]
+                # For methods that affect nothing, it's OK to set movable to our
+                # notion of infallible on the C++ side, without considering
+                # argument conversions, since argument conversions that can
+                # reliably throw would be effectful anyway and the jit doesn't
+                # move effectful things.
+                extendedAttrs = self.descriptor.getExtendedAttributes(self.member)
+                hasInfallibleImpl = "needsErrorResult" not in extendedAttrs
+                # At this point hasInfallibleImpl is true if our method either
+                # can't throw at all, or can only throw OOM.  In both cases, it
+                # may be safe to move, or dead-code-eliminate, the method,
+                # because throwing OOM is not semantically meaningful, so code
+                # can't rely on it happening.  Note that this makes the behavior
+                # consistent for OOM thrown from the method itself and OOM
+                # thrown from the to-JS conversion of the return value (see the
+                # "canOOM" and "infallibleForMember" checks below).
+                movable = self.mayBeMovable() and hasInfallibleImpl
+                eliminatable = self.mayBeEliminatable() and hasInfallibleImpl
+                # XXXbz can we move the smarts about fallibility due to arg
+                # conversions into the JIT, using our new args stuff?
+                if len(sig[1]) != 0 or not infallibleForMember(
+                    self.member, sig[0], self.descriptor
+                ):
+                    # We have arguments or our return-value boxing can fail
+                    methodInfal = False
+                else:
+                    methodInfal = hasInfallibleImpl and "canOOM" not in extendedAttrs
+                # For now, only bother to output args if we're side-effect-free.
+                if self.member.affects == "Nothing":
+                    args = sig[1]
+                else:
+                    args = None
+
+            aliasSet = self.aliasSet()
+            result = self.defineJitInfo(
+                methodinfo,
+                method,
+                "Method",
+                methodInfal,
+                movable,
+                eliminatable,
+                aliasSet,
+                False,
+                False,
+                "0",
+                [s[0] for s in sigs],
+                args,
+            )
+            return result
+        raise TypeError("Illegal member type to CGPropertyJITInfo")
+
+    def mayBeMovable(self):
+        """
+        Returns whether this attribute or method may be movable, just
+        based on Affects/DependsOn annotations.
+        """
+        affects = self.member.affects
+        dependsOn = self.member.dependsOn
+        assert affects in IDLInterfaceMember.AffectsValues
+        assert dependsOn in IDLInterfaceMember.DependsOnValues
+        # Things that are DependsOn=DeviceState are not movable, because we
+        # don't want them coalesced with each other or loop-hoisted, since
+        # their return value can change even if nothing is going on from our
+        # point of view.
+        return affects == "Nothing" and (
+            dependsOn != "Everything" and dependsOn != "DeviceState"
+        )
+
+    def mayBeEliminatable(self):
+        """
+        Returns whether this attribute or method may be eliminatable, just
+        based on Affects/DependsOn annotations.
+        """
+        # dependsOn shouldn't affect this decision at all, except in jitinfo we
+        # have no way to express "Depends on everything, affects nothing",
+        # because we only have three alias set values: AliasNone ("depends on
+        # nothing, affects nothing"), AliasDOMSets ("depends on DOM sets,
+        # affects nothing"), AliasEverything ("depends on everything, affects
+        # everything").  So the [Affects=Nothing, DependsOn=Everything] case
+        # gets encoded as AliasEverything and defineJitInfo asserts that if our
+        # alias state is AliasEverything then we're not eliminatable (because it
+        # thinks we might have side-effects at that point).  Bug 1155796 is
+        # tracking possible solutions for this.
+        affects = self.member.affects
+        dependsOn = self.member.dependsOn
+        assert affects in IDLInterfaceMember.AffectsValues
+        assert dependsOn in IDLInterfaceMember.DependsOnValues
+        return affects == "Nothing" and dependsOn != "Everything"
+
+    def aliasSet(self):
+        """
+        Returns the alias set to store in the jitinfo.  This may not be the
+        effective alias set the JIT uses, depending on whether we have enough
+        information about our args to allow the JIT to prove that effectful
+        argument conversions won't happen.
+        """
+        dependsOn = self.member.dependsOn
+        assert dependsOn in IDLInterfaceMember.DependsOnValues
+
+        if dependsOn == "Nothing" or dependsOn == "DeviceState":
+            assert self.member.affects == "Nothing"
+            return "AliasNone"
+
+        if dependsOn == "DOMState":
+            assert self.member.affects == "Nothing"
+            return "AliasDOMSets"
+
+        return "AliasEverything"
+
+    @staticmethod
+    def getJSReturnTypeTag(t):
+        if t.nullable():
+            # Sometimes it might return null, sometimes not
+            return "JSVAL_TYPE_UNKNOWN"
+        if t.isUndefined():
+            # No return, every time
+            return "JSVAL_TYPE_UNDEFINED"
+        if t.isSequence():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isRecord():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isPromise():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isGeckoInterface():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isString():
+            return "JSVAL_TYPE_STRING"
+        if t.isEnum():
+            return "JSVAL_TYPE_STRING"
+        if t.isCallback():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isAny():
+            # The whole point is to return various stuff
+            return "JSVAL_TYPE_UNKNOWN"
+        if t.isObject():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isSpiderMonkeyInterface():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isUnion():
+            u = t.unroll()
+            if u.hasNullableType:
+                # Might be null or not
+                return "JSVAL_TYPE_UNKNOWN"
+            return functools.reduce(
+                CGMemberJITInfo.getSingleReturnType, u.flatMemberTypes, ""
+            )
+        if t.isDictionary():
+            return "JSVAL_TYPE_OBJECT"
+        if t.isObservableArray():
+            return "JSVAL_TYPE_OBJECT"
+        if not t.isPrimitive():
+            raise TypeError("No idea what type " + str(t) + " is.")
+        tag = t.tag()
+        if tag == IDLType.Tags.bool:
+            return "JSVAL_TYPE_BOOLEAN"
+        if tag in [
+            IDLType.Tags.int8,
+            IDLType.Tags.uint8,
+            IDLType.Tags.int16,
+            IDLType.Tags.uint16,
+            IDLType.Tags.int32,
+        ]:
+            return "JSVAL_TYPE_INT32"
+        if tag in [
+            IDLType.Tags.int64,
+            IDLType.Tags.uint64,
+            IDLType.Tags.unrestricted_float,
+            IDLType.Tags.float,
+            IDLType.Tags.unrestricted_double,
+            IDLType.Tags.double,
+        ]:
+            # These all use JS_NumberValue, which can return int or double.
+            # But TI treats "double" as meaning "int or double", so we're
+            # good to return JSVAL_TYPE_DOUBLE here.
+            return "JSVAL_TYPE_DOUBLE"
+        if tag != IDLType.Tags.uint32:
+            raise TypeError("No idea what type " + str(t) + " is.")
+        # uint32 is sometimes int and sometimes double.
+        return "JSVAL_TYPE_DOUBLE"
+
+    @staticmethod
+    def getSingleReturnType(existingType, t):
+        type = CGMemberJITInfo.getJSReturnTypeTag(t)
+        if existingType == "":
+            # First element of the list; just return its type
+            return type
+
+        if type == existingType:
+            return existingType
+        if (type == "JSVAL_TYPE_DOUBLE" and existingType == "JSVAL_TYPE_INT32") or (
+            existingType == "JSVAL_TYPE_DOUBLE" and type == "JSVAL_TYPE_INT32"
+        ):
+            # Promote INT32 to DOUBLE as needed
+            return "JSVAL_TYPE_DOUBLE"
+        # Different types
+        return "JSVAL_TYPE_UNKNOWN"
+
+    @staticmethod
+    def getJSArgType(t):
+        assert not t.isUndefined()
+        if t.nullable():
+            # Sometimes it might return null, sometimes not
+            return (
+                "JSJitInfo::ArgType(JSJitInfo::Null | %s)"
+                % CGMemberJITInfo.getJSArgType(t.inner)
+            )
+        if t.isSequence():
+            return "JSJitInfo::Object"
+        if t.isPromise():
+            return "JSJitInfo::Object"
+        if t.isGeckoInterface():
+            return "JSJitInfo::Object"
+        if t.isString():
+            return "JSJitInfo::String"
+        if t.isEnum():
+            return "JSJitInfo::String"
+        if t.isCallback():
+            return "JSJitInfo::Object"
+        if t.isAny():
+            # The whole point is to return various stuff
+            return "JSJitInfo::Any"
+        if t.isObject():
+            return "JSJitInfo::Object"
+        if t.isSpiderMonkeyInterface():
+            return "JSJitInfo::Object"
+        if t.isUnion():
+            u = t.unroll()
+            type = "JSJitInfo::Null" if u.hasNullableType else ""
+            return "JSJitInfo::ArgType(%s)" % functools.reduce(
+                CGMemberJITInfo.getSingleArgType, u.flatMemberTypes, type
+            )
+        if t.isDictionary():
+            return "JSJitInfo::Object"
+        if not t.isPrimitive():
+            raise TypeError("No idea what type " + str(t) + " is.")
+        tag = t.tag()
+        if tag == IDLType.Tags.bool:
+            return "JSJitInfo::Boolean"
+        if tag in [
+            IDLType.Tags.int8,
+            IDLType.Tags.uint8,
+            IDLType.Tags.int16,
+            IDLType.Tags.uint16,
+            IDLType.Tags.int32,
+        ]:
+            return "JSJitInfo::Integer"
+        if tag in [
+            IDLType.Tags.int64,
+            IDLType.Tags.uint64,
+            IDLType.Tags.unrestricted_float,
+            IDLType.Tags.float,
+            IDLType.Tags.unrestricted_double,
+            IDLType.Tags.double,
+        ]:
+            # These all use JS_NumberValue, which can return int or double.
+            # But TI treats "double" as meaning "int or double", so we're
+            # good to return JSVAL_TYPE_DOUBLE here.
+            return "JSJitInfo::Double"
+        if tag != IDLType.Tags.uint32:
+            raise TypeError("No idea what type " + str(t) + " is.")
+        # uint32 is sometimes int and sometimes double.
+        return "JSJitInfo::Double"
+
+    @staticmethod
+    def getSingleArgType(existingType, t):
+        type = CGMemberJITInfo.getJSArgType(t)
+        if existingType == "":
+            # First element of the list; just return its type
+            return type
+
+        if type == existingType:
+            return existingType
+        return "%s | %s" % (existingType, type)
+
+
+class CGStaticMethodJitinfo(CGGeneric):
+    """
+    A class for generating the JITInfo for a promise-returning static method.
+    """
+
+    def __init__(self, method):
+        CGGeneric.__init__(
+            self,
+            "\n"
+            "static const JSJitInfo %s_methodinfo = {\n"
+            "  { (JSJitGetterOp)%s },\n"
+            "  { prototypes::id::_ID_Count }, { 0 }, JSJitInfo::StaticMethod,\n"
+            "  JSJitInfo::AliasEverything, JSVAL_TYPE_OBJECT, false, false,\n"
+            "  false, false, 0\n"
+            "};\n"
+            % (
+                IDLToCIdentifier(method.identifier.name),
+                CppKeywords.checkMethodName(IDLToCIdentifier(method.identifier.name)),
+            ),
+        )
+
+
+def getEnumValueName(value):
+    # Some enum values can be empty strings.  Others might have weird
+    # characters in them.  Deal with the former by returning "_empty",
+    # deal with possible name collisions from that by throwing if the
+    # enum value is actually "_empty", and throw on any value
+    # containing non-ASCII chars for now. Replace all chars other than
+    # [0-9A-Za-z_] with '_'.
+    if re.match("[^\x20-\x7E]", value):
+        raise SyntaxError('Enum value "' + value + '" contains non-ASCII characters')
+    if re.match("^[0-9]", value):
+        value = "_" + value
+    value = re.sub(r"[^0-9A-Za-z_]", "_", value)
+    if re.match("^_[A-Z]|__", value):
+        raise SyntaxError('Enum value "' + value + '" is reserved by the C++ spec')
+    if value == "_empty":
+        raise SyntaxError('"_empty" is not an IDL enum value we support yet')
+    if value == "":
+        return "_empty"
+    nativeName = MakeNativeName(value)
+    if nativeName == "EndGuard_":
+        raise SyntaxError(
+            'Enum value "' + value + '" cannot be used because it'
+            " collides with our internal EndGuard_ value.  Please"
+            " rename our internal EndGuard_ to something else"
+        )
+    return nativeName
+
+
+class CGEnumToJSValue(CGAbstractMethod):
+    def __init__(self, enum):
+        enumType = enum.identifier.name
+        self.stringsArray = enumType + "Values::" + ENUM_ENTRY_VARIABLE_NAME
+        CGAbstractMethod.__init__(
+            self,
+            None,
+            "ToJSValue",
+            "bool",
+            [
+                Argument("JSContext*", "aCx"),
+                Argument(enumType, "aArgument"),
+                Argument("JS::MutableHandle<JS::Value>", "aValue"),
+            ],
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            MOZ_ASSERT(uint32_t(aArgument) < ArrayLength(${strings}));
+            JSString* resultStr =
+              JS_NewStringCopyN(aCx, ${strings}[uint32_t(aArgument)].value,
+                                ${strings}[uint32_t(aArgument)].length);
+            if (!resultStr) {
+              return false;
+            }
+            aValue.setString(resultStr);
+            return true;
+            """,
+            strings=self.stringsArray,
+        )
+
+
+class CGEnum(CGThing):
+    def __init__(self, enum):
+        CGThing.__init__(self)
+        self.enum = enum
+        entryDecl = fill(
+            """
+            extern const EnumEntry ${entry_array}[${entry_count}];
+
+            static constexpr size_t Count = ${real_entry_count};
+
+            // Our "${entry_array}" contains an extra entry with a null string.
+            static_assert(mozilla::ArrayLength(${entry_array}) - 1 == Count,
+                          "Mismatch between enum strings and enum count");
+
+            static_assert(static_cast<size_t>(${name}::EndGuard_) == Count,
+                          "Mismatch between enum value and enum count");
+
+            inline auto GetString(${name} stringId) {
+              MOZ_ASSERT(static_cast<${type}>(stringId) < Count);
+              const EnumEntry& entry = ${entry_array}[static_cast<${type}>(stringId)];
+              return Span<const char>{entry.value, entry.length};
+            }
+            """,
+            entry_array=ENUM_ENTRY_VARIABLE_NAME,
+            entry_count=self.nEnumStrings(),
+            # -1 because nEnumStrings() includes a string for EndGuard_
+            real_entry_count=self.nEnumStrings() - 1,
+            name=self.enum.identifier.name,
+            type=self.underlyingType(),
+        )
+        strings = CGNamespace(
+            self.stringsNamespace(),
+            CGGeneric(
+                declare=entryDecl,
+                define=fill(
+                    """
+                          extern const EnumEntry ${name}[${count}] = {
+                            $*{entries}
+                            { nullptr, 0 }
+                          };
+                          """,
+                    name=ENUM_ENTRY_VARIABLE_NAME,
+                    count=self.nEnumStrings(),
+                    entries="".join(
+                        '{"%s", %d},\n' % (val, len(val)) for val in self.enum.values()
+                    ),
+                ),
+            ),
+        )
+        toJSValue = CGEnumToJSValue(enum)
+        self.cgThings = CGList([strings, toJSValue], "\n")
+
+    def stringsNamespace(self):
+        return self.enum.identifier.name + "Values"
+
+    def nEnumStrings(self):
+        return len(self.enum.values()) + 1
+
+    def underlyingType(self):
+        count = self.nEnumStrings()
+        if count <= 256:
+            return "uint8_t"
+        if count <= 65536:
+            return "uint16_t"
+        raise ValueError(
+            "Enum " + self.enum.identifier.name + " has more than 65536 values"
+        )
+
+    def declare(self):
+        decl = fill(
+            """
+            enum class ${name} : ${ty} {
+              $*{enums}
+              EndGuard_
+            };
+            """,
+            name=self.enum.identifier.name,
+            ty=self.underlyingType(),
+            enums=",\n".join(map(getEnumValueName, self.enum.values())) + ",\n",
+        )
+
+        return decl + "\n" + self.cgThings.declare()
+
+    def define(self):
+        return self.cgThings.define()
+
+    def deps(self):
+        return self.enum.getDeps()
+
+
+def getUnionAccessorSignatureType(type, descriptorProvider):
+    """
+    Returns the types that are used in the getter and setter signatures for
+    union types
+    """
+    # Flat member types have already unwrapped nullables.
+    assert not type.nullable()
+
+    # Promise types can never appear in unions, because Promise is not
+    # distinguishable from anything.
+    assert not type.isPromise()
+
+    if type.isSequence() or type.isRecord():
+        if type.isSequence():
+            wrapperType = "Sequence"
+        else:
+            wrapperType = "Record"
+        # We don't use the returned template here, so it's OK to just pass no
+        # sourceDescription.
+        elementInfo = getJSToNativeConversionInfo(
+            type.inner, descriptorProvider, isMember=wrapperType
+        )
+        if wrapperType == "Sequence":
+            innerType = elementInfo.declType
+        else:
+            innerType = [recordKeyDeclType(type), elementInfo.declType]
+
+        return CGTemplatedType(wrapperType, innerType, isConst=True, isReference=True)
+
+    # Nested unions are unwrapped automatically into our flatMemberTypes.
+    assert not type.isUnion()
+
+    if type.isGeckoInterface():
+        descriptor = descriptorProvider.getDescriptor(
+            type.unroll().inner.identifier.name
+        )
+        typeName = CGGeneric(descriptor.nativeType)
+        if not type.unroll().inner.isExternal():
+            typeName = CGWrapper(typeName, post="&")
+        elif descriptor.interface.identifier.name == "WindowProxy":
+            typeName = CGGeneric("WindowProxyHolder const&")
+        else:
+            # Allow null pointers for old-binding classes.
+            typeName = CGWrapper(typeName, post="*")
+        return typeName
+
+    if type.isSpiderMonkeyInterface():
+        typeName = CGGeneric(type.name)
+        return CGWrapper(typeName, post=" const &")
+
+    if type.isJSString():
+        raise TypeError("JSString not supported in unions")
+
+    if type.isDOMString() or type.isUSVString():
+        return CGGeneric("const nsAString&")
+
+    if type.isUTF8String():
+        return CGGeneric("const nsACString&")
+
+    if type.isByteString():
+        return CGGeneric("const nsCString&")
+
+    if type.isEnum():
+        return CGGeneric(type.inner.identifier.name)
+
+    if type.isCallback():
+        return CGGeneric("%s&" % type.unroll().callback.identifier.name)
+
+    if type.isAny():
+        return CGGeneric("JS::Value")
+
+    if type.isObject():
+        return CGGeneric("JSObject*")
+
+    if type.isDictionary():
+        return CGGeneric("const %s&" % type.inner.identifier.name)
+
+    if not type.isPrimitive():
+        raise TypeError("Need native type for argument type '%s'" % str(type))
+
+    return CGGeneric(builtinNames[type.tag()])
+
+
+def getUnionTypeTemplateVars(unionType, type, descriptorProvider, isMember=False):
+    assert not type.isUndefined()
+    assert not isMember or isMember in ("Union", "OwningUnion")
+
+    ownsMembers = isMember == "OwningUnion"
+    name = getUnionMemberName(type)
+    holderName = "m" + name + "Holder"
+
+    # By the time tryNextCode is invoked, we're guaranteed the union has been
+    # constructed as some type, since we've been trying to convert into the
+    # corresponding member.
+    tryNextCode = fill(
+        """
+        Destroy${name}();
+        tryNext = true;
+        return true;
+        """,
+        name=name,
+    )
+
+    sourceDescription = "%s branch of %s" % (type.prettyName(), unionType.prettyName())
+
+    conversionInfo = getJSToNativeConversionInfo(
+        type,
+        descriptorProvider,
+        failureCode=tryNextCode,
+        isDefinitelyObject=not type.isDictionary(),
+        isMember=isMember,
+        sourceDescription=sourceDescription,
+    )
+
+    ctorNeedsCx = conversionInfo.declArgs == "cx"
+    ctorArgs = "cx" if ctorNeedsCx else ""
+
+    structType = conversionInfo.declType.define()
+    externalType = getUnionAccessorSignatureType(type, descriptorProvider).define()
+
+    if type.isObject():
+        if ownsMembers:
+            setValue = "mValue.mObject.SetValue(obj);"
+        else:
+            setValue = "mValue.mObject.SetValue(cx, obj);"
+
+        body = fill(
+            """
+            MOZ_ASSERT(mType == eUninitialized);
+            ${setValue}
+            mType = eObject;
+            """,
+            setValue=setValue,
+        )
+
+        # It's a bit sketchy to do the security check after setting the value,
+        # but it keeps the code cleaner and lets us avoid rooting |obj| over the
+        # call to CallerSubsumes().
+        body = body + fill(
+            """
+            if (passedToJSImpl && !CallerSubsumes(obj)) {
+              cx.ThrowErrorMessage<MSG_PERMISSION_DENIED_TO_PASS_ARG>("${sourceDescription}");
+              return false;
+            }
+            return true;
+            """,
+            sourceDescription=sourceDescription,
+        )
+
+        setters = [
+            ClassMethod(
+                "SetToObject",
+                "bool",
+                [
+                    Argument("BindingCallContext&", "cx"),
+                    Argument("JSObject*", "obj"),
+                    Argument("bool", "passedToJSImpl", default="false"),
+                ],
+                inline=True,
+                bodyInHeader=True,
+                body=body,
+            )
+        ]
+    elif type.isDictionary() and not type.inner.needsConversionFromJS:
+        # In this case we are never initialized from JS to start with
+        setters = None
+    else:
+        # Important: we need to not have our declName involve
+        # maybe-GCing operations.
+        jsConversion = fill(
+            conversionInfo.template,
+            val="value",
+            maybeMutableVal="value",
+            declName="memberSlot",
+            holderName=(holderName if ownsMembers else "%s.ref()" % holderName),
+            passedToJSImpl="passedToJSImpl",
+        )
+
+        jsConversion = fill(
+            """
+            tryNext = false;
+            { // scope for memberSlot
+              ${structType}& memberSlot = RawSetAs${name}(${ctorArgs});
+              $*{jsConversion}
+            }
+            return true;
+            """,
+            structType=structType,
+            name=name,
+            ctorArgs=ctorArgs,
+            jsConversion=jsConversion,
+        )
+
+        needCallContext = idlTypeNeedsCallContext(type)
+        if needCallContext:
+            cxType = "BindingCallContext&"
+        else:
+            cxType = "JSContext*"
+        setters = [
+            ClassMethod(
+                "TrySetTo" + name,
+                "bool",
+                [
+                    Argument(cxType, "cx"),
+                    Argument("JS::Handle<JS::Value>", "value"),
+                    Argument("bool&", "tryNext"),
+                    Argument("bool", "passedToJSImpl", default="false"),
+                ],
+                visibility="private",
+                body=jsConversion,
+            )
+        ]
+        if needCallContext:
+            # Add a method for non-binding uses of unions to allow them to set
+            # things in the union without providing a call context (though if
+            # they want good error reporting they'll provide one anyway).
+            shimBody = fill(
+                """
+                BindingCallContext cx(cx_, nullptr);
+                return TrySetTo${name}(cx, value, tryNext, passedToJSImpl);
+                """,
+                name=name,
+            )
+            setters.append(
+                ClassMethod(
+                    "TrySetTo" + name,
+                    "bool",
+                    [
+                        Argument("JSContext*", "cx_"),
+                        Argument("JS::Handle<JS::Value>", "value"),
+                        Argument("bool&", "tryNext"),
+                        Argument("bool", "passedToJSImpl", default="false"),
+                    ],
+                    visibility="private",
+                    body=shimBody,
+                )
+            )
+
+    return {
+        "name": name,
+        "structType": structType,
+        "externalType": externalType,
+        "setters": setters,
+        "ctorArgs": ctorArgs,
+        "ctorArgList": [Argument("JSContext*", "cx")] if ctorNeedsCx else [],
+    }
+
+
+def getUnionConversionTemplate(type):
+    assert type.isUnion()
+    assert not type.nullable()
+
+    memberTypes = type.flatMemberTypes
+    prettyNames = []
+
+    interfaceMemberTypes = [t for t in memberTypes if t.isNonCallbackInterface()]
+    if len(interfaceMemberTypes) > 0:
+        interfaceObject = []
+        for memberType in interfaceMemberTypes:
+            name = getUnionMemberName(memberType)
+            interfaceObject.append(
+                CGGeneric(
+                    "(failed = !TrySetTo%s(cx, ${val}, tryNext, ${passedToJSImpl})) || !tryNext"
+                    % name
+                )
+            )
+            prettyNames.append(memberType.prettyName())
+        interfaceObject = CGWrapper(
+            CGList(interfaceObject, " ||\n"),
+            pre="done = ",
+            post=";\n",
+            reindent=True,
+        )
+    else:
+        interfaceObject = None
+
+    sequenceObjectMemberTypes = [t for t in memberTypes if t.isSequence()]
+    if len(sequenceObjectMemberTypes) > 0:
+        assert len(sequenceObjectMemberTypes) == 1
+        memberType = sequenceObjectMemberTypes[0]
+        name = getUnionMemberName(memberType)
+        sequenceObject = CGGeneric(
+            "done = (failed = !TrySetTo%s(cx, ${val}, tryNext, ${passedToJSImpl})) || !tryNext;\n"
+            % name
+        )
+        prettyNames.append(memberType.prettyName())
+    else:
+        sequenceObject = None
+
+    callbackMemberTypes = [
+        t for t in memberTypes if t.isCallback() or t.isCallbackInterface()
+    ]
+    if len(callbackMemberTypes) > 0:
+        assert len(callbackMemberTypes) == 1
+        memberType = callbackMemberTypes[0]
+        name = getUnionMemberName(memberType)
+        callbackObject = CGGeneric(
+            "done = (failed = !TrySetTo%s(cx, ${val}, tryNext, ${passedToJSImpl})) || !tryNext;\n"
+            % name
+        )
+        prettyNames.append(memberType.prettyName())
+    else:
+        callbackObject = None
+
+    dictionaryMemberTypes = [t for t in memberTypes if t.isDictionary()]
+    if len(dictionaryMemberTypes) > 0:
+        assert len(dictionaryMemberTypes) == 1
+        memberType = dictionaryMemberTypes[0]
+        name = getUnionMemberName(memberType)
+        setDictionary = CGGeneric(
+            "done = (failed = !TrySetTo%s(cx, ${val}, tryNext, ${passedToJSImpl})) || !tryNext;\n"
+            % name
+        )
+        prettyNames.append(memberType.prettyName())
+    else:
+        setDictionary = None
+
+    recordMemberTypes = [t for t in memberTypes if t.isRecord()]
+    if len(recordMemberTypes) > 0:
+        assert len(recordMemberTypes) == 1
+        memberType = recordMemberTypes[0]
+        name = getUnionMemberName(memberType)
+        recordObject = CGGeneric(
+            "done = (failed = !TrySetTo%s(cx, ${val}, tryNext, ${passedToJSImpl})) || !tryNext;\n"
+            % name
+        )
+        prettyNames.append(memberType.prettyName())
+    else:
+        recordObject = None
+
+    objectMemberTypes = [t for t in memberTypes if t.isObject()]
+    if len(objectMemberTypes) > 0:
+        assert len(objectMemberTypes) == 1
+        # Very important to NOT construct a temporary Rooted here, since the
+        # SetToObject call can call a Rooted constructor and we need to keep
+        # stack discipline for Rooted.
+        object = CGGeneric(
+            "if (!SetToObject(cx, &${val}.toObject(), ${passedToJSImpl})) {\n"
+            "  return false;\n"
+            "}\n"
+            "done = true;\n"
+        )
+        prettyNames.append(objectMemberTypes[0].prettyName())
+    else:
+        object = None
+
+    hasObjectTypes = (
+        interfaceObject or sequenceObject or callbackObject or object or recordObject
+    )
+    if hasObjectTypes:
+        # "object" is not distinguishable from other types
+        assert not object or not (
+            interfaceObject or sequenceObject or callbackObject or recordObject
+        )
+        if sequenceObject or callbackObject:
+            # An object can be both an sequence object and a callback or
+            # dictionary, but we shouldn't have both in the union's members
+            # because they are not distinguishable.
+            assert not (sequenceObject and callbackObject)
+            templateBody = CGElseChain([sequenceObject, callbackObject])
+        else:
+            templateBody = None
+        if interfaceObject:
+            assert not object
+            if templateBody:
+                templateBody = CGIfWrapper(templateBody, "!done")
+            templateBody = CGList([interfaceObject, templateBody])
+        else:
+            templateBody = CGList([templateBody, object])
+
+        if recordObject:
+            templateBody = CGList([templateBody, CGIfWrapper(recordObject, "!done")])
+
+        templateBody = CGIfWrapper(templateBody, "${val}.isObject()")
+    else:
+        templateBody = CGGeneric()
+
+    if setDictionary:
+        assert not object
+        templateBody = CGList([templateBody, CGIfWrapper(setDictionary, "!done")])
+
+    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:
+        assert len(stringTypes) <= 1
+        assert len(numericTypes) <= 1
+        assert len(booleanTypes) <= 1
+
+        # We will wrap all this stuff in a do { } while (0); so we
+        # can use "break" for flow control.
+        def getStringOrPrimitiveConversion(memberType):
+            name = getUnionMemberName(memberType)
+            return CGGeneric(
+                "done = (failed = !TrySetTo%s(cx, ${val}, tryNext)) || !tryNext;\n"
+                "break;\n" % name
+            )
+
+        other = CGList([])
+        stringConversion = [getStringOrPrimitiveConversion(t) for t in stringTypes]
+        numericConversion = [getStringOrPrimitiveConversion(t) for t in numericTypes]
+        booleanConversion = [getStringOrPrimitiveConversion(t) for t in booleanTypes]
+        if stringConversion:
+            if booleanConversion:
+                other.append(CGIfWrapper(booleanConversion[0], "${val}.isBoolean()"))
+            if numericConversion:
+                other.append(CGIfWrapper(numericConversion[0], "${val}.isNumber()"))
+            other.append(stringConversion[0])
+        elif numericConversion:
+            if booleanConversion:
+                other.append(CGIfWrapper(booleanConversion[0], "${val}.isBoolean()"))
+            other.append(numericConversion[0])
+        else:
+            assert booleanConversion
+            other.append(booleanConversion[0])
+
+        other = CGWrapper(CGIndenter(other), pre="do {\n", post="} while (false);\n")
+        if hasObjectTypes or setDictionary:
+            other = CGWrapper(CGIndenter(other), "{\n", post="}\n")
+            if object:
+                templateBody = CGElseChain([templateBody, other])
+            else:
+                other = CGWrapper(other, pre="if (!done) ")
+                templateBody = CGList([templateBody, other])
+        else:
+            assert templateBody.define() == ""
+            templateBody = other
+    else:
+        other = None
+
+    templateBody = CGWrapper(
+        templateBody, pre="bool done = false, failed = false, tryNext;\n"
+    )
+    throw = CGGeneric(
+        fill(
+            """
+            if (failed) {
+              return false;
+            }
+            if (!done) {
+              cx.ThrowErrorMessage<MSG_NOT_IN_UNION>(sourceDescription, "${names}");
+              return false;
+            }
+            """,
+            names=", ".join(prettyNames),
+        )
+    )
+
+    templateBody = CGList([templateBody, throw])
+
+    hasUndefinedType = any(t.isUndefined() for t in memberTypes)
+    elseChain = []
+
+    # The spec does this before anything else, but we do it after checking
+    # for null in the case of a nullable union. In practice this shouldn't
+    # make a difference, but it makes things easier because we first need to
+    # call Construct on our Maybe<...>, before we can set the union type to
+    # undefined, and we do that below after checking for null (see the
+    # 'if nullable:' block below).
+    if hasUndefinedType:
+        elseChain.append(
+            CGIfWrapper(
+                CGGeneric("SetUndefined();\n"),
+                "${val}.isUndefined()",
+            )
+        )
+
+    if type.hasNullableType:
+        nullTest = (
+            "${val}.isNull()" if hasUndefinedType else "${val}.isNullOrUndefined()"
+        )
+        elseChain.append(
+            CGIfWrapper(
+                CGGeneric("SetNull();\n"),
+                nullTest,
+            )
+        )
+
+    if len(elseChain) > 0:
+        elseChain.append(CGWrapper(CGIndenter(templateBody), pre="{\n", post="}\n"))
+        templateBody = CGElseChain(elseChain)
+
+    return templateBody
+
+
+def getUnionInitMethods(type, isOwningUnion=False):
+    assert type.isUnion()
+
+    template = getUnionConversionTemplate(type).define()
+
+    replacements = {
+        "val": "value",
+        "passedToJSImpl": "passedToJSImpl",
+    }
+
+    initBody = fill(
+        """
+        MOZ_ASSERT(mType == eUninitialized);
+
+        $*{conversion}
+        return true;
+        """,
+        conversion=string.Template(template).substitute(replacements),
+    )
+
+    return [
+        ClassMethod(
+            "Init",
+            "bool",
+            [
+                Argument("BindingCallContext&", "cx"),
+                Argument("JS::Handle<JS::Value>", "value"),
+                Argument("const char*", "sourceDescription", default='"Value"'),
+                Argument("bool", "passedToJSImpl", default="false"),
+            ],
+            visibility="public",
+            body=initBody,
+        ),
+        ClassMethod(
+            "Init",
+            "bool",
+            [
+                Argument("JSContext*", "cx_"),
+                Argument("JS::Handle<JS::Value>", "value"),
+                Argument("const char*", "sourceDescription", default='"Value"'),
+                Argument("bool", "passedToJSImpl", default="false"),
+            ],
+            visibility="public",
+            body=dedent(
+                """
+                BindingCallContext cx(cx_, nullptr);
+                return Init(cx, value, sourceDescription, passedToJSImpl);
+                """
+            ),
+        ),
+    ]
+
+
+class CGUnionStruct(CGThing):
+    def __init__(self, type, descriptorProvider, ownsMembers=False):
+        CGThing.__init__(self)
+        self.type = type.unroll()
+        self.descriptorProvider = descriptorProvider
+        self.ownsMembers = ownsMembers
+        self.struct = self.getStruct()
+
+    def declare(self):
+        return self.struct.declare()
+
+    def define(self):
+        return self.struct.define()
+
+    def deps(self):
+        return self.type.getDeps()
+
+    def getStruct(self):
+
+        members = [
+            ClassMember("mType", "TypeOrUninit", body="eUninitialized"),
+            ClassMember("mValue", "Value"),
+        ]
+        ctor = ClassConstructor(
+            [], bodyInHeader=True, visibility="public", explicit=True
+        )
+
+        methods = []
+        enumValues = ["eUninitialized"]
+        toJSValCases = [
+            CGCase(
+                "eUninitialized", CGGeneric("return false;\n"), CGCase.DONT_ADD_BREAK
+            )
+        ]
+        destructorCases = [CGCase("eUninitialized", None)]
+        assignmentCase = CGCase(
+            "eUninitialized",
+            CGGeneric(
+                "MOZ_ASSERT(mType == eUninitialized,\n"
+                '           "We need to destroy ourselves?");\n'
+            ),
+        )
+        assignmentCases = [assignmentCase]
+        moveCases = [assignmentCase]
+        traceCases = []
+        unionValues = []
+
+        def addSpecialType(typename):
+            enumValue = "e" + typename
+            enumValues.append(enumValue)
+            methods.append(
+                ClassMethod(
+                    "Is" + typename,
+                    "bool",
+                    [],
+                    const=True,
+                    inline=True,
+                    body="return mType == %s;\n" % enumValue,
+                    bodyInHeader=True,
+                )
+            )
+            methods.append(
+                ClassMethod(
+                    "Set" + typename,
+                    "void",
+                    [],
+                    inline=True,
+                    body=fill(
+                        """
+                        Uninit();
+                        mType = ${enumValue};
+                        """,
+                        enumValue=enumValue,
+                    ),
+                    bodyInHeader=True,
+                )
+            )
+            destructorCases.append(CGCase(enumValue, None))
+            assignmentCase = CGCase(
+                enumValue,
+                CGGeneric(
+                    fill(
+                        """
+                            MOZ_ASSERT(mType == eUninitialized);
+                            mType = ${enumValue};
+                            """,
+                        enumValue=enumValue,
+                    )
+                ),
+            )
+            assignmentCases.append(assignmentCase)
+            moveCases.append(assignmentCase)
+            toJSValCases.append(
+                CGCase(
+                    enumValue,
+                    CGGeneric(
+                        fill(
+                            """
+                            rval.set${typename}();
+                            return true;
+                            """,
+                            typename=typename,
+                        )
+                    ),
+                    CGCase.DONT_ADD_BREAK,
+                )
+            )
+
+        if self.type.hasNullableType:
+            addSpecialType("Null")
+
+        hasObjectType = any(t.isObject() for t in self.type.flatMemberTypes)
+        skipToJSVal = False
+        for t in self.type.flatMemberTypes:
+            if t.isUndefined():
+                addSpecialType("Undefined")
+                continue
+
+            vars = getUnionTypeTemplateVars(
+                self.type,
+                t,
+                self.descriptorProvider,
+                isMember="OwningUnion" if self.ownsMembers else "Union",
+            )
+            if vars["setters"]:
+                methods.extend(vars["setters"])
+            uninit = "Uninit();"
+            if hasObjectType and not self.ownsMembers:
+                uninit = (
+                    'MOZ_ASSERT(mType != eObject, "This will not play well with Rooted");\n'
+                    + uninit
+                )
+            if not t.isObject() or self.ownsMembers:
+                body = fill(
+                    """
+                    if (mType == e${name}) {
+                      return mValue.m${name}.Value();
+                    }
+                    %s
+                    mType = e${name};
+                    return mValue.m${name}.SetValue(${ctorArgs});
+                    """,
+                    **vars,
+                )
+
+                # bodyInHeader must be false for return values because they own
+                # their union members and we don't want include headers in
+                # UnionTypes.h just to call Addref/Release
+                methods.append(
+                    ClassMethod(
+                        "RawSetAs" + vars["name"],
+                        vars["structType"] + "&",
+                        vars["ctorArgList"],
+                        bodyInHeader=not self.ownsMembers,
+                        body=body % "MOZ_ASSERT(mType == eUninitialized);",
+                    )
+                )
+                methods.append(
+                    ClassMethod(
+                        "SetAs" + vars["name"],
+                        vars["structType"] + "&",
+                        vars["ctorArgList"],
+                        bodyInHeader=not self.ownsMembers,
+                        body=body % uninit,
+                    )
+                )
+
+                # Provide a SetStringLiteral() method to support string defaults.
+                if t.isByteString() or t.isUTF8String():
+                    charType = "const nsCString::char_type"
+                elif t.isString():
+                    charType = "const nsString::char_type"
+                else:
+                    charType = None
+
+                if charType:
+                    methods.append(
+                        ClassMethod(
+                            "SetStringLiteral",
+                            "void",
+                            # Hack, but it works...
+                            [Argument(charType, "(&aData)[N]")],
+                            inline=True,
+                            bodyInHeader=True,
+                            templateArgs=["int N"],
+                            body="RawSetAs%s().AssignLiteral(aData);\n" % t.name,
+                        )
+                    )
+
+            body = fill("return mType == e${name};\n", **vars)
+            methods.append(
+                ClassMethod(
+                    "Is" + vars["name"],
+                    "bool",
+                    [],
+                    const=True,
+                    bodyInHeader=True,
+                    body=body,
+                )
+            )
+
+            body = fill(
+                """
+                MOZ_RELEASE_ASSERT(Is${name}(), "Wrong type!");
+                mValue.m${name}.Destroy();
+                mType = eUninitialized;
+                """,
+                **vars,
+            )
+            methods.append(
+                ClassMethod(
+                    "Destroy" + vars["name"],
+                    "void",
+                    [],
+                    visibility="private",
+                    bodyInHeader=not self.ownsMembers,
+                    body=body,
+                )
+            )
+
+            body = fill(
+                """
+                MOZ_RELEASE_ASSERT(Is${name}(), "Wrong type!");
+                return mValue.m${name}.Value();
+                """,
+                **vars,
+            )
+            # The non-const version of GetAs* returns our internal type
+            getterReturnType = "%s&" % vars["structType"]
+            methods.append(
+                ClassMethod(
+                    "GetAs" + vars["name"],
+                    getterReturnType,
+                    [],
+                    bodyInHeader=True,
+                    body=body,
+                )
+            )
+            # The const version of GetAs* returns our internal type
+            # for owning unions, but our external type for non-owning
+            # ones.
+            if self.ownsMembers:
+                getterReturnType = "%s const &" % vars["structType"]
+            else:
+                getterReturnType = vars["externalType"]
+            methods.append(
+                ClassMethod(
+                    "GetAs" + vars["name"],
+                    getterReturnType,
+                    [],
+                    const=True,
+                    bodyInHeader=True,
+                    body=body,
+                )
+            )
+
+            unionValues.append(fill("UnionMember<${structType} > m${name}", **vars))
+            destructorCases.append(
+                CGCase("e" + vars["name"], CGGeneric("Destroy%s();\n" % vars["name"]))
+            )
+
+            enumValues.append("e" + vars["name"])
+
+            conversionToJS = self.getConversionToJS(vars, t)
+            if conversionToJS:
+                toJSValCases.append(
+                    CGCase("e" + vars["name"], conversionToJS, CGCase.DONT_ADD_BREAK)
+                )
+            else:
+                skipToJSVal = True
+
+            assignmentCases.append(
+                CGCase(
+                    "e" + vars["name"],
+                    CGGeneric(
+                        "SetAs%s() = aOther.GetAs%s();\n" % (vars["name"], vars["name"])
+                    ),
+                )
+            )
+            moveCases.append(
+                CGCase(
+                    "e" + vars["name"],
+                    CGGeneric(
+                        "mType = e%s;\n" % vars["name"]
+                        + "mValue.m%s.SetValue(std::move(aOther.mValue.m%s.Value()));\n"
+                        % (vars["name"], vars["name"])
+                    ),
+                )
+            )
+            if self.ownsMembers and typeNeedsRooting(t):
+                if t.isObject():
+                    traceCases.append(
+                        CGCase(
+                            "e" + vars["name"],
+                            CGGeneric(
+                                'JS::TraceRoot(trc, %s, "%s");\n'
+                                % (
+                                    "&mValue.m" + vars["name"] + ".Value()",
+                                    "mValue.m" + vars["name"],
+                                )
+                            ),
+                        )
+                    )
+                elif t.isDictionary():
+                    traceCases.append(
+                        CGCase(
+                            "e" + vars["name"],
+                            CGGeneric(
+                                "mValue.m%s.Value().TraceDictionary(trc);\n"
+                                % vars["name"]
+                            ),
+                        )
+                    )
+                elif t.isSequence():
+                    traceCases.append(
+                        CGCase(
+                            "e" + vars["name"],
+                            CGGeneric(
+                                "DoTraceSequence(trc, mValue.m%s.Value());\n"
+                                % vars["name"]
+                            ),
+                        )
+                    )
+                elif t.isRecord():
+                    traceCases.append(
+                        CGCase(
+                            "e" + vars["name"],
+                            CGGeneric(
+                                "TraceRecord(trc, mValue.m%s.Value());\n" % vars["name"]
+                            ),
+                        )
+                    )
+                else:
+                    assert t.isSpiderMonkeyInterface()
+                    traceCases.append(
+                        CGCase(
+                            "e" + vars["name"],
+                            CGGeneric(
+                                "mValue.m%s.Value().TraceSelf(trc);\n" % vars["name"]
+                            ),
+                        )
+                    )
+
+        dtor = CGSwitch("mType", destructorCases).define()
+
+        methods.extend(getUnionInitMethods(self.type, isOwningUnion=self.ownsMembers))
+        methods.append(
+            ClassMethod(
+                "Uninit",
+                "void",
+                [],
+                visibility="public",
+                body=dtor,
+                bodyInHeader=not self.ownsMembers,
+                inline=not self.ownsMembers,
+            )
+        )
+
+        if not skipToJSVal:
+            methods.append(
+                ClassMethod(
+                    "ToJSVal",
+                    "bool",
+                    [
+                        Argument("JSContext*", "cx"),
+                        Argument("JS::Handle<JSObject*>", "scopeObj"),
+                        Argument("JS::MutableHandle<JS::Value>", "rval"),
+                    ],
+                    body=CGSwitch(
+                        "mType", toJSValCases, default=CGGeneric("return false;\n")
+                    ).define(),
+                    const=True,
+                )
+            )
+
+        constructors = [ctor]
+        selfName = CGUnionStruct.unionTypeName(self.type, self.ownsMembers)
+        if self.ownsMembers:
+            if traceCases:
+                traceBody = CGSwitch(
+                    "mType", traceCases, default=CGGeneric("")
+                ).define()
+                methods.append(
+                    ClassMethod(
+                        "TraceUnion",
+                        "void",
+                        [Argument("JSTracer*", "trc")],
+                        body=traceBody,
+                    )
+                )
+
+            op_body = CGList([])
+            op_body.append(CGSwitch("aOther.mType", moveCases))
+            constructors.append(
+                ClassConstructor(
+                    [Argument("%s&&" % selfName, "aOther")],
+                    visibility="public",
+                    body=op_body.define(),
+                )
+            )
+
+            methods.append(
+                ClassMethod(
+                    "operator=",
+                    "%s&" % selfName,
+                    [Argument("%s&&" % selfName, "aOther")],
+                    body="this->~%s();\nnew (this) %s (std::move(aOther));\nreturn *this;\n"
+                    % (selfName, selfName),
+                )
+            )
+
+            body = dedent(
+                """
+                MOZ_RELEASE_ASSERT(mType != eUninitialized);
+                return static_cast<Type>(mType);
+                """
+            )
+            methods.append(
+                ClassMethod(
+                    "GetType",
+                    "Type",
+                    [],
+                    bodyInHeader=True,
+                    body=body,
+                    const=True,
+                )
+            )
+
+            if CGUnionStruct.isUnionCopyConstructible(self.type):
+                constructors.append(
+                    ClassConstructor(
+                        [Argument("const %s&" % selfName, "aOther")],
+                        bodyInHeader=True,
+                        visibility="public",
+                        explicit=True,
+                        body="*this = aOther;\n",
+                    )
+                )
+                op_body = CGList([])
+                op_body.append(CGSwitch("aOther.mType", assignmentCases))
+                op_body.append(CGGeneric("return *this;\n"))
+                methods.append(
+                    ClassMethod(
+                        "operator=",
+                        "%s&" % selfName,
+                        [Argument("const %s&" % selfName, "aOther")],
+                        body=op_body.define(),
+                    )
+                )
+                disallowCopyConstruction = False
+            else:
+                disallowCopyConstruction = True
+        else:
+            disallowCopyConstruction = True
+
+        if self.ownsMembers and idlTypeNeedsCycleCollection(self.type):
+            friend = (
+                "  friend void ImplCycleCollectionUnlink(%s& aUnion);\n"
+                % CGUnionStruct.unionTypeName(self.type, True)
+            )
+        else:
+            friend = ""
+
+        enumValuesNoUninit = [x for x in enumValues if x != "eUninitialized"]
+
+        bases = [ClassBase("AllOwningUnionBase")] if self.ownsMembers else []
+        enums = [
+            ClassEnum("TypeOrUninit", enumValues, visibility="private"),
+            ClassEnum(
+                "Type",
+                enumValuesNoUninit,
+                visibility="public",
+                enumClass=True,
+                values=["TypeOrUninit::" + x for x in enumValuesNoUninit],
+            ),
+        ]
+        return CGClass(
+            selfName,
+            bases=bases,
+            members=members,
+            constructors=constructors,
+            methods=methods,
+            disallowCopyConstruction=disallowCopyConstruction,
+            extradeclarations=friend,
+            destructor=ClassDestructor(
+                visibility="public", body="Uninit();\n", bodyInHeader=True
+            ),
+            enums=enums,
+            unions=[ClassUnion("Value", unionValues, visibility="private")],
+        )
+
+    def getConversionToJS(self, templateVars, type):
+        if type.isDictionary() and not type.inner.needsConversionToJS:
+            # We won't be able to convert this dictionary to a JS value, nor
+            # will we need to, since we don't need a ToJSVal method at all.
+            return None
+
+        assert not type.nullable()  # flatMemberTypes never has nullable types
+        val = "mValue.m%(name)s.Value()" % templateVars
+        wrapCode = wrapForType(
+            type,
+            self.descriptorProvider,
+            {
+                "jsvalRef": "rval",
+                "jsvalHandle": "rval",
+                "obj": "scopeObj",
+                "result": val,
+                "spiderMonkeyInterfacesAreStructs": True,
+            },
+        )
+        return CGGeneric(wrapCode)
+
+    @staticmethod
+    def isUnionCopyConstructible(type):
+        return all(isTypeCopyConstructible(t) for t in type.flatMemberTypes)
+
+    @staticmethod
+    def unionTypeName(type, ownsMembers):
+        """
+        Returns a string name for this known union type.
+        """
+        assert type.isUnion() and not type.nullable()
+        return ("Owning" if ownsMembers else "") + type.name
+
+    @staticmethod
+    def unionTypeDecl(type, ownsMembers):
+        """
+        Returns a string for declaring this possibly-nullable union type.
+        """
+        assert type.isUnion()
+        nullable = type.nullable()
+        if nullable:
+            type = type.inner
+        decl = CGGeneric(CGUnionStruct.unionTypeName(type, ownsMembers))
+        if nullable:
+            decl = CGTemplatedType("Nullable", decl)
+        return decl.define()
+
+
+class ClassItem:
+    """Use with CGClass"""
+
+    def __init__(self, name, visibility):
+        self.name = name
+        self.visibility = visibility
+
+    def declare(self, cgClass):
+        assert False
+
+    def define(self, cgClass):
+        assert False
+
+
+class ClassBase(ClassItem):
+    def __init__(self, name, visibility="public"):
+        ClassItem.__init__(self, name, visibility)
+
+    def declare(self, cgClass):
+        return "%s %s" % (self.visibility, self.name)
+
+    def define(self, cgClass):
+        # Only in the header
+        return ""
+
+
+class ClassMethod(ClassItem):
+    def __init__(
+        self,
+        name,
+        returnType,
+        args,
+        inline=False,
+        static=False,
+        virtual=False,
+        const=False,
+        bodyInHeader=False,
+        templateArgs=None,
+        visibility="public",
+        body=None,
+        breakAfterReturnDecl="\n",
+        breakAfterSelf="\n",
+        override=False,
+        canRunScript=False,
+    ):
+        """
+        override indicates whether to flag the method as override
+        """
+        assert not override or virtual
+        assert not (override and static)
+        self.returnType = returnType
+        self.args = args
+        self.inline = inline or bodyInHeader
+        self.static = static
+        self.virtual = virtual
+        self.const = const
+        self.bodyInHeader = bodyInHeader
+        self.templateArgs = templateArgs
+        self.body = body
+        self.breakAfterReturnDecl = breakAfterReturnDecl
+        self.breakAfterSelf = breakAfterSelf
+        self.override = override
+        self.canRunScript = canRunScript
+        ClassItem.__init__(self, name, visibility)
+
+    def getDecorators(self, declaring):
+        decorators = []
+        if self.canRunScript:
+            decorators.append("MOZ_CAN_RUN_SCRIPT")
+        if self.inline:
+            decorators.append("inline")
+        if declaring:
+            if self.static:
+                decorators.append("static")
+            if self.virtual and not self.override:
+                decorators.append("virtual")
+        if decorators:
+            return " ".join(decorators) + " "
+        return ""
+
+    def getBody(self):
+        # Override me or pass a string to constructor
+        assert self.body is not None
+        return self.body
+
+    def declare(self, cgClass):
+        templateClause = (
+            "template <%s>\n" % ", ".join(self.templateArgs)
+            if self.bodyInHeader and self.templateArgs
+            else ""
+        )
+        args = ", ".join([a.declare() for a in self.args])
+        if self.bodyInHeader:
+            body = indent(self.getBody())
+            body = "\n{\n" + body + "}\n"
+        else:
+            body = ";\n"
+
+        return fill(
+            "${templateClause}${decorators}${returnType}${breakAfterReturnDecl}"
+            "${name}(${args})${const}${override}${body}"
+            "${breakAfterSelf}",
+            templateClause=templateClause,
+            decorators=self.getDecorators(True),
+            returnType=self.returnType,
+            breakAfterReturnDecl=self.breakAfterReturnDecl,
+            name=self.name,
+            args=args,
+            const=" const" if self.const else "",
+            override=" override" if self.override else "",
+            body=body,
+            breakAfterSelf=self.breakAfterSelf,
+        )
+
+    def define(self, cgClass):
+        if self.bodyInHeader:
+            return ""
+
+        templateArgs = cgClass.templateArgs
+        if templateArgs:
+            if cgClass.templateSpecialization:
+                templateArgs = templateArgs[len(cgClass.templateSpecialization) :]
+
+        if templateArgs:
+            templateClause = "template <%s>\n" % ", ".join(
+                [str(a) for a in templateArgs]
+            )
+        else:
+            templateClause = ""
+
+        return fill(
+            """
+            ${templateClause}${decorators}${returnType}
+            ${className}::${name}(${args})${const}
+            {
+              $*{body}
+            }
+            """,
+            templateClause=templateClause,
+            decorators=self.getDecorators(False),
+            returnType=self.returnType,
+            className=cgClass.getNameString(),
+            name=self.name,
+            args=", ".join([a.define() for a in self.args]),
+            const=" const" if self.const else "",
+            body=self.getBody(),
+        )
+
+
+class ClassUsingDeclaration(ClassItem):
+    """
+    Used for importing a name from a base class into a CGClass
+
+    baseClass is the name of the base class to import the name from
+
+    name is the name to import
+
+    visibility determines the visibility of the name (public,
+    protected, private), defaults to public.
+    """
+
+    def __init__(self, baseClass, name, visibility="public"):
+        self.baseClass = baseClass
+        ClassItem.__init__(self, name, visibility)
+
+    def declare(self, cgClass):
+        return "using %s::%s;\n\n" % (self.baseClass, self.name)
+
+    def define(self, cgClass):
+        return ""
+
+
+class ClassConstructor(ClassItem):
+    """
+    Used for adding a constructor to a CGClass.
+
+    args is a list of Argument objects that are the arguments taken by the
+    constructor.
+
+    inline should be True if the constructor should be marked inline.
+
+    bodyInHeader should be True if the body should be placed in the class
+    declaration in the header.
+
+    default should be True if the definition of the constructor should be
+    `= default;`.
+
+    visibility determines the visibility of the constructor (public,
+    protected, private), defaults to private.
+
+    explicit should be True if the constructor should be marked explicit.
+
+    baseConstructors is a list of strings containing calls to base constructors,
+    defaults to None.
+
+    body contains a string with the code for the constructor, defaults to empty.
+    """
+
+    def __init__(
+        self,
+        args,
+        inline=False,
+        bodyInHeader=False,
+        default=False,
+        visibility="private",
+        explicit=False,
+        constexpr=False,
+        baseConstructors=None,
+        body="",
+    ):
+        assert not (inline and constexpr)
+        assert not (bodyInHeader and constexpr)
+        assert not (default and body)
+        self.args = args
+        self.inline = inline or bodyInHeader
+        self.bodyInHeader = bodyInHeader or constexpr or default
+        self.default = default
+        self.explicit = explicit
+        self.constexpr = constexpr
+        self.baseConstructors = baseConstructors or []
+        self.body = body
+        ClassItem.__init__(self, None, visibility)
+
+    def getDecorators(self, declaring):
+        decorators = []
+        if declaring:
+            if self.explicit:
+                decorators.append("explicit")
+            if self.inline:
+                decorators.append("inline")
+            if self.constexpr:
+                decorators.append("constexpr")
+        if decorators:
+            return " ".join(decorators) + " "
+        return ""
+
+    def getInitializationList(self, cgClass):
+        items = [str(c) for c in self.baseConstructors]
+        for m in cgClass.members:
+            if not m.static:
+                initialize = m.body
+                if initialize:
+                    items.append(m.name + "(" + initialize + ")")
+
+        if len(items) > 0:
+            return "\n  : " + ",\n    ".join(items)
+        return ""
+
+    def getBody(self):
+        return self.body
+
+    def declare(self, cgClass):
+        args = ", ".join([a.declare() for a in self.args])
+        if self.bodyInHeader:
+            if self.default:
+                body = " = default;\n"
+            else:
+                body = (
+                    self.getInitializationList(cgClass)
+                    + "\n{\n"
+                    + indent(self.getBody())
+                    + "}\n"
+                )
+        else:
+            body = ";\n"
+
+        return fill(
+            "${decorators}${className}(${args})${body}\n",
+            decorators=self.getDecorators(True),
+            className=cgClass.getNameString(),
+            args=args,
+            body=body,
+        )
+
+    def define(self, cgClass):
+        if self.bodyInHeader:
+            return ""
+
+        return fill(
+            """
+            ${decorators}
+            ${className}::${className}(${args})${initializationList}
+            {
+              $*{body}
+            }
+            """,
+            decorators=self.getDecorators(False),
+            className=cgClass.getNameString(),
+            args=", ".join([a.define() for a in self.args]),
+            initializationList=self.getInitializationList(cgClass),
+            body=self.getBody(),
+        )
+
+
+class ClassDestructor(ClassItem):
+    """
+    Used for adding a destructor to a CGClass.
+
+    inline should be True if the destructor should be marked inline.
+
+    bodyInHeader should be True if the body should be placed in the class
+    declaration in the header.
+
+    visibility determines the visibility of the destructor (public,
+    protected, private), defaults to private.
+
+    body contains a string with the code for the destructor, defaults to empty.
+
+    virtual determines whether the destructor is virtual, defaults to False.
+    """
+
+    def __init__(
+        self,
+        inline=False,
+        bodyInHeader=False,
+        visibility="private",
+        body="",
+        virtual=False,
+    ):
+        self.inline = inline or bodyInHeader
+        self.bodyInHeader = bodyInHeader
+        self.body = body
+        self.virtual = virtual
+        ClassItem.__init__(self, None, visibility)
+
+    def getDecorators(self, declaring):
+        decorators = []
+        if self.virtual and declaring:
+            decorators.append("virtual")
+        if self.inline and declaring:
+            decorators.append("inline")
+        if decorators:
+            return " ".join(decorators) + " "
+        return ""
+
+    def getBody(self):
+        return self.body
+
+    def declare(self, cgClass):
+        if self.bodyInHeader:
+            body = "\n{\n" + indent(self.getBody()) + "}\n"
+        else:
+            body = ";\n"
+
+        return fill(
+            "${decorators}~${className}()${body}\n",
+            decorators=self.getDecorators(True),
+            className=cgClass.getNameString(),
+            body=body,
+        )
+
+    def define(self, cgClass):
+        if self.bodyInHeader:
+            return ""
+        return fill(
+            """
+            ${decorators}
+            ${className}::~${className}()
+            {
+              $*{body}
+            }
+            """,
+            decorators=self.getDecorators(False),
+            className=cgClass.getNameString(),
+            body=self.getBody(),
+        )
+
+
+class ClassMember(ClassItem):
+    def __init__(
+        self,
+        name,
+        type,
+        visibility="private",
+        static=False,
+        body=None,
+        hasIgnoreInitCheckFlag=False,
+    ):
+        self.type = type
+        self.static = static
+        self.body = body
+        self.hasIgnoreInitCheckFlag = hasIgnoreInitCheckFlag
+        ClassItem.__init__(self, name, visibility)
+
+    def declare(self, cgClass):
+        return "%s%s%s %s;\n" % (
+            "static " if self.static else "",
+            "MOZ_INIT_OUTSIDE_CTOR " if self.hasIgnoreInitCheckFlag else "",
+            self.type,
+            self.name,
+        )
+
+    def define(self, cgClass):
+        if not self.static:
+            return ""
+        if self.body:
+            body = " = " + self.body
+        else:
+            body = ""
+        return "%s %s::%s%s;\n" % (self.type, cgClass.getNameString(), self.name, body)
+
+
+class ClassEnum(ClassItem):
+    def __init__(
+        self, name, entries, values=None, visibility="public", enumClass=False
+    ):
+        self.entries = entries
+        self.values = values
+        self.enumClass = enumClass
+        ClassItem.__init__(self, name, visibility)
+
+    def declare(self, cgClass):
+        entries = []
+        for i in range(0, len(self.entries)):
+            if not self.values or i >= len(self.values):
+                entry = "%s" % self.entries[i]
+            else:
+                entry = "%s = %s" % (self.entries[i], self.values[i])
+            entries.append(entry)
+
+        decl = ["enum"]
+        self.enumClass and decl.append("class")
+        self.name and decl.append(self.name)
+        decl = " ".join(decl)
+
+        return "%s\n{\n%s\n};\n" % (decl, indent(",\n".join(entries)))
+
+    def define(self, cgClass):
+        # Only goes in the header
+        return ""
+
+
+class ClassUnion(ClassItem):
+    def __init__(self, name, entries, visibility="public"):
+        self.entries = [entry + ";\n" for entry in entries]
+        ClassItem.__init__(self, name, visibility)
+
+    def declare(self, cgClass):
+        return "union %s\n{\n%s\n};\n" % (self.name, indent("".join(self.entries)))
+
+    def define(self, cgClass):
+        # Only goes in the header
+        return ""
+
+
+class CGClass(CGThing):
+    def __init__(
+        self,
+        name,
+        bases=[],
+        members=[],
+        constructors=[],
+        destructor=None,
+        methods=[],
+        enums=[],
+        unions=[],
+        templateArgs=[],
+        templateSpecialization=[],
+        isStruct=False,
+        disallowCopyConstruction=False,
+        indent="",
+        decorators="",
+        extradeclarations="",
+        extradefinitions="",
+    ):
+        CGThing.__init__(self)
+        self.name = name
+        self.bases = bases
+        self.members = members
+        self.constructors = constructors
+        # We store our single destructor in a list, since all of our
+        # code wants lists of members.
+        self.destructors = [destructor] if destructor else []
+        self.methods = methods
+        self.enums = enums
+        self.unions = unions
+        self.templateArgs = templateArgs
+        self.templateSpecialization = templateSpecialization
+        self.isStruct = isStruct
+        self.disallowCopyConstruction = disallowCopyConstruction
+        self.indent = indent
+        self.defaultVisibility = "public" if isStruct else "private"
+        self.decorators = decorators
+        self.extradeclarations = extradeclarations
+        self.extradefinitions = extradefinitions
+
+    def getNameString(self):
+        className = self.name
+        if self.templateSpecialization:
+            className += "<%s>" % ", ".join(
+                [str(a) for a in self.templateSpecialization]
+            )
+        return className
+
+    def declare(self):
+        result = ""
+        if self.templateArgs:
+            templateArgs = [a.declare() for a in self.templateArgs]
+            templateArgs = templateArgs[len(self.templateSpecialization) :]
+            result += "template <%s>\n" % ",".join([str(a) for a in templateArgs])
+
+        type = "struct" if self.isStruct else "class"
+
+        if self.templateSpecialization:
+            specialization = "<%s>" % ", ".join(
+                [str(a) for a in self.templateSpecialization]
+            )
+        else:
+            specialization = ""
+
+        myself = "%s %s%s" % (type, self.name, specialization)
+        if self.decorators != "":
+            myself += " " + self.decorators
+        result += myself
+
+        if self.bases:
+            inherit = " : "
+            result += inherit
+            # Grab our first base
+            baseItems = [CGGeneric(b.declare(self)) for b in self.bases]
+            bases = baseItems[:1]
+            # Indent the rest
+            bases.extend(
+                CGIndenter(b, len(myself) + len(inherit)) for b in baseItems[1:]
+            )
+            result += ",\n".join(b.define() for b in bases)
+
+        result += "\n{\n"
+
+        result += self.extradeclarations
+
+        def declareMembers(cgClass, memberList, defaultVisibility):
+            members = {"private": [], "protected": [], "public": []}
+
+            for member in memberList:
+                members[member.visibility].append(member)
+
+            if defaultVisibility == "public":
+                order = ["public", "protected", "private"]
+            else:
+                order = ["private", "protected", "public"]
+
+            result = ""
+
+            lastVisibility = defaultVisibility
+            for visibility in order:
+                list = members[visibility]
+                if list:
+                    if visibility != lastVisibility:
+                        result += visibility + ":\n"
+                    for member in list:
+                        result += indent(member.declare(cgClass))
+                    lastVisibility = visibility
+            return (result, lastVisibility)
+
+        if self.disallowCopyConstruction:
+
+            class DisallowedCopyConstructor(object):
+                def __init__(self):
+                    self.visibility = "private"
+
+                def declare(self, cgClass):
+                    name = cgClass.getNameString()
+                    return (
+                        "%s(const %s&) = delete;\n"
+                        "%s& operator=(const %s&) = delete;\n"
+                        % (name, name, name, name)
+                    )
+
+            disallowedCopyConstructors = [DisallowedCopyConstructor()]
+        else:
+            disallowedCopyConstructors = []
+
+        order = [
+            self.enums,
+            self.unions,
+            self.members,
+            self.constructors + disallowedCopyConstructors,
+            self.destructors,
+            self.methods,
+        ]
+
+        lastVisibility = self.defaultVisibility
+        pieces = []
+        for memberList in order:
+            code, lastVisibility = declareMembers(self, memberList, lastVisibility)
+
+            if code:
+                code = code.rstrip() + "\n"  # remove extra blank lines at the end
+                pieces.append(code)
+
+        result += "\n".join(pieces)
+        result += "};\n"
+        result = indent(result, len(self.indent))
+        return result
+
+    def define(self):
+        def defineMembers(cgClass, memberList, itemCount, separator=""):
+            result = ""
+            for member in memberList:
+                if itemCount != 0:
+                    result = result + separator
+                definition = member.define(cgClass)
+                if definition:
+                    # Member variables would only produce empty lines here.
+                    result += definition
+                    itemCount += 1
+            return (result, itemCount)
+
+        order = [
+            (self.members, ""),
+            (self.constructors, "\n"),
+            (self.destructors, "\n"),
+            (self.methods, "\n"),
+        ]
+
+        result = self.extradefinitions
+        itemCount = 0
+        for memberList, separator in order:
+            memberString, itemCount = defineMembers(
+                self, memberList, itemCount, separator
+            )
+            result = result + memberString
+        return result
+
+
+class CGResolveOwnPropertyViaResolve(CGAbstractBindingMethod):
+    """
+    An implementation of Xray ResolveOwnProperty stuff for things that have a
+    resolve hook.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "wrapper"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::MutableHandle<Maybe<JS::PropertyDescriptor>>", "desc"),
+        ]
+        CGAbstractBindingMethod.__init__(
+            self,
+            descriptor,
+            "ResolveOwnPropertyViaResolve",
+            args,
+            getThisObj="",
+            callArgs="",
+        )
+
+    def generate_code(self):
+        return CGGeneric(
+            dedent(
+                """
+            {
+              // Since we're dealing with an Xray, do the resolve on the
+              // underlying object first.  That gives it a chance to
+              // define properties on the actual object as needed, and
+              // then use the fact that it created the objects as a flag
+              // to avoid re-resolving the properties if someone deletes
+              // them.
+              JSAutoRealm ar(cx, obj);
+              JS_MarkCrossZoneId(cx, id);
+              JS::Rooted<mozilla::Maybe<JS::PropertyDescriptor>> objDesc(cx);
+              if (!self->DoResolve(cx, obj, id, &objDesc)) {
+                return false;
+              }
+              // If desc->value() is undefined, then the DoResolve call
+              // has already defined the property on the object.  Don't
+              // try to also define it.
+              if (objDesc.isSome() &&
+                  !objDesc->value().isUndefined()) {
+                JS::Rooted<JS::PropertyDescriptor> defineDesc(cx, *objDesc);
+                if (!JS_DefinePropertyById(cx, obj, id, defineDesc)) {
+                  return false;
+                }
+              }
+            }
+            return self->DoResolve(cx, wrapper, id, desc);
+            """
+            )
+        )
+
+
+class CGEnumerateOwnPropertiesViaGetOwnPropertyNames(CGAbstractBindingMethod):
+    """
+    An implementation of Xray EnumerateOwnProperties stuff for things
+    that have a resolve hook.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "wrapper"),
+            Argument("JS::Handle<JSObject*>", "obj"),
+            Argument("JS::MutableHandleVector<jsid>", "props"),
+        ]
+        CGAbstractBindingMethod.__init__(
+            self,
+            descriptor,
+            "EnumerateOwnPropertiesViaGetOwnPropertyNames",
+            args,
+            getThisObj="",
+            callArgs="",
+        )
+
+    def generate_code(self):
+        return CGGeneric(
+            dedent(
+                """
+            FastErrorResult rv;
+            // This wants all own props, not just enumerable ones.
+            self->GetOwnPropertyNames(cx, props, false, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return false;
+            }
+            return true;
+            """
+            )
+        )
+
+
+class CGPrototypeTraitsClass(CGClass):
+    def __init__(self, descriptor, indent=""):
+        templateArgs = [Argument("prototypes::ID", "PrototypeID")]
+        templateSpecialization = ["prototypes::id::" + descriptor.name]
+        enums = [ClassEnum("", ["Depth"], [descriptor.interface.inheritanceDepth()])]
+        CGClass.__init__(
+            self,
+            "PrototypeTraits",
+            indent=indent,
+            templateArgs=templateArgs,
+            templateSpecialization=templateSpecialization,
+            enums=enums,
+            isStruct=True,
+        )
+
+    def deps(self):
+        return set()
+
+
+class CGClassForwardDeclare(CGThing):
+    def __init__(self, name, isStruct=False):
+        CGThing.__init__(self)
+        self.name = name
+        self.isStruct = isStruct
+
+    def declare(self):
+        type = "struct" if self.isStruct else "class"
+        return "%s %s;\n" % (type, self.name)
+
+    def define(self):
+        # Header only
+        return ""
+
+    def deps(self):
+        return set()
+
+
+class CGProxySpecialOperation(CGPerSignatureCall):
+    """
+    Base class for classes for calling an indexed or named special operation
+    (don't use this directly, use the derived classes below).
+
+    If checkFound is False, will just assert that the prop is found instead of
+    checking that it is before wrapping the value.
+
+    resultVar: See the docstring for CGCallGenerator.
+
+    foundVar: For getters and deleters, the generated code can also set a bool
+    variable, declared by the caller, if the given indexed or named property
+    already existed. If the caller wants this, it should pass the name of the
+    bool variable as the foundVar keyword argument to the constructor. The
+    caller is responsible for declaring the variable and initializing it to
+    false.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        operation,
+        checkFound=True,
+        argumentHandleValue=None,
+        resultVar=None,
+        foundVar=None,
+    ):
+        self.checkFound = checkFound
+        self.foundVar = foundVar or "found"
+
+        nativeName = MakeNativeName(descriptor.binaryNameFor(operation, False))
+        operation = descriptor.operations[operation]
+        assert len(operation.signatures()) == 1
+        signature = operation.signatures()[0]
+
+        returnType, arguments = signature
+
+        # We pass len(arguments) as the final argument so that the
+        # CGPerSignatureCall won't do any argument conversion of its own.
+        CGPerSignatureCall.__init__(
+            self,
+            returnType,
+            arguments,
+            nativeName,
+            False,
+            descriptor,
+            operation,
+            len(arguments),
+            resultVar=resultVar,
+            objectName="proxy",
+        )
+
+        if operation.isSetter():
+            # arguments[0] is the index or name of the item that we're setting.
+            argument = arguments[1]
+            info = getJSToNativeConversionInfo(
+                argument.type,
+                descriptor,
+                sourceDescription=(
+                    "value being assigned to %s setter"
+                    % descriptor.interface.identifier.name
+                ),
+            )
+            if argumentHandleValue is None:
+                argumentHandleValue = "desc.value()"
+            rootedValue = fill(
+                """
+                JS::Rooted<JS::Value> rootedValue(cx, ${argumentHandleValue});
+                """,
+                argumentHandleValue=argumentHandleValue,
+            )
+            templateValues = {
+                "declName": argument.identifier.name,
+                "holderName": argument.identifier.name + "_holder",
+                "val": argumentHandleValue,
+                "maybeMutableVal": "&rootedValue",
+                "obj": "obj",
+                "passedToJSImpl": "false",
+            }
+            self.cgRoot.prepend(instantiateJSToNativeConversion(info, templateValues))
+            # rootedValue needs to come before the conversion, so we
+            # need to prepend it last.
+            self.cgRoot.prepend(CGGeneric(rootedValue))
+        elif operation.isGetter() or operation.isDeleter():
+            if foundVar is None:
+                self.cgRoot.prepend(CGGeneric("bool found = false;\n"))
+
+    def getArguments(self):
+        args = [(a, a.identifier.name) for a in self.arguments]
+        if self.idlNode.isGetter() or self.idlNode.isDeleter():
+            args.append(
+                (
+                    FakeArgument(BuiltinTypes[IDLBuiltinType.Types.boolean]),
+                    self.foundVar,
+                )
+            )
+        return args
+
+    def wrap_return_value(self):
+        if not self.idlNode.isGetter() or self.templateValues is None:
+            return ""
+
+        wrap = CGGeneric(
+            wrapForType(self.returnType, self.descriptor, self.templateValues)
+        )
+        if self.checkFound:
+            wrap = CGIfWrapper(wrap, self.foundVar)
+        else:
+            wrap = CGList([CGGeneric("MOZ_ASSERT(" + self.foundVar + ");\n"), wrap])
+        return "\n" + wrap.define()
+
+
+class CGProxyIndexedOperation(CGProxySpecialOperation):
+    """
+    Class to generate a call to an indexed operation.
+
+    If doUnwrap is False, the caller is responsible for making sure a variable
+    named 'self' holds the C++ object somewhere where the code we generate
+    will see it.
+
+    If checkFound is False, will just assert that the prop is found instead of
+    checking that it is before wrapping the value.
+
+    resultVar: See the docstring for CGCallGenerator.
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        name,
+        doUnwrap=True,
+        checkFound=True,
+        argumentHandleValue=None,
+        resultVar=None,
+        foundVar=None,
+    ):
+        self.doUnwrap = doUnwrap
+        CGProxySpecialOperation.__init__(
+            self,
+            descriptor,
+            name,
+            checkFound,
+            argumentHandleValue=argumentHandleValue,
+            resultVar=resultVar,
+            foundVar=foundVar,
+        )
+
+    def define(self):
+        # Our first argument is the id we're getting.
+        argName = self.arguments[0].identifier.name
+        if argName == "index":
+            # We already have our index in a variable with that name
+            setIndex = ""
+        else:
+            setIndex = "uint32_t %s = index;\n" % argName
+        if self.doUnwrap:
+            unwrap = "%s* self = UnwrapProxy(proxy);\n" % self.descriptor.nativeType
+        else:
+            unwrap = ""
+        return setIndex + unwrap + CGProxySpecialOperation.define(self)
+
+
+class CGProxyIndexedGetter(CGProxyIndexedOperation):
+    """
+    Class to generate a call to an indexed getter. If templateValues is not None
+    the returned value will be wrapped with wrapForType using templateValues.
+
+    If doUnwrap is False, the caller is responsible for making sure a variable
+    named 'self' holds the C++ object somewhere where the code we generate
+    will see it.
+
+    If checkFound is False, will just assert that the prop is found instead of
+    checking that it is before wrapping the value.
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        templateValues=None,
+        doUnwrap=True,
+        checkFound=True,
+        foundVar=None,
+    ):
+        self.templateValues = templateValues
+        CGProxyIndexedOperation.__init__(
+            self, descriptor, "IndexedGetter", doUnwrap, checkFound, foundVar=foundVar
+        )
+
+
+class CGProxyIndexedPresenceChecker(CGProxyIndexedGetter):
+    """
+    Class to generate a call that checks whether an indexed property exists.
+
+    For now, we just delegate to CGProxyIndexedGetter
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(self, descriptor, foundVar):
+        CGProxyIndexedGetter.__init__(self, descriptor, foundVar=foundVar)
+        self.cgRoot.append(CGGeneric("(void)result;\n"))
+
+
+class CGProxyIndexedSetter(CGProxyIndexedOperation):
+    """
+    Class to generate a call to an indexed setter.
+    """
+
+    def __init__(self, descriptor, argumentHandleValue=None):
+        CGProxyIndexedOperation.__init__(
+            self, descriptor, "IndexedSetter", argumentHandleValue=argumentHandleValue
+        )
+
+
+class CGProxyNamedOperation(CGProxySpecialOperation):
+    """
+    Class to generate a call to a named operation.
+
+    'value' is the jsval to use for the name; None indicates that it should be
+    gotten from the property id.
+
+    resultVar: See the docstring for CGCallGenerator.
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+
+    tailCode: if we end up with a non-symbol string id, run this code after
+              we do all our other work.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        name,
+        value=None,
+        argumentHandleValue=None,
+        resultVar=None,
+        foundVar=None,
+        tailCode="",
+    ):
+        CGProxySpecialOperation.__init__(
+            self,
+            descriptor,
+            name,
+            argumentHandleValue=argumentHandleValue,
+            resultVar=resultVar,
+            foundVar=foundVar,
+        )
+        self.value = value
+        self.tailCode = tailCode
+
+    def define(self):
+        # Our first argument is the id we're getting.
+        argName = self.arguments[0].identifier.name
+        if argName == "id":
+            # deal with the name collision
+            decls = "JS::Rooted<jsid> id_(cx, id);\n"
+            idName = "id_"
+        else:
+            decls = ""
+            idName = "id"
+
+        decls += "FakeString<char16_t> %s;\n" % argName
+
+        main = fill(
+            """
+            ${nativeType}* self = UnwrapProxy(proxy);
+            $*{op}
+            $*{tailCode}
+            """,
+            nativeType=self.descriptor.nativeType,
+            op=CGProxySpecialOperation.define(self),
+            tailCode=self.tailCode,
+        )
+
+        if self.value is None:
+            return fill(
+                """
+                $*{decls}
+                bool isSymbol;
+                if (!ConvertIdToString(cx, ${idName}, ${argName}, isSymbol)) {
+                  return false;
+                }
+                if (!isSymbol) {
+                  $*{main}
+                }
+                """,
+                decls=decls,
+                idName=idName,
+                argName=argName,
+                main=main,
+            )
+
+        # Sadly, we have to set up nameVal even if we have an atom id,
+        # because we don't know for sure, and we can end up needing it
+        # so it needs to be higher up the stack.  Using a Maybe here
+        # seems like probable overkill.
+        return fill(
+            """
+            $*{decls}
+            JS::Rooted<JS::Value> nameVal(cx, ${value});
+            if (!nameVal.isSymbol()) {
+              if (!ConvertJSValueToString(cx, nameVal, eStringify, eStringify,
+                                          ${argName})) {
+                return false;
+              }
+              $*{main}
+            }
+            """,
+            decls=decls,
+            value=self.value,
+            argName=argName,
+            main=main,
+        )
+
+
+class CGProxyNamedGetter(CGProxyNamedOperation):
+    """
+    Class to generate a call to an named getter. If templateValues is not None
+    the returned value will be wrapped with wrapForType using templateValues.
+    'value' is the jsval to use for the name; None indicates that it should be
+    gotten from the property id.
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(self, descriptor, templateValues=None, value=None, foundVar=None):
+        self.templateValues = templateValues
+        CGProxyNamedOperation.__init__(
+            self, descriptor, "NamedGetter", value, foundVar=foundVar
+        )
+
+
+class CGProxyNamedPresenceChecker(CGProxyNamedGetter):
+    """
+    Class to generate a call that checks whether a named property exists.
+
+    For now, we just delegate to CGProxyNamedGetter
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(self, descriptor, foundVar=None):
+        CGProxyNamedGetter.__init__(self, descriptor, foundVar=foundVar)
+        self.cgRoot.append(CGGeneric("(void)result;\n"))
+
+
+class CGProxyNamedSetter(CGProxyNamedOperation):
+    """
+    Class to generate a call to a named setter.
+    """
+
+    def __init__(self, descriptor, tailCode, argumentHandleValue=None):
+        CGProxyNamedOperation.__init__(
+            self,
+            descriptor,
+            "NamedSetter",
+            argumentHandleValue=argumentHandleValue,
+            tailCode=tailCode,
+        )
+
+
+class CGProxyNamedDeleter(CGProxyNamedOperation):
+    """
+    Class to generate a call to a named deleter.
+
+    resultVar: See the docstring for CGCallGenerator.
+
+    foundVar: See the docstring for CGProxySpecialOperation.
+    """
+
+    def __init__(self, descriptor, resultVar=None, foundVar=None):
+        CGProxyNamedOperation.__init__(
+            self, descriptor, "NamedDeleter", resultVar=resultVar, foundVar=foundVar
+        )
+
+
+class CGProxyIsProxy(CGAbstractMethod):
+    def __init__(self, descriptor):
+        args = [Argument("JSObject*", "obj")]
+        CGAbstractMethod.__init__(
+            self, descriptor, "IsProxy", "bool", args, alwaysInline=True
+        )
+
+    def declare(self):
+        return ""
+
+    def definition_body(self):
+        return "return js::IsProxy(obj) && js::GetProxyHandler(obj) == DOMProxyHandler::getInstance();\n"
+
+
+class CGProxyUnwrap(CGAbstractMethod):
+    def __init__(self, descriptor):
+        args = [Argument("JSObject*", "obj")]
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "UnwrapProxy",
+            descriptor.nativeType + "*",
+            args,
+            alwaysInline=True,
+        )
+
+    def declare(self):
+        return ""
+
+    def definition_body(self):
+        return fill(
+            """
+            MOZ_ASSERT(js::IsProxy(obj));
+            if (js::GetProxyHandler(obj) != DOMProxyHandler::getInstance()) {
+              MOZ_ASSERT(xpc::WrapperFactory::IsXrayWrapper(obj));
+              obj = js::UncheckedUnwrap(obj);
+            }
+            MOZ_ASSERT(IsProxy(obj));
+            return static_cast<${type}*>(js::GetProxyReservedSlot(obj, DOM_OBJECT_SLOT).toPrivate());
+            """,
+            type=self.descriptor.nativeType,
+        )
+
+
+MISSING_PROP_PREF = "dom.missing_prop_counters.enabled"
+
+
+def missingPropUseCountersForDescriptor(desc):
+    if not desc.needsMissingPropUseCounters:
+        return ""
+
+    return fill(
+        """
+        if (StaticPrefs::${pref}() && id.isAtom()) {
+          CountMaybeMissingProperty(proxy, id);
+        }
+
+        """,
+        pref=prefIdentifier(MISSING_PROP_PREF),
+    )
+
+
+def findAncestorWithInstrumentedProps(desc):
+    """
+    Find an ancestor of desc.interface (not including desc.interface
+    itself) that has instrumented properties on it.  May return None
+    if there is no such ancestor.
+    """
+    ancestor = desc.interface.parent
+    while ancestor:
+        if ancestor.getExtendedAttribute("InstrumentedProps"):
+            return ancestor
+        ancestor = ancestor.parent
+    return None
+
+
+class CGCountMaybeMissingProperty(CGAbstractMethod):
+    def __init__(self, descriptor):
+        """
+        Returns whether we counted the property involved.
+        """
+        CGAbstractMethod.__init__(
+            self,
+            descriptor,
+            "CountMaybeMissingProperty",
+            "bool",
+            [
+                Argument("JS::Handle<JSObject*>", "proxy"),
+                Argument("JS::Handle<jsid>", "id"),
+            ],
+        )
+
+    def gen_switch(self, switchDecriptor):
+        """
+        Generate a switch from the switch descriptor.  The descriptor
+        dictionary must have the following properties:
+
+        1) A "precondition" property that contains code to run before the
+           switch statement.  Its value ie a string.
+        2) A "condition" property for the condition.  Its value is a string.
+        3) A "cases" property.  Its value is an object that has property names
+           corresponding to the case labels.  The values of those properties
+           are either new switch descriptor dictionaries (which will then
+           generate nested switches) or strings to use for case bodies.
+        """
+        cases = []
+        for label, body in sorted(six.iteritems(switchDecriptor["cases"])):
+            if isinstance(body, dict):
+                body = self.gen_switch(body)
+            cases.append(
+                fill(
+                    """
+                case ${label}: {
+                  $*{body}
+                  break;
+                }
+                """,
+                    label=label,
+                    body=body,
+                )
+            )
+        return fill(
+            """
+            $*{precondition}
+            switch (${condition}) {
+              $*{cases}
+            }
+            """,
+            precondition=switchDecriptor["precondition"],
+            condition=switchDecriptor["condition"],
+            cases="".join(cases),
+        )
+
+    def charSwitch(self, props, charIndex):
+        """
+        Create a switch for the given props, based on the first char where
+        they start to differ at index charIndex or more.  Each prop is a tuple
+        containing interface name and prop name.
+
+        Incoming props should be a sorted list.
+        """
+        if len(props) == 1:
+            # We're down to one string: just check whether we match it.
+            return fill(
+                """
+                if (JS_LinearStringEqualsLiteral(str, "${name}")) {
+                  counter.emplace(eUseCounter_${iface}_${name});
+                }
+                """,
+                iface=self.descriptor.name,
+                name=props[0],
+            )
+
+        switch = dict()
+        if charIndex == 0:
+            switch["precondition"] = "StringIdChars chars(nogc, str);\n"
+        else:
+            switch["precondition"] = ""
+
+        # Find the first place where we might actually have a difference.
+        while all(prop[charIndex] == props[0][charIndex] for prop in props):
+            charIndex += 1
+
+        switch["condition"] = "chars[%d]" % charIndex
+        switch["cases"] = dict()
+        current_props = None
+        curChar = None
+        idx = 0
+        while idx < len(props):
+            nextChar = "'%s'" % props[idx][charIndex]
+            if nextChar != curChar:
+                if curChar:
+                    switch["cases"][curChar] = self.charSwitch(
+                        current_props, charIndex + 1
+                    )
+                current_props = []
+                curChar = nextChar
+            current_props.append(props[idx])
+            idx += 1
+        switch["cases"][curChar] = self.charSwitch(current_props, charIndex + 1)
+        return switch
+
+    def definition_body(self):
+        ancestor = findAncestorWithInstrumentedProps(self.descriptor)
+
+        if ancestor:
+            body = fill(
+                """
+                if (${ancestor}_Binding::CountMaybeMissingProperty(proxy, id)) {
+                  return true;
+                }
+
+                """,
+                ancestor=ancestor.identifier.name,
+            )
+        else:
+            body = ""
+
+        instrumentedProps = self.descriptor.instrumentedProps
+        if not instrumentedProps:
+            return body + dedent(
+                """
+                return false;
+                """
+            )
+
+        lengths = set(len(prop) for prop in instrumentedProps)
+        switchDesc = {"condition": "JS::GetLinearStringLength(str)", "precondition": ""}
+        switchDesc["cases"] = dict()
+        for length in sorted(lengths):
+            switchDesc["cases"][str(length)] = self.charSwitch(
+                list(sorted(prop for prop in instrumentedProps if len(prop) == length)),
+                0,
+            )
+
+        return body + fill(
+            """
+            MOZ_ASSERT(StaticPrefs::${pref}() && id.isAtom());
+            Maybe<UseCounter> counter;
+            {
+              // Scope for our no-GC section, so we don't need to rely on SetUseCounter not GCing.
+              JS::AutoCheckCannotGC nogc;
+              JSLinearString* str = JS::AtomToLinearString(id.toAtom());
+              // Don't waste time fetching the chars until we've done the length switch.
+              $*{switch}
+            }
+            if (counter) {
+              SetUseCounter(proxy, *counter);
+              return true;
+            }
+
+            return false;
+            """,
+            pref=prefIdentifier(MISSING_PROP_PREF),
+            switch=self.gen_switch(switchDesc),
+        )
+
+
+class CGDOMJSProxyHandler_getOwnPropDescriptor(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("bool", "ignoreNamedProps"),
+            Argument("JS::MutableHandle<Maybe<JS::PropertyDescriptor>>", "desc"),
+        ]
+        ClassMethod.__init__(
+            self,
+            "getOwnPropDescriptor",
+            "bool",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        indexedSetter = self.descriptor.operations["IndexedSetter"]
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            xrayDecl = dedent(
+                """
+                MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy));
+                MOZ_ASSERT(IsPlatformObjectSameOrigin(cx, proxy),
+                           "getOwnPropertyDescriptor() and set() should have dealt");
+                MOZ_ASSERT(js::IsObjectInContextCompartment(proxy, cx),
+                           "getOwnPropertyDescriptor() and set() should have dealt");
+
+                """
+            )
+            xrayCheck = ""
+        else:
+            xrayDecl = "bool isXray = xpc::WrapperFactory::IsXrayWrapper(proxy);\n"
+            xrayCheck = "!isXray &&"
+
+        if self.descriptor.supportsIndexedProperties():
+            attributes = [
+                "JS::PropertyAttribute::Configurable",
+                "JS::PropertyAttribute::Enumerable",
+            ]
+            if indexedSetter is not None:
+                attributes.append("JS::PropertyAttribute::Writable")
+            setDescriptor = (
+                "desc.set(mozilla::Some(JS::PropertyDescriptor::Data(value, { %s })));\nreturn true;\n"
+                % ", ".join(attributes)
+            )
+            templateValues = {
+                "jsvalRef": "value",
+                "jsvalHandle": "&value",
+                "obj": "proxy",
+                "successCode": setDescriptor,
+            }
+            getIndexed = fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  JS::Rooted<JS::Value> value(cx);
+                  $*{callGetter}
+                }
+
+                """,
+                callGetter=CGProxyIndexedGetter(
+                    self.descriptor, templateValues
+                ).define(),
+            )
+        else:
+            getIndexed = ""
+
+        missingPropUseCounters = missingPropUseCountersForDescriptor(self.descriptor)
+
+        if self.descriptor.supportsNamedProperties():
+            operations = self.descriptor.operations
+            attributes = ["JS::PropertyAttribute::Configurable"]
+            if self.descriptor.namedPropertiesEnumerable:
+                attributes.append("JS::PropertyAttribute::Enumerable")
+            if operations["NamedSetter"] is not None:
+                attributes.append("JS::PropertyAttribute::Writable")
+            setDescriptor = (
+                "desc.set(mozilla::Some(JS::PropertyDescriptor::Data(value, { %s })));\nreturn true;\n"
+                % ", ".join(attributes)
+            )
+            templateValues = {
+                "jsvalRef": "value",
+                "jsvalHandle": "&value",
+                "obj": "proxy",
+                "successCode": setDescriptor,
+            }
+
+            computeCondition = dedent(
+                """
+                bool hasOnProto;
+                if (!HasPropertyOnPrototype(cx, proxy, id, &hasOnProto)) {
+                  return false;
+                }
+                callNamedGetter = !hasOnProto;
+                """
+            )
+            if self.descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+                computeCondition = fill(
+                    """
+                    if (!isXray) {
+                      callNamedGetter = true;
+                    } else {
+                      $*{hasOnProto}
+                    }
+                    """,
+                    hasOnProto=computeCondition,
+                )
+
+            outerCondition = "!ignoreNamedProps"
+            if self.descriptor.supportsIndexedProperties():
+                outerCondition = "!IsArrayIndex(index) && " + outerCondition
+
+            namedGetCode = CGProxyNamedGetter(self.descriptor, templateValues).define()
+            namedGet = fill(
+                """
+                bool callNamedGetter = false;
+                if (${outerCondition}) {
+                  $*{computeCondition}
+                }
+                if (callNamedGetter) {
+                  JS::Rooted<JS::Value> value(cx);
+                  $*{namedGetCode}
+                }
+                """,
+                outerCondition=outerCondition,
+                computeCondition=computeCondition,
+                namedGetCode=namedGetCode,
+            )
+            namedGet += "\n"
+        else:
+            namedGet = ""
+
+        return fill(
+            """
+            $*{xrayDecl}
+            $*{getIndexed}
+            $*{missingPropUseCounters}
+            JS::Rooted<JSObject*> expando(cx);
+            if (${xrayCheck}(expando = GetExpandoObject(proxy))) {
+              if (!JS_GetOwnPropertyDescriptorById(cx, expando, id, desc)) {
+                return false;
+              }
+              if (desc.isSome()) {
+                return true;
+              }
+            }
+
+            $*{namedGet}
+            desc.reset();
+            return true;
+            """,
+            xrayDecl=xrayDecl,
+            xrayCheck=xrayCheck,
+            getIndexed=getIndexed,
+            missingPropUseCounters=missingPropUseCounters,
+            namedGet=namedGet,
+        )
+
+
+class CGDOMJSProxyHandler_defineProperty(ClassMethod):
+    def __init__(self, descriptor):
+        # The usual convention is to name the ObjectOpResult out-parameter
+        # `result`, but that name is a bit overloaded around here.
+        args = [
+            Argument("JSContext*", "cx_"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::Handle<JS::PropertyDescriptor>", "desc"),
+            Argument("JS::ObjectOpResult&", "opresult"),
+            Argument("bool*", "done"),
+        ]
+        ClassMethod.__init__(
+            self,
+            "defineProperty",
+            "bool",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        set = ""
+
+        indexedSetter = self.descriptor.operations["IndexedSetter"]
+        if indexedSetter:
+            error_label = CGSpecializedMethod.error_reporting_label_helper(
+                self.descriptor, indexedSetter, isConstructor=False
+            )
+            if error_label:
+                cxDecl = fill(
+                    """
+                    BindingCallContext cx(cx_, "${error_label}");
+                    """,
+                    error_label=error_label,
+                )
+            else:
+                cxDecl = dedent(
+                    """
+                    JSContext* cx = cx_;
+                    """
+                )
+            set += fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  $*{cxDecl}
+                  *done = true;
+                  // https://heycam.github.io/webidl/#legacy-platform-object-defineownproperty
+                  // Step 1.1.  The no-indexed-setter case is handled by step 1.2.
+                  if (!desc.isDataDescriptor()) {
+                    return opresult.failNotDataDescriptor();
+                  }
+
+                  $*{callSetter}
+                  return opresult.succeed();
+                }
+                """,
+                cxDecl=cxDecl,
+                callSetter=CGProxyIndexedSetter(self.descriptor).define(),
+            )
+        elif self.descriptor.supportsIndexedProperties():
+            # We allow untrusted content to prevent Xrays from setting a
+            # property if that property is an indexed property and we have no
+            # indexed setter.  That's how the object would normally behave if
+            # you tried to set the property on it.  That means we don't need to
+            # do anything special for Xrays here.
+            set += dedent(
+                """
+                if (IsArrayIndex(GetArrayIndexFromId(id))) {
+                  *done = true;
+                  return opresult.failNoIndexedSetter();
+                }
+                """
+            )
+
+        namedSetter = self.descriptor.operations["NamedSetter"]
+        if namedSetter:
+            error_label = CGSpecializedMethod.error_reporting_label_helper(
+                self.descriptor, namedSetter, isConstructor=False
+            )
+            if error_label:
+                set += fill(
+                    """
+                    BindingCallContext cx(cx_, "${error_label}");
+                    """,
+                    error_label=error_label,
+                )
+            else:
+                set += dedent(
+                    """
+                    JSContext* cx = cx_;
+                    """
+                )
+            if self.descriptor.hasLegacyUnforgeableMembers:
+                raise TypeError(
+                    "Can't handle a named setter on an interface "
+                    "that has unforgeables.  Figure out how that "
+                    "should work!"
+                )
+            tailCode = dedent(
+                """
+                *done = true;
+                return opresult.succeed();
+                """
+            )
+            set += CGProxyNamedSetter(self.descriptor, tailCode).define()
+        else:
+            # We allow untrusted content to prevent Xrays from setting a
+            # property if that property is already a named property on the
+            # object and we have no named setter.  That's how the object would
+            # normally behave if you tried to set the property on it.  That
+            # means we don't need to do anything special for Xrays here.
+            if self.descriptor.supportsNamedProperties():
+                set += fill(
+                    """
+                    JSContext* cx = cx_;
+                    bool found = false;
+                    $*{presenceChecker}
+
+                    if (found) {
+                      *done = true;
+                      return opresult.failNoNamedSetter();
+                    }
+                    """,
+                    presenceChecker=CGProxyNamedPresenceChecker(
+                        self.descriptor, foundVar="found"
+                    ).define(),
+                )
+            if self.descriptor.isMaybeCrossOriginObject():
+                set += dedent(
+                    """
+                    MOZ_ASSERT(IsPlatformObjectSameOrigin(cx_, proxy),
+                               "Why did the MaybeCrossOriginObject defineProperty override fail?");
+                    MOZ_ASSERT(js::IsObjectInContextCompartment(proxy, cx_),
+                               "Why did the MaybeCrossOriginObject defineProperty override fail?");
+                    """
+                )
+
+        # In all cases we want to tail-call to our base class; we can
+        # always land here for symbols.
+        set += (
+            "return mozilla::dom::DOMProxyHandler::defineProperty(%s);\n"
+            % ", ".join(a.name for a in self.args)
+        )
+        return set
+
+
+def getDeleterBody(descriptor, type, foundVar=None):
+    """
+    type should be "Named" or "Indexed"
+
+    The possible outcomes:
+    - an error happened                       (the emitted code returns false)
+    - own property not found                  (foundVar=false, deleteSucceeded=true)
+    - own property found and deleted          (foundVar=true,  deleteSucceeded=true)
+    - own property found but can't be deleted (foundVar=true,  deleteSucceeded=false)
+    """
+    assert type in ("Named", "Indexed")
+    deleter = descriptor.operations[type + "Deleter"]
+    if deleter:
+        assert type == "Named"
+        assert foundVar is not None
+        if descriptor.hasLegacyUnforgeableMembers:
+            raise TypeError(
+                "Can't handle a deleter on an interface "
+                "that has unforgeables.  Figure out how "
+                "that should work!"
+            )
+        # See if the deleter method is fallible.
+        t = deleter.signatures()[0][0]
+        if t.isPrimitive() and not t.nullable() and t.tag() == IDLType.Tags.bool:
+            # The deleter method has a boolean return value. When a
+            # property is found, the return value indicates whether it
+            # was successfully deleted.
+            setDS = fill(
+                """
+                if (!${foundVar}) {
+                  deleteSucceeded = true;
+                }
+                """,
+                foundVar=foundVar,
+            )
+        else:
+            # No boolean return value: if a property is found,
+            # deleting it always succeeds.
+            setDS = "deleteSucceeded = true;\n"
+
+        body = (
+            CGProxyNamedDeleter(
+                descriptor, resultVar="deleteSucceeded", foundVar=foundVar
+            ).define()
+            + setDS
+        )
+    elif getattr(descriptor, "supports%sProperties" % type)():
+        presenceCheckerClass = globals()["CGProxy%sPresenceChecker" % type]
+        foundDecl = ""
+        if foundVar is None:
+            foundVar = "found"
+            foundDecl = "bool found = false;\n"
+        body = fill(
+            """
+            $*{foundDecl}
+            $*{presenceChecker}
+            deleteSucceeded = !${foundVar};
+            """,
+            foundDecl=foundDecl,
+            presenceChecker=presenceCheckerClass(
+                descriptor, foundVar=foundVar
+            ).define(),
+            foundVar=foundVar,
+        )
+    else:
+        body = None
+    return body
+
+
+class CGDeleteNamedProperty(CGAbstractStaticMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "xray"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::ObjectOpResult&", "opresult"),
+        ]
+        CGAbstractStaticMethod.__init__(
+            self, descriptor, "DeleteNamedProperty", "bool", args
+        )
+
+    def definition_body(self):
+        return fill(
+            """
+            MOZ_ASSERT(xpc::WrapperFactory::IsXrayWrapper(xray));
+            MOZ_ASSERT(js::IsProxy(proxy));
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy));
+            JSAutoRealm ar(cx, proxy);
+            bool deleteSucceeded = false;
+            bool found = false;
+            $*{namedBody}
+            if (!found || deleteSucceeded) {
+              return opresult.succeed();
+            }
+            return opresult.failCantDelete();
+            """,
+            namedBody=getDeleterBody(self.descriptor, "Named", foundVar="found"),
+        )
+
+
+class CGDOMJSProxyHandler_delete(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::ObjectOpResult&", "opresult"),
+        ]
+        ClassMethod.__init__(
+            self, "delete_", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        delete = dedent(
+            """
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
+                      "Should not have a XrayWrapper here");
+
+            """
+        )
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            delete += dedent(
+                """
+                if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+                  return ReportCrossOriginDenial(cx, id, "delete"_ns);
+                }
+
+                // Safe to enter the Realm of proxy now.
+                JSAutoRealm ar(cx, proxy);
+                JS_MarkCrossZoneId(cx, id);
+                """
+            )
+
+        indexedBody = getDeleterBody(self.descriptor, "Indexed")
+        if indexedBody is not None:
+            # Can't handle cross-origin objects here.
+            assert not self.descriptor.isMaybeCrossOriginObject()
+            delete += fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  bool deleteSucceeded;
+                  $*{indexedBody}
+                  return deleteSucceeded ? opresult.succeed() : opresult.failCantDelete();
+                }
+                """,
+                indexedBody=indexedBody,
+            )
+
+        namedBody = getDeleterBody(self.descriptor, "Named", foundVar="found")
+        if namedBody is not None:
+            delete += dedent(
+                """
+                // Try named delete only if the named property visibility
+                // algorithm says the property is visible.
+                bool tryNamedDelete = true;
+                { // Scope for expando
+                  JS::Rooted<JSObject*> expando(cx, DOMProxyHandler::GetExpandoObject(proxy));
+                  if (expando) {
+                    bool hasProp;
+                    if (!JS_HasPropertyById(cx, expando, id, &hasProp)) {
+                      return false;
+                    }
+                    tryNamedDelete = !hasProp;
+                  }
+                }
+                """
+            )
+
+            if not self.descriptor.interface.getExtendedAttribute(
+                "LegacyOverrideBuiltIns"
+            ):
+                delete += dedent(
+                    """
+                    if (tryNamedDelete) {
+                      bool hasOnProto;
+                      if (!HasPropertyOnPrototype(cx, proxy, id, &hasOnProto)) {
+                        return false;
+                      }
+                      tryNamedDelete = !hasOnProto;
+                    }
+                    """
+                )
+
+            # We always return above for an index id in the case when we support
+            # indexed properties, so we can just treat the id as a name
+            # unconditionally here.
+            delete += fill(
+                """
+                if (tryNamedDelete) {
+                  bool found = false;
+                  bool deleteSucceeded;
+                  $*{namedBody}
+                  if (found) {
+                    return deleteSucceeded ? opresult.succeed() : opresult.failCantDelete();
+                  }
+                }
+                """,
+                namedBody=namedBody,
+            )
+
+        delete += dedent(
+            """
+
+            return dom::DOMProxyHandler::delete_(cx, proxy, id, opresult);
+            """
+        )
+
+        return delete
+
+
+class CGDOMJSProxyHandler_ownPropNames(ClassMethod):
+    def __init__(
+        self,
+        descriptor,
+    ):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("unsigned", "flags"),
+            Argument("JS::MutableHandleVector<jsid>", "props"),
+        ]
+        ClassMethod.__init__(
+            self, "ownPropNames", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        if self.descriptor.isMaybeCrossOriginObject():
+            xrayDecl = dedent(
+                """
+                MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy));
+                if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+                  if (!(flags & JSITER_HIDDEN)) {
+                    // There are no enumerable cross-origin props, so we're done.
+                    return true;
+                  }
+
+                  JS::Rooted<JSObject*> holder(cx);
+                  if (!EnsureHolder(cx, proxy, &holder)) {
+                    return false;
+                  }
+
+                  if (!js::GetPropertyKeys(cx, holder, flags, props)) {
+                    return false;
+                  }
+
+                  return xpc::AppendCrossOriginWhitelistedPropNames(cx, props);
+                }
+
+                """
+            )
+            xrayCheck = ""
+        else:
+            xrayDecl = "bool isXray = xpc::WrapperFactory::IsXrayWrapper(proxy);\n"
+            xrayCheck = "!isXray &&"
+
+        # Per spec, we do indices, then named props, then everything else.
+        if self.descriptor.supportsIndexedProperties():
+            if self.descriptor.lengthNeedsCallerType():
+                callerType = callerTypeGetterForDescriptor(self.descriptor)
+            else:
+                callerType = ""
+            addIndices = fill(
+                """
+
+                uint32_t length = UnwrapProxy(proxy)->Length(${callerType});
+                MOZ_ASSERT(int32_t(length) >= 0);
+                for (int32_t i = 0; i < int32_t(length); ++i) {
+                  if (!props.append(JS::PropertyKey::Int(i))) {
+                    return false;
+                  }
+                }
+                """,
+                callerType=callerType,
+            )
+        else:
+            addIndices = ""
+
+        if self.descriptor.supportsNamedProperties():
+            if self.descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+                shadow = "!isXray"
+            else:
+                shadow = "false"
+
+            if self.descriptor.supportedNamesNeedCallerType():
+                callerType = ", " + callerTypeGetterForDescriptor(self.descriptor)
+            else:
+                callerType = ""
+
+            addNames = fill(
+                """
+                nsTArray<nsString> names;
+                UnwrapProxy(proxy)->GetSupportedNames(names${callerType});
+                if (!AppendNamedPropertyIds(cx, proxy, names, ${shadow}, props)) {
+                  return false;
+                }
+                """,
+                callerType=callerType,
+                shadow=shadow,
+            )
+            if not self.descriptor.namedPropertiesEnumerable:
+                addNames = CGIfWrapper(
+                    CGGeneric(addNames), "flags & JSITER_HIDDEN"
+                ).define()
+            addNames = "\n" + addNames
+        else:
+            addNames = ""
+
+        addExpandoProps = fill(
+            """
+            JS::Rooted<JSObject*> expando(cx);
+            if (${xrayCheck}(expando = DOMProxyHandler::GetExpandoObject(proxy)) &&
+                !js::GetPropertyKeys(cx, expando, flags, props)) {
+              return false;
+            }
+            """,
+            xrayCheck=xrayCheck,
+        )
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            # We need to enter our compartment (which we might not be
+            # in right now) to get the expando props.
+            addExpandoProps = fill(
+                """
+                {  // Scope for accessing the expando.
+                  // Safe to enter our compartment, because IsPlatformObjectSameOrigin tested true.
+                  JSAutoRealm ar(cx, proxy);
+                  $*{addExpandoProps}
+                }
+                for (auto& id : props) {
+                  JS_MarkCrossZoneId(cx, id);
+                }
+                """,
+                addExpandoProps=addExpandoProps,
+            )
+
+        return fill(
+            """
+            $*{xrayDecl}
+            $*{addIndices}
+            $*{addNames}
+
+            $*{addExpandoProps}
+
+            return true;
+            """,
+            xrayDecl=xrayDecl,
+            addIndices=addIndices,
+            addNames=addNames,
+            addExpandoProps=addExpandoProps,
+        )
+
+
+class CGDOMJSProxyHandler_hasOwn(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("bool*", "bp"),
+        ]
+        ClassMethod.__init__(
+            self, "hasOwn", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        if self.descriptor.isMaybeCrossOriginObject():
+            maybeCrossOrigin = dedent(
+                """
+                if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+                  // Just hand this off to BaseProxyHandler to do the slow-path thing.
+                  // The BaseProxyHandler code is OK with this happening without entering the
+                  // compartment of "proxy", which is important to get the right answers.
+                  return js::BaseProxyHandler::hasOwn(cx, proxy, id, bp);
+                }
+
+                // Now safe to enter the Realm of proxy and do the rest of the work there.
+                JSAutoRealm ar(cx, proxy);
+                JS_MarkCrossZoneId(cx, id);
+                """
+            )
+        else:
+            maybeCrossOrigin = ""
+
+        if self.descriptor.supportsIndexedProperties():
+            indexed = fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  bool found = false;
+                  $*{presenceChecker}
+
+                  *bp = found;
+                  return true;
+                }
+
+                """,
+                presenceChecker=CGProxyIndexedPresenceChecker(
+                    self.descriptor, foundVar="found"
+                ).define(),
+            )
+        else:
+            indexed = ""
+
+        if self.descriptor.supportsNamedProperties():
+            # If we support indexed properties we always return above for index
+            # property names, so no need to check for those here.
+            named = fill(
+                """
+                bool found = false;
+                $*{presenceChecker}
+
+                *bp = found;
+                """,
+                presenceChecker=CGProxyNamedPresenceChecker(
+                    self.descriptor, foundVar="found"
+                ).define(),
+            )
+            if not self.descriptor.interface.getExtendedAttribute(
+                "LegacyOverrideBuiltIns"
+            ):
+                named = fill(
+                    """
+                    bool hasOnProto;
+                    if (!HasPropertyOnPrototype(cx, proxy, id, &hasOnProto)) {
+                      return false;
+                    }
+                    if (!hasOnProto) {
+                      $*{protoLacksProperty}
+                      return true;
+                    }
+                    """,
+                    protoLacksProperty=named,
+                )
+                named += "*bp = false;\n"
+            else:
+                named += "\n"
+        else:
+            named = "*bp = false;\n"
+
+        missingPropUseCounters = missingPropUseCountersForDescriptor(self.descriptor)
+
+        return fill(
+            """
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
+                      "Should not have a XrayWrapper here");
+            $*{maybeCrossOrigin}
+            $*{indexed}
+
+            $*{missingPropUseCounters}
+            JS::Rooted<JSObject*> expando(cx, GetExpandoObject(proxy));
+            if (expando) {
+              bool b = true;
+              bool ok = JS_HasPropertyById(cx, expando, id, &b);
+              *bp = !!b;
+              if (!ok || *bp) {
+                return ok;
+              }
+            }
+
+            $*{named}
+            return true;
+            """,
+            maybeCrossOrigin=maybeCrossOrigin,
+            indexed=indexed,
+            missingPropUseCounters=missingPropUseCounters,
+            named=named,
+        )
+
+
+class CGDOMJSProxyHandler_get(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<JS::Value>", "receiver"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::MutableHandle<JS::Value>", "vp"),
+        ]
+        ClassMethod.__init__(
+            self, "get", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        missingPropUseCounters = missingPropUseCountersForDescriptor(self.descriptor)
+
+        getUnforgeableOrExpando = dedent(
+            """
+            bool expandoHasProp = false;
+            { // Scope for expando
+              JS::Rooted<JSObject*> expando(cx, DOMProxyHandler::GetExpandoObject(proxy));
+              if (expando) {
+                if (!JS_HasPropertyById(cx, expando, id, &expandoHasProp)) {
+                  return false;
+                }
+
+                if (expandoHasProp) {
+                  // Forward the get to the expando object, but our receiver is whatever our
+                  // receiver is.
+                  if (!JS_ForwardGetPropertyTo(cx, expando, id, ${receiver}, vp)) {
+                    return false;
+                  }
+                }
+              }
+            }
+            """
+        )
+
+        getOnPrototype = dedent(
+            """
+            bool foundOnPrototype;
+            if (!GetPropertyOnPrototype(cx, proxy, ${receiver}, id, &foundOnPrototype, vp)) {
+              return false;
+            }
+            """
+        )
+
+        if self.descriptor.isMaybeCrossOriginObject():
+            # We can't handle these for cross-origin objects
+            assert not self.descriptor.supportsIndexedProperties()
+            assert not self.descriptor.supportsNamedProperties()
+
+            return fill(
+                """
+                MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
+                            "Should not have a XrayWrapper here");
+
+                if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+                  return CrossOriginGet(cx, proxy, receiver, id, vp);
+                }
+
+                $*{missingPropUseCounters}
+                { // Scope for the JSAutoRealm accessing expando and prototype.
+                  JSAutoRealm ar(cx, proxy);
+                  JS::Rooted<JS::Value> wrappedReceiver(cx, receiver);
+                  if (!MaybeWrapValue(cx, &wrappedReceiver)) {
+                    return false;
+                  }
+                  JS_MarkCrossZoneId(cx, id);
+
+                  $*{getUnforgeableOrExpando}
+                  if (!expandoHasProp) {
+                    $*{getOnPrototype}
+                    if (!foundOnPrototype) {
+                      MOZ_ASSERT(vp.isUndefined());
+                      return true;
+                    }
+                  }
+                }
+
+                return MaybeWrapValue(cx, vp);
+                """,
+                missingPropUseCounters=missingPropUseCountersForDescriptor(
+                    self.descriptor
+                ),
+                getUnforgeableOrExpando=fill(
+                    getUnforgeableOrExpando, receiver="wrappedReceiver"
+                ),
+                getOnPrototype=fill(getOnPrototype, receiver="wrappedReceiver"),
+            )
+
+        templateValues = {"jsvalRef": "vp", "jsvalHandle": "vp", "obj": "proxy"}
+
+        getUnforgeableOrExpando = fill(
+            getUnforgeableOrExpando, receiver="receiver"
+        ) + dedent(
+            """
+
+            if (expandoHasProp) {
+              return true;
+            }
+            """
+        )
+        if self.descriptor.supportsIndexedProperties():
+            getIndexedOrExpando = fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  $*{callGetter}
+                  // Even if we don't have this index, we don't forward the
+                  // get on to our expando object.
+                } else {
+                  $*{getUnforgeableOrExpando}
+                }
+                """,
+                callGetter=CGProxyIndexedGetter(
+                    self.descriptor, templateValues
+                ).define(),
+                getUnforgeableOrExpando=getUnforgeableOrExpando,
+            )
+        else:
+            getIndexedOrExpando = getUnforgeableOrExpando
+
+        if self.descriptor.supportsNamedProperties():
+            getNamed = CGProxyNamedGetter(self.descriptor, templateValues)
+            if self.descriptor.supportsIndexedProperties():
+                getNamed = CGIfWrapper(getNamed, "!IsArrayIndex(index)")
+            getNamed = getNamed.define() + "\n"
+        else:
+            getNamed = ""
+
+        getOnPrototype = fill(getOnPrototype, receiver="receiver") + dedent(
+            """
+
+            if (foundOnPrototype) {
+              return true;
+            }
+
+            MOZ_ASSERT(vp.isUndefined());
+            """
+        )
+
+        if self.descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+            getNamedOrOnPrototype = getNamed + getOnPrototype
+        else:
+            getNamedOrOnPrototype = getOnPrototype + getNamed
+
+        return fill(
+            """
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
+                        "Should not have a XrayWrapper here");
+
+            $*{missingPropUseCounters}
+            $*{indexedOrExpando}
+
+            $*{namedOrOnPropotype}
+            return true;
+            """,
+            missingPropUseCounters=missingPropUseCounters,
+            indexedOrExpando=getIndexedOrExpando,
+            namedOrOnPropotype=getNamedOrOnPrototype,
+        )
+
+
+class CGDOMJSProxyHandler_setCustom(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx_"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::Handle<JS::Value>", "v"),
+            Argument("bool*", "done"),
+        ]
+        ClassMethod.__init__(
+            self, "setCustom", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        assertion = (
+            "MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),\n"
+            '           "Should not have a XrayWrapper here");\n'
+        )
+
+        # Correctness first. If we have a NamedSetter and [LegacyOverrideBuiltIns],
+        # always call the NamedSetter and never do anything else.
+        namedSetter = self.descriptor.operations["NamedSetter"]
+        if namedSetter is not None and self.descriptor.interface.getExtendedAttribute(
+            "LegacyOverrideBuiltIns"
+        ):
+            # Check assumptions.
+            if self.descriptor.supportsIndexedProperties():
+                raise ValueError(
+                    "In interface "
+                    + self.descriptor.name
+                    + ": "
+                    + "Can't cope with [LegacyOverrideBuiltIns] and an indexed getter"
+                )
+            if self.descriptor.hasLegacyUnforgeableMembers:
+                raise ValueError(
+                    "In interface "
+                    + self.descriptor.name
+                    + ": "
+                    + "Can't cope with [LegacyOverrideBuiltIns] and unforgeable members"
+                )
+
+            tailCode = dedent(
+                """
+                *done = true;
+                return true;
+                """
+            )
+            callSetter = CGProxyNamedSetter(
+                self.descriptor, tailCode, argumentHandleValue="v"
+            )
+            error_label = CGSpecializedMethod.error_reporting_label_helper(
+                self.descriptor, namedSetter, isConstructor=False
+            )
+            if error_label:
+                cxDecl = fill(
+                    """
+                    BindingCallContext cx(cx_, "${error_label}");
+                    """,
+                    error_label=error_label,
+                )
+            else:
+                cxDecl = dedent(
+                    """
+                    JSContext* cx = cx_;
+                    """
+                )
+            return fill(
+                """
+                $*{assertion}
+                $*{cxDecl}
+                $*{callSetter}
+                *done = false;
+                return true;
+                """,
+                assertion=assertion,
+                cxDecl=cxDecl,
+                callSetter=callSetter.define(),
+            )
+
+        # As an optimization, if we are going to call an IndexedSetter, go
+        # ahead and call it and have done.
+        indexedSetter = self.descriptor.operations["IndexedSetter"]
+        if indexedSetter is not None:
+            error_label = CGSpecializedMethod.error_reporting_label_helper(
+                self.descriptor, indexedSetter, isConstructor=False
+            )
+            if error_label:
+                cxDecl = fill(
+                    """
+                    BindingCallContext cx(cx_, "${error_label}");
+                    """,
+                    error_label=error_label,
+                )
+            else:
+                cxDecl = dedent(
+                    """
+                    JSContext* cx = cx_;
+                    """
+                )
+            setIndexed = fill(
+                """
+                uint32_t index = GetArrayIndexFromId(id);
+                if (IsArrayIndex(index)) {
+                  $*{cxDecl}
+                  $*{callSetter}
+                  *done = true;
+                  return true;
+                }
+
+                """,
+                cxDecl=cxDecl,
+                callSetter=CGProxyIndexedSetter(
+                    self.descriptor, argumentHandleValue="v"
+                ).define(),
+            )
+        else:
+            setIndexed = ""
+
+        return assertion + setIndexed + "*done = false;\n" "return true;\n"
+
+
+class CGDOMJSProxyHandler_className(ClassMethod):
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+        ]
+        ClassMethod.__init__(
+            self,
+            "className",
+            "const char*",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        if self.descriptor.isMaybeCrossOriginObject():
+            crossOrigin = dedent(
+                """
+                if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+                  return "Object";
+                }
+
+                """
+            )
+        else:
+            crossOrigin = ""
+        return fill(
+            """
+            $*{crossOrigin}
+            return "${name}";
+            """,
+            crossOrigin=crossOrigin,
+            name=self.descriptor.name,
+        )
+
+
+class CGDOMJSProxyHandler_finalizeInBackground(ClassMethod):
+    def __init__(self, descriptor):
+        args = [Argument("const JS::Value&", "priv")]
+        ClassMethod.__init__(
+            self,
+            "finalizeInBackground",
+            "bool",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return "return false;\n"
+
+
+class CGDOMJSProxyHandler_finalize(ClassMethod):
+    def __init__(self, descriptor):
+        args = [Argument("JS::GCContext*", "gcx"), Argument("JSObject*", "proxy")]
+        ClassMethod.__init__(
+            self, "finalize", "void", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return (
+            "%s* self = UnwrapPossiblyNotInitializedDOMObject<%s>(proxy);\n"
+            % (self.descriptor.nativeType, self.descriptor.nativeType)
+        ) + finalizeHook(
+            self.descriptor,
+            FINALIZE_HOOK_NAME,
+            self.args[0].name,
+            self.args[1].name,
+        ).define()
+
+
+class CGDOMJSProxyHandler_objectMoved(ClassMethod):
+    def __init__(self, descriptor):
+        args = [Argument("JSObject*", "obj"), Argument("JSObject*", "old")]
+        ClassMethod.__init__(
+            self, "objectMoved", "size_t", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return (
+            "%s* self = UnwrapPossiblyNotInitializedDOMObject<%s>(obj);\n"
+            % (self.descriptor.nativeType, self.descriptor.nativeType)
+        ) + objectMovedHook(
+            self.descriptor,
+            OBJECT_MOVED_HOOK_NAME,
+            self.args[0].name,
+            self.args[1].name,
+        )
+
+
+class CGDOMJSProxyHandler_getElements(ClassMethod):
+    def __init__(self, descriptor):
+        assert descriptor.supportsIndexedProperties()
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("uint32_t", "begin"),
+            Argument("uint32_t", "end"),
+            Argument("js::ElementAdder*", "adder"),
+        ]
+        ClassMethod.__init__(
+            self, "getElements", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        # Just like ownPropertyKeys we'll assume that we have no holes, so
+        # we have all properties from 0 to length.  If that ever changes
+        # (unlikely), we'll need to do something a bit more clever with how we
+        # forward on to our ancestor.
+
+        templateValues = {
+            "jsvalRef": "temp",
+            "jsvalHandle": "&temp",
+            "obj": "proxy",
+            "successCode": (
+                "if (!adder->append(cx, temp)) return false;\n" "continue;\n"
+            ),
+        }
+        get = CGProxyIndexedGetter(
+            self.descriptor, templateValues, False, False
+        ).define()
+
+        if self.descriptor.lengthNeedsCallerType():
+            callerType = callerTypeGetterForDescriptor(self.descriptor)
+        else:
+            callerType = ""
+
+        return fill(
+            """
+            JS::Rooted<JS::Value> temp(cx);
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy),
+                       "Should not have a XrayWrapper here");
+
+            ${nativeType}* self = UnwrapProxy(proxy);
+            uint32_t length = self->Length(${callerType});
+            // Compute the end of the indices we'll get ourselves
+            uint32_t ourEnd = std::max(begin, std::min(end, length));
+
+            for (uint32_t index = begin; index < ourEnd; ++index) {
+              $*{get}
+            }
+
+            if (end > ourEnd) {
+              JS::Rooted<JSObject*> proto(cx);
+              if (!js::GetObjectProto(cx, proxy, &proto)) {
+                return false;
+              }
+              return js::GetElementsWithAdder(cx, proto, proxy, ourEnd, end, adder);
+            }
+
+            return true;
+            """,
+            nativeType=self.descriptor.nativeType,
+            callerType=callerType,
+            get=get,
+        )
+
+
+class CGJSProxyHandler_getInstance(ClassMethod):
+    def __init__(self, type):
+        self.type = type
+        ClassMethod.__init__(
+            self, "getInstance", "const %s*" % self.type, [], static=True
+        )
+
+    def getBody(self):
+        return fill(
+            """
+            static const ${type} instance;
+            return &instance;
+            """,
+            type=self.type,
+        )
+
+
+class CGDOMJSProxyHandler_call(ClassMethod):
+    def __init__(self):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("const JS::CallArgs&", "args"),
+        ]
+
+        ClassMethod.__init__(
+            self, "call", "bool", args, virtual=True, override=True, const=True
+        )
+
+    def getBody(self):
+        return fill(
+            """
+            return js::ForwardToNative(cx, ${legacyCaller}, args);
+            """,
+            legacyCaller=LEGACYCALLER_HOOK_NAME,
+        )
+
+
+class CGDOMJSProxyHandler_isCallable(ClassMethod):
+    def __init__(self):
+        ClassMethod.__init__(
+            self,
+            "isCallable",
+            "bool",
+            [Argument("JSObject*", "obj")],
+            virtual=True,
+            override=True,
+            const=True,
+        )
+
+    def getBody(self):
+        return dedent(
+            """
+            return true;
+        """
+        )
+
+
+class CGDOMJSProxyHandler_canNurseryAllocate(ClassMethod):
+    """
+    Override the default canNurseryAllocate in BaseProxyHandler, for cases when
+    we should be nursery-allocated.
+    """
+
+    def __init__(self):
+        ClassMethod.__init__(
+            self,
+            "canNurseryAllocate",
+            "bool",
+            [],
+            virtual=True,
+            override=True,
+            const=True,
+        )
+
+    def getBody(self):
+        return dedent(
+            """
+            return true;
+        """
+        )
+
+
+class CGDOMJSProxyHandler_getOwnPropertyDescriptor(ClassMethod):
+    """
+    Implementation of getOwnPropertyDescriptor.  We only use this for
+    cross-origin objects.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.isMaybeCrossOriginObject()
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::MutableHandle<Maybe<JS::PropertyDescriptor>>", "desc"),
+        ]
+        ClassMethod.__init__(
+            self,
+            "getOwnPropertyDescriptor",
+            "bool",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return dedent(
+            """
+            // Implementation of <https://html.spec.whatwg.org/multipage/history.html#location-getownproperty>.
+            MOZ_ASSERT(!xpc::WrapperFactory::IsXrayWrapper(proxy));
+
+            // Step 1.
+            if (IsPlatformObjectSameOrigin(cx, proxy)) {
+              { // Scope so we can wrap our PropertyDescriptor back into
+                // the caller compartment.
+                // Enter the Realm of "proxy" so we can work with it.
+                JSAutoRealm ar(cx, proxy);
+
+                JS_MarkCrossZoneId(cx, id);
+
+                // The spec messes around with configurability of the returned
+                // descriptor here, but it's not clear what should actually happen
+                // here.  See <https://github.com/whatwg/html/issues/4157>.  For
+                // now, keep our old behavior and don't do any magic.
+                if (!dom::DOMProxyHandler::getOwnPropertyDescriptor(cx, proxy, id, desc)) {
+                  return false;
+                }
+              }
+              return JS_WrapPropertyDescriptor(cx, desc);
+            }
+
+            // Step 2.
+            if (!CrossOriginGetOwnPropertyHelper(cx, proxy, id, desc)) {
+              return false;
+            }
+
+            // Step 3.
+            if (desc.isSome()) {
+              return true;
+            }
+
+            // And step 4.
+            return CrossOriginPropertyFallback(cx, proxy, id, desc);
+            """
+        )
+
+
+class CGDOMJSProxyHandler_getSameOriginPrototype(ClassMethod):
+    """
+    Implementation of getSameOriginPrototype.  We only use this for
+    cross-origin objects.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.isMaybeCrossOriginObject()
+
+        args = [Argument("JSContext*", "cx")]
+        ClassMethod.__init__(
+            self,
+            "getSameOriginPrototype",
+            "JSObject*",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return dedent(
+            """
+            return GetProtoObjectHandle(cx);
+            """
+        )
+
+
+class CGDOMJSProxyHandler_definePropertySameOrigin(ClassMethod):
+    """
+    Implementation of definePropertySameOrigin.  We only use this for
+    cross-origin objects.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.isMaybeCrossOriginObject()
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::Handle<JS::PropertyDescriptor>", "desc"),
+            Argument("JS::ObjectOpResult&", "result"),
+        ]
+        ClassMethod.__init__(
+            self,
+            "definePropertySameOrigin",
+            "bool",
+            args,
+            virtual=True,
+            override=True,
+            const=True,
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return dedent(
+            """
+            return dom::DOMProxyHandler::defineProperty(cx, proxy, id, desc, result);
+            """
+        )
+
+
+class CGDOMJSProxyHandler_set(ClassMethod):
+    """
+    Implementation of set().  We only use this for cross-origin objects.
+    """
+
+    def __init__(self, descriptor):
+        assert descriptor.isMaybeCrossOriginObject()
+
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::Handle<jsid>", "id"),
+            Argument("JS::Handle<JS::Value>", "v"),
+            Argument("JS::Handle<JS::Value>", "receiver"),
+            Argument("JS::ObjectOpResult&", "result"),
+        ]
+        ClassMethod.__init__(
+            self, "set", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return dedent(
+            """
+            if (!IsPlatformObjectSameOrigin(cx, proxy)) {
+              return CrossOriginSet(cx, proxy, id, v, receiver, result);
+            }
+
+            // Safe to enter the Realm of proxy now, since it's same-origin with us.
+            JSAutoRealm ar(cx, proxy);
+            JS::Rooted<JS::Value> wrappedReceiver(cx, receiver);
+            if (!MaybeWrapValue(cx, &wrappedReceiver)) {
+              return false;
+            }
+
+            JS::Rooted<JS::Value> wrappedValue(cx, v);
+            if (!MaybeWrapValue(cx, &wrappedValue)) {
+              return false;
+            }
+
+            JS_MarkCrossZoneId(cx, id);
+
+            return dom::DOMProxyHandler::set(cx, proxy, id, wrappedValue, wrappedReceiver, result);
+            """
+        )
+
+
+class CGDOMJSProxyHandler_EnsureHolder(ClassMethod):
+    """
+    Implementation of EnsureHolder().  We only use this for cross-origin objects.
+    """
+
+    def __init__(self, descriptor):
+        args = [
+            Argument("JSContext*", "cx"),
+            Argument("JS::Handle<JSObject*>", "proxy"),
+            Argument("JS::MutableHandle<JSObject*>", "holder"),
+        ]
+        ClassMethod.__init__(
+            self, "EnsureHolder", "bool", args, virtual=True, override=True, const=True
+        )
+        self.descriptor = descriptor
+
+    def getBody(self):
+        return dedent(
+            """
+            return EnsureHolder(cx, proxy,
+                                JSCLASS_RESERVED_SLOTS(JS::GetClass(proxy)) - 1,
+                                sCrossOriginProperties, holder);
+            """
+        )
+
+
+class CGDOMJSProxyHandler(CGClass):
+    def __init__(self, descriptor):
+        assert (
+            descriptor.supportsIndexedProperties()
+            or descriptor.supportsNamedProperties()
+            or descriptor.isMaybeCrossOriginObject()
+        )
+        methods = [
+            CGDOMJSProxyHandler_getOwnPropDescriptor(descriptor),
+            CGDOMJSProxyHandler_defineProperty(descriptor),
+            ClassUsingDeclaration("mozilla::dom::DOMProxyHandler", "defineProperty"),
+            CGDOMJSProxyHandler_ownPropNames(descriptor),
+            CGDOMJSProxyHandler_hasOwn(descriptor),
+            CGDOMJSProxyHandler_get(descriptor),
+            CGDOMJSProxyHandler_className(descriptor),
+            CGDOMJSProxyHandler_finalizeInBackground(descriptor),
+            CGDOMJSProxyHandler_finalize(descriptor),
+            CGJSProxyHandler_getInstance("DOMProxyHandler"),
+            CGDOMJSProxyHandler_delete(descriptor),
+        ]
+        constructors = [
+            ClassConstructor([], constexpr=True, visibility="public", explicit=True)
+        ]
+
+        if descriptor.supportsIndexedProperties():
+            methods.append(CGDOMJSProxyHandler_getElements(descriptor))
+        if descriptor.operations["IndexedSetter"] is not None or (
+            descriptor.operations["NamedSetter"] is not None
+            and descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns")
+        ):
+            methods.append(CGDOMJSProxyHandler_setCustom(descriptor))
+        if descriptor.operations["LegacyCaller"]:
+            methods.append(CGDOMJSProxyHandler_call())
+            methods.append(CGDOMJSProxyHandler_isCallable())
+        if descriptor.interface.hasProbablyShortLivingWrapper():
+            if not descriptor.wrapperCache:
+                raise TypeError(
+                    "Need a wrapper cache to support nursery "
+                    "allocation of DOM objects"
+                )
+            methods.append(CGDOMJSProxyHandler_canNurseryAllocate())
+        if descriptor.wrapperCache:
+            methods.append(CGDOMJSProxyHandler_objectMoved(descriptor))
+
+        if descriptor.isMaybeCrossOriginObject():
+            methods.extend(
+                [
+                    CGDOMJSProxyHandler_getOwnPropertyDescriptor(descriptor),
+                    CGDOMJSProxyHandler_getSameOriginPrototype(descriptor),
+                    CGDOMJSProxyHandler_definePropertySameOrigin(descriptor),
+                    CGDOMJSProxyHandler_set(descriptor),
+                    CGDOMJSProxyHandler_EnsureHolder(descriptor),
+                    ClassUsingDeclaration(
+                        "MaybeCrossOriginObjectMixins", "EnsureHolder"
+                    ),
+                ]
+            )
+
+        if descriptor.interface.getExtendedAttribute("LegacyOverrideBuiltIns"):
+            assert not descriptor.isMaybeCrossOriginObject()
+            parentClass = "ShadowingDOMProxyHandler"
+        elif descriptor.isMaybeCrossOriginObject():
+            parentClass = "MaybeCrossOriginObject<mozilla::dom::DOMProxyHandler>"
+        else:
+            parentClass = "mozilla::dom::DOMProxyHandler"
+
+        CGClass.__init__(
+            self,
+            "DOMProxyHandler",
+            bases=[ClassBase(parentClass)],
+            constructors=constructors,
+            methods=methods,
+        )
+
+
+class CGDOMJSProxyHandlerDeclarer(CGThing):
+    """
+    A class for declaring a DOMProxyHandler.
+    """
+
+    def __init__(self, handlerThing):
+        self.handlerThing = handlerThing
+
+    def declare(self):
+        # Our class declaration should happen when we're defining
+        return ""
+
+    def define(self):
+        return self.handlerThing.declare()
+
+
+class CGDOMJSProxyHandlerDefiner(CGThing):
+    """
+    A class for defining a DOMProxyHandler.
+    """
+
+    def __init__(self, handlerThing):
+        self.handlerThing = handlerThing
+
+    def declare(self):
+        return ""
+
+    def define(self):
+        return self.handlerThing.define()
+
+
+def stripTrailingWhitespace(text):
+    tail = "\n" if text.endswith("\n") else ""
+    lines = text.splitlines()
+    return "\n".join(line.rstrip() for line in lines) + tail
+
+
+class MemberProperties:
+    def __init__(self):
+        self.isCrossOriginMethod = False
+        self.isCrossOriginGetter = False
+        self.isCrossOriginSetter = False
+
+
+def memberProperties(m, descriptor):
+    props = MemberProperties()
+    if m.isMethod():
+        if not m.isIdentifierLess() or m == descriptor.operations["Stringifier"]:
+            if not m.isStatic() and descriptor.interface.hasInterfacePrototypeObject():
+                if m.getExtendedAttribute("CrossOriginCallable"):
+                    props.isCrossOriginMethod = True
+    elif m.isAttr():
+        if not m.isStatic() and descriptor.interface.hasInterfacePrototypeObject():
+            if m.getExtendedAttribute("CrossOriginReadable"):
+                props.isCrossOriginGetter = True
+        if not m.readonly:
+            if not m.isStatic() and descriptor.interface.hasInterfacePrototypeObject():
+                if m.getExtendedAttribute("CrossOriginWritable"):
+                    props.isCrossOriginSetter = True
+        elif m.getExtendedAttribute("PutForwards"):
+            if m.getExtendedAttribute("CrossOriginWritable"):
+                props.isCrossOriginSetter = True
+        elif m.getExtendedAttribute("Replaceable") or m.getExtendedAttribute(
+            "LegacyLenientSetter"
+        ):
+            if m.getExtendedAttribute("CrossOriginWritable"):
+                props.isCrossOriginSetter = True
+
+    return props
+
+
+class CGDescriptor(CGThing):
+    def __init__(self, descriptor):
+        CGThing.__init__(self)
+
+        assert (
+            not descriptor.concrete
+            or descriptor.interface.hasInterfacePrototypeObject()
+        )
+
+        self._deps = descriptor.interface.getDeps()
+
+        iteratorCGThings = None
+        if (
+            descriptor.interface.isIterable()
+            and descriptor.interface.maplikeOrSetlikeOrIterable.isPairIterator()
+        ) or descriptor.interface.isAsyncIterable():
+            # We need the Wrap function when using the [Async]IterableIterator type, so we want to declare it before we need it. We don't really want to expose it in the header file, so we make it static too.
+            iteratorCGThings = []
+            itr_iface = (
+                descriptor.interface.maplikeOrSetlikeOrIterable.iteratorType.inner
+            )
+            iteratorDescriptor = descriptor.getDescriptor(itr_iface.identifier.name)
+            iteratorCGThings.append(
+                CGWrapNonWrapperCacheMethod(
+                    iteratorDescriptor, static=True, signatureOnly=True
+                )
+            )
+            iteratorCGThings = CGList(
+                (CGIndenter(t, declareOnly=True) for t in iteratorCGThings), "\n"
+            )
+            iteratorCGThings = CGWrapper(iteratorCGThings, pre="\n", post="\n")
+            iteratorCGThings = CGWrapper(
+                CGNamespace(
+                    toBindingNamespace(iteratorDescriptor.name), iteratorCGThings
+                ),
+                post="\n",
+            )
+
+        cgThings = []
+
+        isIteratorInterface = (
+            descriptor.interface.isIteratorInterface()
+            or descriptor.interface.isAsyncIteratorInterface()
+        )
+        if not isIteratorInterface:
+            cgThings.append(
+                CGGeneric(declare="typedef %s NativeType;\n" % descriptor.nativeType)
+            )
+            parent = descriptor.interface.parent
+            if parent:
+                cgThings.append(
+                    CGGeneric(
+                        "static_assert(IsRefcounted<NativeType>::value == IsRefcounted<%s::NativeType>::value,\n"
+                        '              "Can\'t inherit from an interface with a different ownership model.");\n'
+                        % toBindingNamespace(descriptor.parentPrototypeName)
+                    )
+                )
+
+        defaultToJSONMethod = None
+        needCrossOriginPropertyArrays = False
+        unscopableNames = list()
+        for n in descriptor.interface.legacyFactoryFunctions:
+            cgThings.append(
+                CGClassConstructor(descriptor, n, LegacyFactoryFunctionName(n))
+            )
+        for m in descriptor.interface.members:
+            if m.isMethod() and m.identifier.name == "QueryInterface":
+                continue
+
+            props = memberProperties(m, descriptor)
+
+            if m.isMethod():
+                if m.getExtendedAttribute("Unscopable"):
+                    assert not m.isStatic()
+                    unscopableNames.append(m.identifier.name)
+                if m.isDefaultToJSON():
+                    defaultToJSONMethod = m
+                elif (
+                    not m.isIdentifierLess()
+                    or m == descriptor.operations["Stringifier"]
+                ):
+                    if m.isStatic():
+                        assert descriptor.interface.hasInterfaceObject()
+                        cgThings.append(CGStaticMethod(descriptor, m))
+                        if m.returnsPromise():
+                            cgThings.append(CGStaticMethodJitinfo(m))
+                    elif descriptor.interface.hasInterfacePrototypeObject():
+                        specializedMethod = CGSpecializedMethod(descriptor, m)
+                        cgThings.append(specializedMethod)
+                        if m.returnsPromise():
+                            cgThings.append(
+                                CGMethodPromiseWrapper(descriptor, specializedMethod)
+                            )
+                        cgThings.append(CGMemberJITInfo(descriptor, m))
+                        if props.isCrossOriginMethod:
+                            needCrossOriginPropertyArrays = True
+            # If we've hit the maplike/setlike member itself, go ahead and
+            # generate its convenience functions.
+            elif m.isMaplikeOrSetlike():
+                cgThings.append(CGMaplikeOrSetlikeHelperGenerator(descriptor, m))
+            elif m.isAttr():
+                if m.type.isObservableArray():
+                    cgThings.append(
+                        CGObservableArrayProxyHandlerGenerator(descriptor, m)
+                    )
+                    cgThings.append(CGObservableArrayHelperGenerator(descriptor, m))
+                if m.getExtendedAttribute("Unscopable"):
+                    assert not m.isStatic()
+                    unscopableNames.append(m.identifier.name)
+                if m.isStatic():
+                    assert descriptor.interface.hasInterfaceObject()
+                    cgThings.append(CGStaticGetter(descriptor, m))
+                elif descriptor.interface.hasInterfacePrototypeObject():
+                    specializedGetter = CGSpecializedGetter(descriptor, m)
+                    cgThings.append(specializedGetter)
+                    if m.type.isPromise():
+                        cgThings.append(
+                            CGGetterPromiseWrapper(descriptor, specializedGetter)
+                        )
+                    if props.isCrossOriginGetter:
+                        needCrossOriginPropertyArrays = True
+                if not m.readonly:
+                    if m.isStatic():
+                        assert descriptor.interface.hasInterfaceObject()
+                        cgThings.append(CGStaticSetter(descriptor, m))
+                    elif descriptor.interface.hasInterfacePrototypeObject():
+                        cgThings.append(CGSpecializedSetter(descriptor, m))
+                        if props.isCrossOriginSetter:
+                            needCrossOriginPropertyArrays = True
+                elif m.getExtendedAttribute("PutForwards"):
+                    cgThings.append(CGSpecializedForwardingSetter(descriptor, m))
+                    if props.isCrossOriginSetter:
+                        needCrossOriginPropertyArrays = True
+                elif m.getExtendedAttribute("Replaceable"):
+                    cgThings.append(CGSpecializedReplaceableSetter(descriptor, m))
+                elif m.getExtendedAttribute("LegacyLenientSetter"):
+                    # XXX In this case, we need to add an include for mozilla/dom/Document.h to the generated cpp file.
+                    cgThings.append(CGSpecializedLenientSetter(descriptor, m))
+                if (
+                    not m.isStatic()
+                    and descriptor.interface.hasInterfacePrototypeObject()
+                ):
+                    cgThings.append(CGMemberJITInfo(descriptor, m))
+            if m.isConst() and m.type.isPrimitive():
+                cgThings.append(CGConstDefinition(m))
+
+        if defaultToJSONMethod:
+            cgThings.append(CGDefaultToJSONMethod(descriptor, defaultToJSONMethod))
+            cgThings.append(CGMemberJITInfo(descriptor, defaultToJSONMethod))
+
+        if descriptor.concrete and not descriptor.proxy:
+            if wantsAddProperty(descriptor):
+                cgThings.append(CGAddPropertyHook(descriptor))
+
+            # Always have a finalize hook, regardless of whether the class
+            # wants a custom hook.
+            cgThings.append(CGClassFinalizeHook(descriptor))
+
+        if wantsGetWrapperCache(descriptor):
+            cgThings.append(CGGetWrapperCacheHook(descriptor))
+
+        if descriptor.concrete and descriptor.wrapperCache and not descriptor.proxy:
+            cgThings.append(CGClassObjectMovedHook(descriptor))
+
+        properties = PropertyArrays(descriptor)
+        cgThings.append(CGGeneric(define=str(properties)))
+        cgThings.append(CGNativeProperties(descriptor, properties))
+
+        if defaultToJSONMethod:
+            # Now that we know about our property arrays, we can
+            # output our "collect attribute values" method, which uses those.
+            cgThings.append(
+                CGCollectJSONAttributesMethod(descriptor, defaultToJSONMethod)
+            )
+
+        # Declare our DOMProxyHandler.
+        if descriptor.concrete and descriptor.proxy:
+            cgThings.append(
+                CGGeneric(
+                    fill(
+                        """
+                        static_assert(std::is_base_of_v<nsISupports, ${nativeType}>,
+                                      "We don't support non-nsISupports native classes for "
+                                      "proxy-based bindings yet");
+
+                        """,
+                        nativeType=descriptor.nativeType,
+                    )
+                )
+            )
+            if not descriptor.wrapperCache:
+                raise TypeError(
+                    "We need a wrappercache to support expandos for proxy-based "
+                    "bindings (" + descriptor.name + ")"
+                )
+            handlerThing = CGDOMJSProxyHandler(descriptor)
+            cgThings.append(CGDOMJSProxyHandlerDeclarer(handlerThing))
+            cgThings.append(CGProxyIsProxy(descriptor))
+            cgThings.append(CGProxyUnwrap(descriptor))
+
+        # Set up our Xray callbacks as needed.  This needs to come
+        # after we have our DOMProxyHandler defined.
+        if descriptor.wantsXrays:
+            if descriptor.concrete and descriptor.proxy:
+                if descriptor.needsXrayNamedDeleterHook():
+                    cgThings.append(CGDeleteNamedProperty(descriptor))
+            elif descriptor.needsXrayResolveHooks():
+                cgThings.append(CGResolveOwnPropertyViaResolve(descriptor))
+                cgThings.append(
+                    CGEnumerateOwnPropertiesViaGetOwnPropertyNames(descriptor)
+                )
+            if descriptor.wantsXrayExpandoClass:
+                cgThings.append(CGXrayExpandoJSClass(descriptor))
+
+            # Now that we have our ResolveOwnProperty/EnumerateOwnProperties stuff
+            # done, set up our NativePropertyHooks.
+            cgThings.append(CGNativePropertyHooks(descriptor, properties))
+
+        if descriptor.interface.hasInterfaceObject():
+            cgThings.append(CGClassConstructor(descriptor, descriptor.interface.ctor()))
+            cgThings.append(CGInterfaceObjectJSClass(descriptor, properties))
+            cgThings.append(CGLegacyFactoryFunctions(descriptor))
+
+        cgThings.append(CGLegacyCallHook(descriptor))
+        if descriptor.interface.getExtendedAttribute("NeedResolve"):
+            cgThings.append(CGResolveHook(descriptor))
+            cgThings.append(CGMayResolveHook(descriptor))
+            cgThings.append(CGEnumerateHook(descriptor))
+
+        if descriptor.hasNamedPropertiesObject:
+            cgThings.append(CGGetNamedPropertiesObjectMethod(descriptor))
+
+        if descriptor.interface.hasInterfacePrototypeObject():
+            cgThings.append(CGPrototypeJSClass(descriptor, properties))
+
+        if (
+            descriptor.interface.hasInterfaceObject()
+            and not descriptor.interface.isExternal()
+            and descriptor.isExposedConditionally()
+        ):
+            cgThings.append(CGConstructorEnabled(descriptor))
+
+        if (
+            descriptor.interface.hasMembersInSlots()
+            and descriptor.interface.hasChildInterfaces()
+        ):
+            raise TypeError(
+                "We don't support members in slots on "
+                "non-leaf interfaces like %s" % descriptor.interface.identifier.name
+            )
+
+        if descriptor.needsMissingPropUseCounters:
+            cgThings.append(CGCountMaybeMissingProperty(descriptor))
+
+        if descriptor.concrete:
+            if descriptor.interface.isSerializable():
+                cgThings.append(CGSerializer(descriptor))
+                cgThings.append(CGDeserializer(descriptor))
+
+            # CGDOMProxyJSClass/CGDOMJSClass need GetProtoObjectHandle, but we don't want to export it for the iterator interfaces, so declare it here.
+            if isIteratorInterface:
+                cgThings.append(
+                    CGGetProtoObjectHandleMethod(
+                        descriptor, static=True, signatureOnly=True
+                    )
+                )
+
+            if descriptor.proxy:
+                cgThings.append(CGDOMJSProxyHandlerDefiner(handlerThing))
+                cgThings.append(CGDOMProxyJSClass(descriptor))
+            else:
+                cgThings.append(CGDOMJSClass(descriptor))
+
+            if descriptor.interface.hasMembersInSlots():
+                cgThings.append(CGUpdateMemberSlotsMethod(descriptor))
+
+            if descriptor.isGlobal():
+                assert descriptor.wrapperCache
+                cgThings.append(CGWrapGlobalMethod(descriptor, properties))
+            elif descriptor.wrapperCache:
+                cgThings.append(CGWrapWithCacheMethod(descriptor))
+                cgThings.append(CGWrapMethod(descriptor))
+            else:
+                cgThings.append(
+                    CGWrapNonWrapperCacheMethod(descriptor, static=isIteratorInterface)
+                )
+
+        # If we're not wrappercached, we don't know how to clear our
+        # cached values, since we can't get at the JSObject.
+        if descriptor.wrapperCache:
+            cgThings.extend(
+                CGClearCachedValueMethod(descriptor, m)
+                for m in clearableCachedAttrs(descriptor)
+            )
+
+        haveUnscopables = (
+            len(unscopableNames) != 0
+            and descriptor.interface.hasInterfacePrototypeObject()
+        )
+        if haveUnscopables:
+            cgThings.append(
+                CGList(
+                    [
+                        CGGeneric("static const char* const unscopableNames[] = {"),
+                        CGIndenter(
+                            CGList(
+                                [CGGeneric('"%s"' % name) for name in unscopableNames]
+                                + [CGGeneric("nullptr")],
+                                ",\n",
+                            )
+                        ),
+                        CGGeneric("};\n"),
+                    ],
+                    "\n",
+                )
+            )
+
+        legacyWindowAliases = descriptor.interface.legacyWindowAliases
+        haveLegacyWindowAliases = len(legacyWindowAliases) != 0
+        if haveLegacyWindowAliases:
+            cgThings.append(
+                CGList(
+                    [
+                        CGGeneric("static const char* const legacyWindowAliases[] = {"),
+                        CGIndenter(
+                            CGList(
+                                [
+                                    CGGeneric('"%s"' % name)
+                                    for name in legacyWindowAliases
+                                ]
+                                + [CGGeneric("nullptr")],
+                                ",\n",
+                            )
+                        ),
+                        CGGeneric("};\n"),
+                    ],
+                    "\n",
+                )
+            )
+
+        # CGCreateInterfaceObjectsMethod needs to come after our
+        # CGDOMJSClass and unscopables, if any.
+        cgThings.append(
+            CGCreateInterfaceObjectsMethod(
+                descriptor,
+                properties,
+                haveUnscopables,
+                haveLegacyWindowAliases,
+                static=isIteratorInterface,
+            )
+        )
+
+        # CGGetProtoObjectMethod and CGGetConstructorObjectMethod need
+        # to come after CGCreateInterfaceObjectsMethod.
+        if (
+            descriptor.interface.hasInterfacePrototypeObject()
+            and not descriptor.hasOrdinaryObjectPrototype
+        ):
+            cgThings.append(
+                CGGetProtoObjectHandleMethod(descriptor, static=isIteratorInterface)
+            )
+            if descriptor.interface.hasChildInterfaces():
+                assert not isIteratorInterface
+                cgThings.append(CGGetProtoObjectMethod(descriptor))
+        if descriptor.interface.hasInterfaceObject():
+            cgThings.append(CGGetConstructorObjectHandleMethod(descriptor))
+            cgThings.append(CGGetConstructorObjectMethod(descriptor))
+
+        # See whether we need to generate cross-origin property arrays.
+        if needCrossOriginPropertyArrays:
+            cgThings.append(CGCrossOriginProperties(descriptor))
+
+        cgThings = CGList((CGIndenter(t, declareOnly=True) for t in cgThings), "\n")
+        cgThings = CGWrapper(cgThings, pre="\n", post="\n")
+        cgThings = CGWrapper(
+            CGNamespace(toBindingNamespace(descriptor.name), cgThings), post="\n"
+        )
+        self.cgRoot = CGList([iteratorCGThings, cgThings], "\n")
+
+    def declare(self):
+        return self.cgRoot.declare()
+
+    def define(self):
+        return self.cgRoot.define()
+
+    def deps(self):
+        return self._deps
+
+
+class CGNamespacedEnum(CGThing):
+    def __init__(self, namespace, enumName, names, values, comment=""):
+
+        if not values:
+            values = []
+
+        # Account for explicit enum values.
+        entries = []
+        for i in range(0, len(names)):
+            if len(values) > i and values[i] is not None:
+                entry = "%s = %s" % (names[i], values[i])
+            else:
+                entry = names[i]
+            entries.append(entry)
+
+        # Append a Count.
+        entries.append("_" + enumName + "_Count")
+
+        # Indent.
+        entries = ["  " + e for e in entries]
+
+        # Build the enum body.
+        enumstr = comment + "enum %s : uint16_t\n{\n%s\n};\n" % (
+            enumName,
+            ",\n".join(entries),
+        )
+        curr = CGGeneric(declare=enumstr)
+
+        # Add some whitespace padding.
+        curr = CGWrapper(curr, pre="\n", post="\n")
+
+        # Add the namespace.
+        curr = CGNamespace(namespace, curr)
+
+        # Add the typedef
+        typedef = "\ntypedef %s::%s %s;\n\n" % (namespace, enumName, enumName)
+        curr = CGList([curr, CGGeneric(declare=typedef)])
+
+        # Save the result.
+        self.node = curr
+
+    def declare(self):
+        return self.node.declare()
+
+    def define(self):
+        return ""
+
+
+def initIdsClassMethod(identifiers, atomCacheName):
+    idinit = [
+        '!atomsCache->%s.init(cx, "%s")' % (CGDictionary.makeIdName(id), id)
+        for id in identifiers
+    ]
+    idinit.reverse()
+    body = fill(
+        """
+        MOZ_ASSERT(reinterpret_cast<jsid*>(atomsCache)->isVoid());
+
+        // Initialize these in reverse order so that any failure leaves the first one
+        // uninitialized.
+        if (${idinit}) {
+          return false;
+        }
+        return true;
+        """,
+        idinit=" ||\n    ".join(idinit),
+    )
+    return ClassMethod(
+        "InitIds",
+        "bool",
+        [Argument("JSContext*", "cx"), Argument("%s*" % atomCacheName, "atomsCache")],
+        static=True,
+        body=body,
+        visibility="private",
+    )
+
+
+class CGDictionary(CGThing):
+    def __init__(self, dictionary, descriptorProvider):
+        self.dictionary = dictionary
+        self.descriptorProvider = descriptorProvider
+        self.needToInitIds = len(dictionary.members) > 0
+        self.memberInfo = [
+            (
+                member,
+                getJSToNativeConversionInfo(
+                    member.type,
+                    descriptorProvider,
+                    isMember="Dictionary",
+                    isOptional=member.canHaveMissingValue(),
+                    isKnownMissing=not dictionary.needsConversionFromJS,
+                    defaultValue=member.defaultValue,
+                    sourceDescription=self.getMemberSourceDescription(member),
+                ),
+            )
+            for member in dictionary.members
+        ]
+
+        # If we have a union member which is going to be declared in a different
+        # header but contains something that will be declared in the same header
+        # as us, bail: the C++ includes won't work out.
+        for member in dictionary.members:
+            type = member.type.unroll()
+            if type.isUnion() and CGHeaders.getUnionDeclarationFilename(
+                descriptorProvider.getConfig(), type
+            ) != CGHeaders.getDeclarationFilename(dictionary):
+                for t in type.flatMemberTypes:
+                    if t.isDictionary() and CGHeaders.getDeclarationFilename(
+                        t.inner
+                    ) == CGHeaders.getDeclarationFilename(dictionary):
+                        raise TypeError(
+                            "Dictionary contains a union that will live in a different "
+                            "header that contains a dictionary from the same header as "
+                            "the original dictionary.  This won't compile.  Move the "
+                            "inner dictionary to a different Web IDL file to move it "
+                            "to a different header.\n%s\n%s"
+                            % (t.location, t.inner.location)
+                        )
+        self.structs = self.getStructs()
+
+    def declare(self):
+        return self.structs.declare()
+
+    def define(self):
+        return self.structs.define()
+
+    def base(self):
+        if self.dictionary.parent:
+            return self.makeClassName(self.dictionary.parent)
+        return "DictionaryBase"
+
+    def initMethod(self):
+        """
+        This function outputs the body of the Init() method for the dictionary.
+
+        For the most part, this is some bookkeeping for our atoms so
+        we can avoid atomizing strings all the time, then we just spit
+        out the getMemberConversion() output for each member,
+        separated by newlines.
+
+        """
+        body = dedent(
+            """
+            // Passing a null JSContext is OK only if we're initing from null,
+            // Since in that case we will not have to do any property gets
+            // Also evaluate isNullOrUndefined in order to avoid false-positive
+            // checkers by static analysis tools
+            MOZ_ASSERT_IF(!cx, val.isNull() && val.isNullOrUndefined());
+            """
+        )
+
+        if self.needToInitIds:
+            body += fill(
+                """
+                ${dictName}Atoms* atomsCache = nullptr;
+                if (cx) {
+                  atomsCache = GetAtomCache<${dictName}Atoms>(cx);
+                  if (reinterpret_cast<jsid*>(atomsCache)->isVoid() &&
+                      !InitIds(cx, atomsCache)) {
+                    return false;
+                  }
+                }
+
+                """,
+                dictName=self.makeClassName(self.dictionary),
+            )
+
+        if self.dictionary.parent:
+            body += fill(
+                """
+                // Per spec, we init the parent's members first
+                if (!${dictName}::Init(cx, val)) {
+                  return false;
+                }
+
+                """,
+                dictName=self.makeClassName(self.dictionary.parent),
+            )
+        else:
+            body += dedent(
+                """
+                if (!IsConvertibleToDictionary(val)) {
+                  return cx.ThrowErrorMessage<MSG_CONVERSION_ERROR>(sourceDescription, "dictionary");
+                }
+
+                """
+            )
+
+        memberInits = [self.getMemberConversion(m).define() for m in self.memberInfo]
+        if memberInits:
+            body += fill(
+                """
+                bool isNull = val.isNullOrUndefined();
+                // We only need these if !isNull, in which case we have |cx|.
+                Maybe<JS::Rooted<JSObject *> > object;
+                Maybe<JS::Rooted<JS::Value> > temp;
+                if (!isNull) {
+                  MOZ_ASSERT(cx);
+                  object.emplace(cx, &val.toObject());
+                  temp.emplace(cx);
+                }
+                $*{memberInits}
+                """,
+                memberInits="\n".join(memberInits),
+            )
+
+        body += "return true;\n"
+
+        return ClassMethod(
+            "Init",
+            "bool",
+            [
+                Argument("BindingCallContext&", "cx"),
+                Argument("JS::Handle<JS::Value>", "val"),
+                Argument("const char*", "sourceDescription", default='"Value"'),
+                Argument("bool", "passedToJSImpl", default="false"),
+            ],
+            body=body,
+        )
+
+    def initWithoutCallContextMethod(self):
+        """
+        This function outputs the body of an Init() method for the dictionary
+        that takes just a JSContext*.  This is needed for non-binding consumers.
+        """
+        body = dedent(
+            """
+            // We don't want to use sourceDescription for our context here;
+            // that's not really what it's formatted for.
+            BindingCallContext cx(cx_, nullptr);
+            return Init(cx, val, sourceDescription, passedToJSImpl);
+            """
+        )
+        return ClassMethod(
+            "Init",
+            "bool",
+            [
+                Argument("JSContext*", "cx_"),
+                Argument("JS::Handle<JS::Value>", "val"),
+                Argument("const char*", "sourceDescription", default='"Value"'),
+                Argument("bool", "passedToJSImpl", default="false"),
+            ],
+            body=body,
+        )
+
+    def simpleInitMethod(self):
+        """
+        This function outputs the body of the Init() method for the dictionary,
+        for cases when we are just default-initializing it.
+
+        """
+        relevantMembers = [
+            m
+            for m in self.memberInfo
+            # We only need to init the things that can have
+            # default values.
+            if m[0].optional and m[0].defaultValue
+        ]
+
+        # We mostly avoid outputting code that uses cx in our native-to-JS
+        # conversions, but there is one exception: we may have a
+        # dictionary-typed member that _does_ generally support conversion from
+        # JS.  If we have such a thing, we can pass it a null JSContext and
+        # JS::NullHandleValue to default-initialize it, but since the
+        # native-to-JS templates hardcode `cx` as the JSContext value, we're
+        # going to need to provide that.
+        haveMemberThatNeedsCx = any(
+            m[0].type.isDictionary() and m[0].type.unroll().inner.needsConversionFromJS
+            for m in relevantMembers
+        )
+        if haveMemberThatNeedsCx:
+            body = dedent(
+                """
+                JSContext* cx = nullptr;
+                """
+            )
+        else:
+            body = ""
+
+        if self.dictionary.parent:
+            if self.dictionary.parent.needsConversionFromJS:
+                args = "nullptr, JS::NullHandleValue"
+            else:
+                args = ""
+            body += fill(
+                """
+                // We init the parent's members first
+                if (!${dictName}::Init(${args})) {
+                  return false;
+                }
+
+                """,
+                dictName=self.makeClassName(self.dictionary.parent),
+                args=args,
+            )
+
+        memberInits = [
+            self.getMemberConversion(m, isKnownMissing=True).define()
+            for m in relevantMembers
+        ]
+        if memberInits:
+            body += fill(
+                """
+                $*{memberInits}
+                """,
+                memberInits="\n".join(memberInits),
+            )
+
+        body += "return true;\n"
+
+        return ClassMethod(
+            "Init",
+            "bool",
+            [
+                Argument("const char*", "sourceDescription", default='"Value"'),
+                Argument("bool", "passedToJSImpl", default="false"),
+            ],
+            body=body,
+        )
+
+    def initFromJSONMethod(self):
+        return ClassMethod(
+            "Init",
+            "bool",
+            [Argument("const nsAString&", "aJSON")],
+            body=dedent(
+                """
+                AutoJSAPI jsapi;
+                JSObject* cleanGlobal = SimpleGlobalObject::Create(SimpleGlobalObject::GlobalType::BindingDetail);
+                if (!cleanGlobal) {
+                  return false;
+                }
+                if (!jsapi.Init(cleanGlobal)) {
+                  return false;
+                }
+                JSContext* cx = jsapi.cx();
+                JS::Rooted<JS::Value> json(cx);
+                bool ok = ParseJSON(cx, aJSON, &json);
+                NS_ENSURE_TRUE(ok, false);
+                return Init(cx, json);
+                """
+            ),
+        )
+
+    def toJSONMethod(self):
+        return ClassMethod(
+            "ToJSON",
+            "bool",
+            [Argument("nsAString&", "aJSON")],
+            body=dedent(
+                """
+                AutoJSAPI jsapi;
+                jsapi.Init();
+                JSContext *cx = jsapi.cx();
+                // It's safe to use UnprivilegedJunkScopeOrWorkerGlobal here
+                // because we'll only be creating objects, in ways that have no
+                // side-effects, followed by a call to JS::ToJSONMaybeSafely,
+                // which likewise guarantees no side-effects for the sorts of
+                // things we will pass it.
+                JSObject* scope = UnprivilegedJunkScopeOrWorkerGlobal(fallible);
+                if (!scope) {
+                  JS_ReportOutOfMemory(cx);
+                  return false;
+                }
+                JSAutoRealm ar(cx, scope);
+                JS::Rooted<JS::Value> val(cx);
+                if (!ToObjectInternal(cx, &val)) {
+                  return false;
+                }
+                JS::Rooted<JSObject*> obj(cx, &val.toObject());
+                return StringifyToJSON(cx, obj, aJSON);
+            """
+            ),
+            const=True,
+        )
+
+    def toObjectInternalMethod(self):
+        body = ""
+        if self.needToInitIds:
+            body += fill(
+                """
+                ${dictName}Atoms* atomsCache = GetAtomCache<${dictName}Atoms>(cx);
+                if (reinterpret_cast<jsid*>(atomsCache)->isVoid() &&
+                    !InitIds(cx, atomsCache)) {
+                  return false;
+                }
+
+                """,
+                dictName=self.makeClassName(self.dictionary),
+            )
+
+        if self.dictionary.parent:
+            body += fill(
+                """
+                // Per spec, we define the parent's members first
+                if (!${dictName}::ToObjectInternal(cx, rval)) {
+                  return false;
+                }
+                JS::Rooted<JSObject*> obj(cx, &rval.toObject());
+
+                """,
+                dictName=self.makeClassName(self.dictionary.parent),
+            )
+        else:
+            body += dedent(
+                """
+                JS::Rooted<JSObject*> obj(cx, JS_NewPlainObject(cx));
+                if (!obj) {
+                  return false;
+                }
+                rval.set(JS::ObjectValue(*obj));
+
+                """
+            )
+
+        if self.memberInfo:
+            body += "\n".join(
+                self.getMemberDefinition(m).define() for m in self.memberInfo
+            )
+        body += "\nreturn true;\n"
+
+        return ClassMethod(
+            "ToObjectInternal",
+            "bool",
+            [
+                Argument("JSContext*", "cx"),
+                Argument("JS::MutableHandle<JS::Value>", "rval"),
+            ],
+            const=True,
+            body=body,
+        )
+
+    def initIdsMethod(self):
+        assert self.needToInitIds
+        return initIdsClassMethod(
+            [m.identifier.name for m in self.dictionary.members],
+            "%sAtoms" % self.makeClassName(self.dictionary),
+        )
+
+    def traceDictionaryMethod(self):
+        body = ""
+        if self.dictionary.parent:
+            cls = self.makeClassName(self.dictionary.parent)
+            body += "%s::TraceDictionary(trc);\n" % cls
+
+        memberTraces = [
+            self.getMemberTrace(m)
+            for m in self.dictionary.members
+            if typeNeedsRooting(m.type)
+        ]
+
+        if memberTraces:
+            body += "\n".join(memberTraces)
+
+        return ClassMethod(
+            "TraceDictionary",
+            "void",
+            [
+                Argument("JSTracer*", "trc"),
+            ],
+            body=body,
+        )
+
+    @staticmethod
+    def dictionaryNeedsCycleCollection(dictionary):
+        return any(idlTypeNeedsCycleCollection(m.type) for m in dictionary.members) or (
+            dictionary.parent
+            and CGDictionary.dictionaryNeedsCycleCollection(dictionary.parent)
+        )
+
+    def traverseForCCMethod(self):
+        body = ""
+        if self.dictionary.parent and self.dictionaryNeedsCycleCollection(
+            self.dictionary.parent
+        ):
+            cls = self.makeClassName(self.dictionary.parent)
+            body += "%s::TraverseForCC(aCallback, aFlags);\n" % cls
+
+        for m, _ in self.memberInfo:
+            if idlTypeNeedsCycleCollection(m.type):
+                memberName = self.makeMemberName(m.identifier.name)
+                body += (
+                    'ImplCycleCollectionTraverse(aCallback, %s, "%s", aFlags);\n'
+                    % (memberName, memberName)
+                )
+
+        return ClassMethod(
+            "TraverseForCC",
+            "void",
+            [
+                Argument("nsCycleCollectionTraversalCallback&", "aCallback"),
+                Argument("uint32_t", "aFlags"),
+            ],
+            body=body,
+            # Inline so we don't pay a codesize hit unless someone actually uses
+            # this traverse method.
+            inline=True,
+            bodyInHeader=True,
+        )
+
+    def unlinkForCCMethod(self):
+        body = ""
+        if self.dictionary.parent and self.dictionaryNeedsCycleCollection(
+            self.dictionary.parent
+        ):
+            cls = self.makeClassName(self.dictionary.parent)
+            body += "%s::UnlinkForCC();\n" % cls
+
+        for m, _ in self.memberInfo:
+            if idlTypeNeedsCycleCollection(m.type):
+                memberName = self.makeMemberName(m.identifier.name)
+                body += "ImplCycleCollectionUnlink(%s);\n" % memberName
+
+        return ClassMethod(
+            "UnlinkForCC",
+            "void",
+            [],
+            body=body,
+            # Inline so we don't pay a codesize hit unless someone actually uses
+            # this unlink method.
+            inline=True,
+            bodyInHeader=True,
+        )
+
+    def assignmentOperator(self):
+        body = CGList([])
+        body.append(CGGeneric("%s::operator=(aOther);\n" % self.base()))
+
+        for m, _ in self.memberInfo:
+            memberName = self.makeMemberName(m.identifier.name)
+            if m.canHaveMissingValue():
+                memberAssign = CGGeneric(
+                    fill(
+                        """
+                    ${name}.Reset();
+                    if (aOther.${name}.WasPassed()) {
+                      ${name}.Construct(aOther.${name}.Value());
+                    }
+                    """,
+                        name=memberName,
+                    )
+                )
+            else:
+                memberAssign = CGGeneric("%s = aOther.%s;\n" % (memberName, memberName))
+            body.append(memberAssign)
+        body.append(CGGeneric("return *this;\n"))
+        return ClassMethod(
+            "operator=",
+            "%s&" % self.makeClassName(self.dictionary),
+            [Argument("const %s&" % self.makeClassName(self.dictionary), "aOther")],
+            body=body.define(),
+        )
+
+    def canHaveEqualsOperator(self):
+        return all(
+            m.type.isString() or m.type.isPrimitive() for (m, _) in self.memberInfo
+        )
+
+    def equalsOperator(self):
+        body = CGList([])
+
+        for m, _ in self.memberInfo:
+            memberName = self.makeMemberName(m.identifier.name)
+            memberTest = CGGeneric(
+                fill(
+                    """
+                if (${memberName} != aOther.${memberName}) {
+                    return false;
+                }
+                """,
+                    memberName=memberName,
+                )
+            )
+            body.append(memberTest)
+        body.append(CGGeneric("return true;\n"))
+        return ClassMethod(
+            "operator==",
+            "bool",
+            [Argument("const %s&" % self.makeClassName(self.dictionary), "aOther")],
+            const=True,
+            body=body.define(),
+        )
+
+    def getStructs(self):
+        d = self.dictionary
+        selfName = self.makeClassName(d)
+        members = [
+            ClassMember(
+                self.makeMemberName(m[0].identifier.name),
+                self.getMemberType(m),
+                visibility="public",
+                body=self.getMemberInitializer(m),
+                hasIgnoreInitCheckFlag=True,
+            )
+            for m in self.memberInfo
+        ]
+        if d.parent:
+            # We always want to init our parent with our non-initializing
+            # constructor arg, because either we're about to init ourselves (and
+            # hence our parent) or we don't want any init happening.
+            baseConstructors = [
+                "%s(%s)"
+                % (self.makeClassName(d.parent), self.getNonInitializingCtorArg())
+            ]
+        else:
+            baseConstructors = None
+
+        if d.needsConversionFromJS:
+            initArgs = "nullptr, JS::NullHandleValue"
+        else:
+            initArgs = ""
+        ctors = [
+            ClassConstructor(
+                [],
+                visibility="public",
+                baseConstructors=baseConstructors,
+                body=(
+                    "// Safe to pass a null context if we pass a null value\n"
+                    "Init(%s);\n" % initArgs
+                ),
+            ),
+            ClassConstructor(
+                [Argument("const FastDictionaryInitializer&", "")],
+                visibility="public",
+                baseConstructors=baseConstructors,
+                explicit=True,
+                bodyInHeader=True,
+                body='// Do nothing here; this is used by our "Fast" subclass\n',
+            ),
+        ]
+        methods = []
+
+        if self.needToInitIds:
+            methods.append(self.initIdsMethod())
+
+        if d.needsConversionFromJS:
+            methods.append(self.initMethod())
+            methods.append(self.initWithoutCallContextMethod())
+        else:
+            methods.append(self.simpleInitMethod())
+
+        canBeRepresentedAsJSON = self.dictionarySafeToJSONify(d)
+        if canBeRepresentedAsJSON and d.getExtendedAttribute("GenerateInitFromJSON"):
+            methods.append(self.initFromJSONMethod())
+
+        if d.needsConversionToJS:
+            methods.append(self.toObjectInternalMethod())
+
+        if canBeRepresentedAsJSON and d.getExtendedAttribute("GenerateToJSON"):
+            methods.append(self.toJSONMethod())
+
+        methods.append(self.traceDictionaryMethod())
+
+        try:
+            if self.dictionaryNeedsCycleCollection(d):
+                methods.append(self.traverseForCCMethod())
+                methods.append(self.unlinkForCCMethod())
+        except CycleCollectionUnsupported:
+            # We have some member that we don't know how to CC.  Don't output
+            # our cycle collection overloads, so attempts to CC us will fail to
+            # compile instead of misbehaving.
+            pass
+
+        ctors.append(
+            ClassConstructor(
+                [Argument("%s&&" % selfName, "aOther")],
+                default=True,
+                visibility="public",
+                baseConstructors=baseConstructors,
+            )
+        )
+
+        if CGDictionary.isDictionaryCopyConstructible(d):
+            disallowCopyConstruction = False
+            # Note: gcc's -Wextra has a warning against not initializng our
+            # base explicitly. If we have one. Use our non-initializing base
+            # constructor to get around that.
+            ctors.append(
+                ClassConstructor(
+                    [Argument("const %s&" % selfName, "aOther")],
+                    bodyInHeader=True,
+                    visibility="public",
+                    baseConstructors=baseConstructors,
+                    explicit=True,
+                    body="*this = aOther;\n",
+                )
+            )
+            methods.append(self.assignmentOperator())
+        else:
+            disallowCopyConstruction = True
+
+        if self.canHaveEqualsOperator():
+            methods.append(self.equalsOperator())
+
+        struct = CGClass(
+            selfName,
+            bases=[ClassBase(self.base())],
+            members=members,
+            constructors=ctors,
+            methods=methods,
+            isStruct=True,
+            disallowCopyConstruction=disallowCopyConstruction,
+        )
+
+        fastDictionaryCtor = ClassConstructor(
+            [],
+            visibility="public",
+            bodyInHeader=True,
+            baseConstructors=["%s(%s)" % (selfName, self.getNonInitializingCtorArg())],
+            body="// Doesn't matter what int we pass to the parent constructor\n",
+        )
+
+        fastStruct = CGClass(
+            "Fast" + selfName,
+            bases=[ClassBase(selfName)],
+            constructors=[fastDictionaryCtor],
+            isStruct=True,
+        )
+
+        return CGList([struct, CGNamespace("binding_detail", fastStruct)], "\n")
+
+    def deps(self):
+        return self.dictionary.getDeps()
+
+    @staticmethod
+    def makeDictionaryName(dictionary):
+        return dictionary.identifier.name
+
+    def makeClassName(self, dictionary):
+        return self.makeDictionaryName(dictionary)
+
+    @staticmethod
+    def makeMemberName(name):
+        return "m" + name[0].upper() + IDLToCIdentifier(name[1:])
+
+    def getMemberType(self, memberInfo):
+        _, conversionInfo = memberInfo
+        # We can't handle having a holderType here
+        assert conversionInfo.holderType is None
+        declType = conversionInfo.declType
+        if conversionInfo.dealWithOptional:
+            declType = CGTemplatedType("Optional", declType)
+        return declType.define()
+
+    def getMemberConversion(self, memberInfo, isKnownMissing=False):
+        """
+        A function that outputs the initialization of a single dictionary
+        member from the given dictionary value.
+
+        We start with our conversionInfo, which tells us how to
+        convert a JS::Value to whatever type this member is.  We
+        substiture the template from the conversionInfo with values
+        that point to our "temp" JS::Value and our member (which is
+        the C++ value we want to produce).  The output is a string of
+        code to do the conversion.  We store this string in
+        conversionReplacements["convert"].
+
+        Now we have three different ways we might use (or skip) this
+        string of code, depending on whether the value is required,
+        optional with default value, or optional without default
+        value.  We set up a template in the 'conversion' variable for
+        exactly how to do this, then substitute into it from the
+        conversionReplacements dictionary.
+        """
+        member, conversionInfo = memberInfo
+
+        # We should only be initializing things with default values if
+        # we're always-missing.
+        assert not isKnownMissing or (member.optional and member.defaultValue)
+
+        replacements = {
+            "declName": self.makeMemberName(member.identifier.name),
+            # We need a holder name for external interfaces, but
+            # it's scoped down to the conversion so we can just use
+            # anything we want.
+            "holderName": "holder",
+            "passedToJSImpl": "passedToJSImpl",
+        }
+
+        if isKnownMissing:
+            replacements["val"] = "(JS::NullHandleValue)"
+        else:
+            replacements["val"] = "temp.ref()"
+            replacements["maybeMutableVal"] = "temp.ptr()"
+
+        # We can't handle having a holderType here
+        assert conversionInfo.holderType is None
+        if conversionInfo.dealWithOptional:
+            replacements["declName"] = "(" + replacements["declName"] + ".Value())"
+        if member.defaultValue:
+            if isKnownMissing:
+                replacements["haveValue"] = "false"
+            else:
+                replacements["haveValue"] = "!isNull && !temp->isUndefined()"
+
+        propId = self.makeIdName(member.identifier.name)
+        propGet = "JS_GetPropertyById(cx, *object, atomsCache->%s, temp.ptr())" % propId
+
+        conversionReplacements = {
+            "prop": self.makeMemberName(member.identifier.name),
+            "convert": string.Template(conversionInfo.template).substitute(
+                replacements
+            ),
+            "propGet": propGet,
+        }
+        # The conversion code will only run where a default value or a value passed
+        # by the author needs to get converted, so we can remember if we have any
+        # members present here.
+        conversionReplacements["convert"] += "mIsAnyMemberPresent = true;\n"
+        if isKnownMissing:
+            conversion = ""
+        else:
+            setTempValue = CGGeneric(
+                dedent(
+                    """
+                if (!${propGet}) {
+                  return false;
+                }
+                """
+                )
+            )
+            conditions = getConditionList(member, "cx", "*object")
+            if len(conditions) != 0:
+                setTempValue = CGIfElseWrapper(
+                    conditions.define(),
+                    setTempValue,
+                    CGGeneric("temp->setUndefined();\n"),
+                )
+            setTempValue = CGIfWrapper(setTempValue, "!isNull")
+            conversion = setTempValue.define()
+
+        if member.defaultValue:
+            if member.type.isUnion() and (
+                not member.type.nullable()
+                or not isinstance(member.defaultValue, IDLNullValue)
+            ):
+                # Since this has a default value, it might have been initialized
+                # already.  Go ahead and uninit it before we try to init it
+                # again.
+                memberName = self.makeMemberName(member.identifier.name)
+                if member.type.nullable():
+                    conversion += fill(
+                        """
+                        if (!${memberName}.IsNull()) {
+                          ${memberName}.Value().Uninit();
+                        }
+                        """,
+                        memberName=memberName,
+                    )
+                else:
+                    conversion += "%s.Uninit();\n" % memberName
+            conversion += "${convert}"
+        elif not conversionInfo.dealWithOptional:
+            # We're required, but have no default value.  Make sure
+            # that we throw if we have no value provided.
+            conversion += dedent(
+                """
+                if (!isNull && !temp->isUndefined()) {
+                ${convert}
+                } else if (cx) {
+                  // Don't error out if we have no cx.  In that
+                  // situation the caller is default-constructing us and we'll
+                  // just assume they know what they're doing.
+                  return cx.ThrowErrorMessage<MSG_MISSING_REQUIRED_DICTIONARY_MEMBER>("%s");
+                }
+                """
+                % self.getMemberSourceDescription(member)
+            )
+            conversionReplacements["convert"] = indent(
+                conversionReplacements["convert"]
+            ).rstrip()
+        else:
+            conversion += (
+                "if (!isNull && !temp->isUndefined()) {\n"
+                "  ${prop}.Construct();\n"
+                "${convert}"
+                "}\n"
+            )
+            conversionReplacements["convert"] = indent(
+                conversionReplacements["convert"]
+            )
+
+        return CGGeneric(string.Template(conversion).substitute(conversionReplacements))
+
+    def getMemberDefinition(self, memberInfo):
+        member = memberInfo[0]
+        declType = memberInfo[1].declType
+        memberLoc = self.makeMemberName(member.identifier.name)
+        if not member.canHaveMissingValue():
+            memberData = memberLoc
+        else:
+            # The data is inside the Optional<>
+            memberData = "%s.InternalValue()" % memberLoc
+
+        # If you have to change this list (which you shouldn't!), make sure it
+        # continues to match the list in test_Object.prototype_props.html
+        if member.identifier.name in [
+            "constructor",
+            "toString",
+            "toLocaleString",
+            "valueOf",
+            "hasOwnProperty",
+            "isPrototypeOf",
+            "propertyIsEnumerable",
+            "__defineGetter__",
+            "__defineSetter__",
+            "__lookupGetter__",
+            "__lookupSetter__",
+            "__proto__",
+        ]:
+            raise TypeError(
+                "'%s' member of %s dictionary shadows "
+                "a property of Object.prototype, and Xrays to "
+                "Object can't handle that.\n"
+                "%s"
+                % (
+                    member.identifier.name,
+                    self.dictionary.identifier.name,
+                    member.location,
+                )
+            )
+
+        propDef = (
+            "JS_DefinePropertyById(cx, obj, atomsCache->%s, temp, JSPROP_ENUMERATE)"
+            % self.makeIdName(member.identifier.name)
+        )
+
+        innerTemplate = wrapForType(
+            member.type,
+            self.descriptorProvider,
+            {
+                "result": "currentValue",
+                "successCode": (
+                    "if (!%s) {\n" "  return false;\n" "}\n" "break;\n" % propDef
+                ),
+                "jsvalRef": "temp",
+                "jsvalHandle": "&temp",
+                "returnsNewObject": False,
+                # 'obj' can just be allowed to be the string "obj", since that
+                # will be our dictionary object, which is presumably itself in
+                # the right scope.
+                "spiderMonkeyInterfacesAreStructs": True,
+            },
+        )
+        conversion = CGGeneric(innerTemplate)
+        conversion = CGWrapper(
+            conversion,
+            pre=(
+                "JS::Rooted<JS::Value> temp(cx);\n"
+                "%s const & currentValue = %s;\n" % (declType.define(), memberData)
+            ),
+        )
+
+        # Now make sure that our successCode can actually break out of the
+        # conversion.  This incidentally gives us a scope for 'temp' and
+        # 'currentValue'.
+        conversion = CGWrapper(
+            CGIndenter(conversion),
+            pre=(
+                "do {\n"
+                "  // block for our 'break' successCode and scope for 'temp' and 'currentValue'\n"
+            ),
+            post="} while(false);\n",
+        )
+        if member.canHaveMissingValue():
+            # Only do the conversion if we have a value
+            conversion = CGIfWrapper(conversion, "%s.WasPassed()" % memberLoc)
+        conditions = getConditionList(member, "cx", "obj")
+        if len(conditions) != 0:
+            conversion = CGIfWrapper(conversion, conditions.define())
+        return conversion
+
+    def getMemberTrace(self, member):
+        type = member.type
+        assert typeNeedsRooting(type)
+        memberLoc = self.makeMemberName(member.identifier.name)
+        if not member.canHaveMissingValue():
+            memberData = memberLoc
+        else:
+            # The data is inside the Optional<>
+            memberData = "%s.Value()" % memberLoc
+
+        memberName = "%s.%s" % (self.makeClassName(self.dictionary), memberLoc)
+
+        if type.isObject():
+            trace = CGGeneric(
+                'JS::TraceRoot(trc, %s, "%s");\n' % ("&" + memberData, memberName)
+            )
+            if type.nullable():
+                trace = CGIfWrapper(trace, memberData)
+        elif type.isAny():
+            trace = CGGeneric(
+                'JS::TraceRoot(trc, %s, "%s");\n' % ("&" + memberData, memberName)
+            )
+        elif (
+            type.isSequence()
+            or type.isDictionary()
+            or type.isSpiderMonkeyInterface()
+            or type.isUnion()
+            or type.isRecord()
+        ):
+            if type.nullable():
+                memberNullable = memberData
+                memberData = "%s.Value()" % memberData
+            if type.isSequence():
+                trace = CGGeneric("DoTraceSequence(trc, %s);\n" % memberData)
+            elif type.isDictionary():
+                trace = CGGeneric("%s.TraceDictionary(trc);\n" % memberData)
+            elif type.isUnion():
+                trace = CGGeneric("%s.TraceUnion(trc);\n" % memberData)
+            elif type.isRecord():
+                trace = CGGeneric("TraceRecord(trc, %s);\n" % memberData)
+            else:
+                assert type.isSpiderMonkeyInterface()
+                trace = CGGeneric("%s.TraceSelf(trc);\n" % memberData)
+            if type.nullable():
+                trace = CGIfWrapper(trace, "!%s.IsNull()" % memberNullable)
+        else:
+            assert False  # unknown type
+
+        if member.canHaveMissingValue():
+            trace = CGIfWrapper(trace, "%s.WasPassed()" % memberLoc)
+
+        return trace.define()
+
+    def getMemberInitializer(self, memberInfo):
+        """
+        Get the right initializer for the member.  Most members don't need one,
+        but we need to pre-initialize 'object' that have a default value or are
+        required (and hence are not inside Optional), so they're safe to trace
+        at all times.  And we can optimize a bit for dictionary-typed members.
+        """
+        member, _ = memberInfo
+        if member.canHaveMissingValue():
+            # Allowed missing value means no need to set it up front, since it's
+            # inside an Optional and won't get traced until it's actually set
+            # up.
+            return None
+        type = member.type
+        if type.isDictionary():
+            # When we construct ourselves, we don't want to init our member
+            # dictionaries.  Either we're being constructed-but-not-initialized
+            # ourselves (and then we don't want to init them) or we're about to
+            # init ourselves and then we'll init them anyway.
+            return CGDictionary.getNonInitializingCtorArg()
+        return initializerForType(type)
+
+    def getMemberSourceDescription(self, member):
+        return "'%s' member of %s" % (
+            member.identifier.name,
+            self.dictionary.identifier.name,
+        )
+
+    @staticmethod
+    def makeIdName(name):
+        return IDLToCIdentifier(name) + "_id"
+
+    @staticmethod
+    def getNonInitializingCtorArg():
+        return "FastDictionaryInitializer()"
+
+    @staticmethod
+    def isDictionaryCopyConstructible(dictionary):
+        if dictionary.parent and not CGDictionary.isDictionaryCopyConstructible(
+            dictionary.parent
+        ):
+            return False
+        return all(isTypeCopyConstructible(m.type) for m in dictionary.members)
+
+    @staticmethod
+    def typeSafeToJSONify(type):
+        """
+        Determine whether the given type is safe to convert to JSON.  The
+        restriction is that this needs to be safe while in a global controlled
+        by an adversary, and "safe" means no side-effects when the JS
+        representation of this type is converted to JSON.  That means that we
+        have to be pretty restrictive about what things we can allow.  For
+        example, "object" is out, because it may have accessor properties on it.
+        """
+        if type.nullable():
+            # Converting null to JSON is always OK.
+            return CGDictionary.typeSafeToJSONify(type.inner)
+
+        if type.isSequence():
+            # Sequences are arrays we create ourselves, with no holes.  They
+            # should be safe if their contents are safe, as long as we suppress
+            # invocation of .toJSON on objects.
+            return CGDictionary.typeSafeToJSONify(type.inner)
+
+        if type.isUnion():
+            # OK if everything in it is ok.
+            return all(CGDictionary.typeSafeToJSONify(t) for t in type.flatMemberTypes)
+
+        if type.isDictionary():
+            # OK if the dictionary is OK
+            return CGDictionary.dictionarySafeToJSONify(type.inner)
+
+        if type.isUndefined() or type.isString() or type.isEnum():
+            # Strings are always OK.
+            return True
+
+        if type.isPrimitive():
+            # Primitives (numbers and booleans) are ok, as long as
+            # they're not unrestricted float/double.
+            return not type.isFloat() or not type.isUnrestricted()
+
+        if type.isRecord():
+            # Records are okay, as long as the value type is.
+            # Per spec, only strings are allowed as keys.
+            return CGDictionary.typeSafeToJSONify(type.inner)
+
+        return False
+
+    @staticmethod
+    def dictionarySafeToJSONify(dictionary):
+        # The dictionary itself is OK, so we're good if all our types are.
+        return all(CGDictionary.typeSafeToJSONify(m.type) for m in dictionary.members)
+
+
+class CGRegisterWorkerBindings(CGAbstractMethod):
+    def __init__(self, config):
+        CGAbstractMethod.__init__(
+            self,
+            None,
+            "RegisterWorkerBindings",
+            "bool",
+            [Argument("JSContext*", "aCx"), Argument("JS::Handle<JSObject*>", "aObj")],
+        )
+        self.config = config
+
+    def definition_body(self):
+        descriptors = self.config.getDescriptors(
+            hasInterfaceObject=True, isExposedInAnyWorker=True, register=True
+        )
+        conditions = []
+        for desc in descriptors:
+            bindingNS = toBindingNamespace(desc.name)
+            condition = "!%s::GetConstructorObject(aCx)" % bindingNS
+            if desc.isExposedConditionally():
+                condition = (
+                    "%s::ConstructorEnabled(aCx, aObj) && " % bindingNS + condition
+                )
+            conditions.append(condition)
+        lines = [
+            CGIfWrapper(CGGeneric("return false;\n"), condition)
+            for condition in conditions
+        ]
+        lines.append(CGGeneric("return true;\n"))
+        return CGList(lines, "\n").define()
+
+
+class CGRegisterWorkerDebuggerBindings(CGAbstractMethod):
+    def __init__(self, config):
+        CGAbstractMethod.__init__(
+            self,
+            None,
+            "RegisterWorkerDebuggerBindings",
+            "bool",
+            [Argument("JSContext*", "aCx"), Argument("JS::Handle<JSObject*>", "aObj")],
+        )
+        self.config = config
+
+    def definition_body(self):
+        descriptors = self.config.getDescriptors(
+            hasInterfaceObject=True, isExposedInWorkerDebugger=True, register=True
+        )
+        conditions = []
+        for desc in descriptors:
+            bindingNS = toBindingNamespace(desc.name)
+            condition = "!%s::GetConstructorObject(aCx)" % bindingNS
+            if desc.isExposedConditionally():
+                condition = (
+                    "%s::ConstructorEnabled(aCx, aObj) && " % bindingNS + condition
+                )
+            conditions.append(condition)
+        lines = [
+            CGIfWrapper(CGGeneric("return false;\n"), condition)
+            for condition in conditions
+        ]
+        lines.append(CGGeneric("return true;\n"))
+        return CGList(lines, "\n").define()
+
+
+class CGRegisterWorkletBindings(CGAbstractMethod):
+    def __init__(self, config):
+        CGAbstractMethod.__init__(
+            self,
+            None,
+            "RegisterWorkletBindings",
+            "bool",
+            [Argument("JSContext*", "aCx"), Argument("JS::Handle<JSObject*>", "aObj")],
+        )
+        self.config = config
+
+    def definition_body(self):
+        descriptors = self.config.getDescriptors(
+            hasInterfaceObject=True, isExposedInAnyWorklet=True, register=True
+        )
+        conditions = []
+        for desc in descriptors:
+            bindingNS = toBindingNamespace(desc.name)
+            condition = "!%s::GetConstructorObject(aCx)" % bindingNS
+            if desc.isExposedConditionally():
+                condition = (
+                    "%s::ConstructorEnabled(aCx, aObj) && " % bindingNS + condition
+                )
+            conditions.append(condition)
+        lines = [
+            CGIfWrapper(CGGeneric("return false;\n"), condition)
+            for condition in conditions
+        ]
+        lines.append(CGGeneric("return true;\n"))
+        return CGList(lines, "\n").define()
+
+
+class CGRegisterShadowRealmBindings(CGAbstractMethod):
+    def __init__(self, config):
+        CGAbstractMethod.__init__(
+            self,
+            None,
+            "RegisterShadowRealmBindings",
+            "bool",
+            [Argument("JSContext*", "aCx"), Argument("JS::Handle<JSObject*>", "aObj")],
+        )
+        self.config = config
+
+    def definition_body(self):
+        descriptors = self.config.getDescriptors(
+            hasInterfaceObject=True, isExposedInShadowRealms=True, register=True
+        )
+        conditions = []
+        for desc in descriptors:
+            bindingNS = toBindingNamespace(desc.name)
+            condition = "!%s::GetConstructorObject(aCx)" % bindingNS
+            if desc.isExposedConditionally():
+                condition = (
+                    "%s::ConstructorEnabled(aCx, aObj) && " % bindingNS + condition
+                )
+            conditions.append(condition)
+        lines = [
+            CGIfWrapper(CGGeneric("return false;\n"), condition)
+            for condition in conditions
+        ]
+        lines.append(CGGeneric("return true;\n"))
+        return CGList(lines, "\n").define()
+
+
+def BindingNamesOffsetEnum(name):
+    return CppKeywords.checkMethodName(name.replace(" ", "_"))
+
+
+class CGGlobalNames(CGGeneric):
+    def __init__(self, names):
+        """
+        names is expected to be a list of tuples of the name and the descriptor it refers to.
+        """
+
+        strings = []
+        entries = []
+        for name, desc in names:
+            # Generate the entry declaration
+            # XXX(nika): mCreate & mEnabled require relocations. If we want to
+            # reduce those, we could move them into separate tables.
+            nativeEntry = fill(
+                """
+                {
+                  /* mNameOffset */ BindingNamesOffset::${nameOffset},
+                  /* mNameLength */ ${nameLength},
+                  /* mConstructorId */ constructors::id::${realname},
+                  /* mCreate */ ${realname}_Binding::CreateInterfaceObjects,
+                  /* mEnabled */ ${enabled}
+                }
+                """,
+                nameOffset=BindingNamesOffsetEnum(name),
+                nameLength=len(name),
+                name=name,
+                realname=desc.name,
+                enabled=(
+                    "%s_Binding::ConstructorEnabled" % desc.name
+                    if desc.isExposedConditionally()
+                    else "nullptr"
+                ),
+            )
+
+            entries.append((name, nativeEntry))
+
+        # Unfortunately, when running tests, we may have no entries.
+        # PerfectHash will assert if we give it an empty set of entries, so we
+        # just generate a dummy value.
+        if len(entries) == 0:
+            CGGeneric.__init__(
+                self,
+                define=dedent(
+                    """
+                static_assert(false, "No WebIDL global name entries!");
+                """
+                ),
+            )
+            return
+
+        # Build the perfect hash function.
+        phf = PerfectHash(entries, GLOBAL_NAMES_PHF_SIZE)
+
+        # Generate code for the PHF
+        phfCodegen = phf.codegen(
+            "WebIDLGlobalNameHash::sEntries", "WebIDLNameTableEntry"
+        )
+        entries = phfCodegen.gen_entries(lambda e: e[1])
+        getter = phfCodegen.gen_jslinearstr_getter(
+            name="WebIDLGlobalNameHash::GetEntry",
+            return_type="const WebIDLNameTableEntry*",
+            return_entry=dedent(
+                """
+                if (JS_LinearStringEqualsAscii(aKey, BindingName(entry.mNameOffset), entry.mNameLength)) {
+                  return &entry;
+                }
+                return nullptr;
+                """
+            ),
+        )
+
+        define = fill(
+            """
+            const uint32_t WebIDLGlobalNameHash::sCount = ${count};
+
+            $*{entries}
+
+            $*{getter}
+            """,
+            count=len(phf.entries),
+            strings="\n".join(strings) + ";\n",
+            entries=entries,
+            getter=getter,
+        )
+        CGGeneric.__init__(self, define=define)
+
+
+def dependencySortObjects(objects, dependencyGetter, nameGetter):
+    """
+    Sort IDL objects with dependencies on each other such that if A
+    depends on B then B will come before A.  This is needed for
+    declaring C++ classes in the right order, for example.  Objects
+    that have no dependencies are just sorted by name.
+
+    objects should be something that can produce a set of objects
+    (e.g. a set, iterator, list, etc).
+
+    dependencyGetter is something that, given an object, should return
+    the set of objects it depends on.
+    """
+    # XXXbz this will fail if we have two webidl files F1 and F2 such that F1
+    # declares an object which depends on an object in F2, and F2 declares an
+    # object (possibly a different one!) that depends on an object in F1.  The
+    # good news is that I expect this to never happen.
+    sortedObjects = []
+    objects = set(objects)
+    while len(objects) != 0:
+        # Find the dictionaries that don't depend on anything else
+        # anymore and move them over.
+        toMove = [o for o in objects if len(dependencyGetter(o) & objects) == 0]
+        if len(toMove) == 0:
+            raise TypeError(
+                "Loop in dependency graph\n" + "\n".join(o.location for o in objects)
+            )
+        objects = objects - set(toMove)
+        sortedObjects.extend(sorted(toMove, key=nameGetter))
+    return sortedObjects
+
+
+class ForwardDeclarationBuilder:
+    """
+    Create a canonical representation of a set of namespaced forward
+    declarations.
+    """
+
+    def __init__(self):
+        """
+        The set of declarations is represented as a tree of nested namespaces.
+        Each tree node has a set of declarations |decls| and a dict |children|.
+        Each declaration is a pair consisting of the class name and a boolean
+        that is true iff the class is really a struct. |children| maps the
+        names of inner namespaces to the declarations in that namespace.
+        """
+        self.decls = set()
+        self.children = {}
+
+    def _ensureNonTemplateType(self, type):
+        if "<" in type:
+            # This is a templated type.  We don't really know how to
+            # forward-declare those, and trying to do it naively is not going to
+            # go well (e.g. we may have :: characters inside the type we're
+            # templated on!).  Just bail out.
+            raise TypeError(
+                "Attempt to use ForwardDeclarationBuilder on "
+                "templated type %s.  We don't know how to do that "
+                "yet." % type
+            )
+
+    def _listAdd(self, namespaces, name, isStruct=False):
+        """
+        Add a forward declaration, where |namespaces| is a list of namespaces.
+        |name| should not contain any other namespaces.
+        """
+        if namespaces:
+            child = self.children.setdefault(namespaces[0], ForwardDeclarationBuilder())
+            child._listAdd(namespaces[1:], name, isStruct)
+        else:
+            assert "::" not in name
+            self.decls.add((name, isStruct))
+
+    def addInMozillaDom(self, name, isStruct=False):
+        """
+        Add a forward declaration to the mozilla::dom:: namespace. |name| should not
+        contain any other namespaces.
+        """
+        self._ensureNonTemplateType(name)
+        self._listAdd(["mozilla", "dom"], name, isStruct)
+
+    def add(self, nativeType, isStruct=False):
+        """
+        Add a forward declaration, where |nativeType| is a string containing
+        the type and its namespaces, in the usual C++ way.
+        """
+        self._ensureNonTemplateType(nativeType)
+        components = nativeType.split("::")
+        self._listAdd(components[:-1], components[-1], isStruct)
+
+    def _build(self, atTopLevel):
+        """
+        Return a codegenerator for the forward declarations.
+        """
+        decls = []
+        if self.decls:
+            decls.append(
+                CGList(
+                    [
+                        CGClassForwardDeclare(cname, isStruct)
+                        for cname, isStruct in sorted(self.decls)
+                    ]
+                )
+            )
+        for namespace, child in sorted(six.iteritems(self.children)):
+            decls.append(CGNamespace(namespace, child._build(atTopLevel=False)))
+
+        cg = CGList(decls, "\n")
+        if not atTopLevel and len(decls) + len(self.decls) > 1:
+            cg = CGWrapper(cg, pre="\n", post="\n")
+        return cg
+
+    def build(self):
+        return self._build(atTopLevel=True)
+
+    def forwardDeclareForType(self, t, config):
+        t = t.unroll()
+        if t.isGeckoInterface():
+            name = t.inner.identifier.name
+            try:
+                desc = config.getDescriptor(name)
+                self.add(desc.nativeType)
+            except NoSuchDescriptorError:
+                pass
+
+        # Note: SpiderMonkey interfaces are typedefs, so can't be
+        # forward-declared
+        elif t.isPromise():
+            self.addInMozillaDom("Promise")
+        elif t.isCallback():
+            self.addInMozillaDom(t.callback.identifier.name)
+        elif t.isDictionary():
+            self.addInMozillaDom(t.inner.identifier.name, isStruct=True)
+        elif t.isCallbackInterface():
+            self.addInMozillaDom(t.inner.identifier.name)
+        elif t.isUnion():
+            # Forward declare both the owning and non-owning version,
+            # since we don't know which one we might want
+            self.addInMozillaDom(CGUnionStruct.unionTypeName(t, False))
+            self.addInMozillaDom(CGUnionStruct.unionTypeName(t, True))
+        elif t.isRecord():
+            self.forwardDeclareForType(t.inner, config)
+        # Don't need to do anything for void, primitive, string, any or object.
+        # There may be some other cases we are missing.
+
+
+class CGForwardDeclarations(CGWrapper):
+    """
+    Code generate the forward declarations for a header file.
+    additionalDeclarations is a list of tuples containing a classname and a
+    boolean. If the boolean is true we will declare a struct, otherwise we'll
+    declare a class.
+    """
+
+    def __init__(
+        self,
+        config,
+        descriptors,
+        callbacks,
+        dictionaries,
+        callbackInterfaces,
+        additionalDeclarations=[],
+    ):
+        builder = ForwardDeclarationBuilder()
+
+        # Needed for at least Wrap.
+        for d in descriptors:
+            # If this is a generated iterator interface, we only create these
+            # in the generated bindings, and don't need to forward declare.
+            if (
+                d.interface.isIteratorInterface()
+                or d.interface.isAsyncIteratorInterface()
+            ):
+                continue
+            builder.add(d.nativeType)
+            if d.interface.isSerializable():
+                builder.add("nsIGlobalObject")
+            # If we're an interface and we have a maplike/setlike declaration,
+            # we'll have helper functions exposed to the native side of our
+            # bindings, which will need to show up in the header. If either of
+            # our key/value types are interfaces, they'll be passed as
+            # arguments to helper functions, and they'll need to be forward
+            # declared in the header.
+            if d.interface.maplikeOrSetlikeOrIterable:
+                if d.interface.maplikeOrSetlikeOrIterable.hasKeyType():
+                    builder.forwardDeclareForType(
+                        d.interface.maplikeOrSetlikeOrIterable.keyType, config
+                    )
+                if d.interface.maplikeOrSetlikeOrIterable.hasValueType():
+                    builder.forwardDeclareForType(
+                        d.interface.maplikeOrSetlikeOrIterable.valueType, config
+                    )
+
+            for m in d.interface.members:
+                if m.isAttr() and m.type.isObservableArray():
+                    builder.forwardDeclareForType(m.type, config)
+
+        # We just about always need NativePropertyHooks
+        builder.addInMozillaDom("NativePropertyHooks", isStruct=True)
+        builder.addInMozillaDom("ProtoAndIfaceCache")
+
+        for callback in callbacks:
+            builder.addInMozillaDom(callback.identifier.name)
+            for t in getTypesFromCallback(callback):
+                builder.forwardDeclareForType(t, config)
+
+        for d in callbackInterfaces:
+            builder.add(d.nativeType)
+            builder.add(d.nativeType + "Atoms", isStruct=True)
+            for t in getTypesFromDescriptor(d):
+                builder.forwardDeclareForType(t, config)
+            if d.hasCEReactions():
+                builder.addInMozillaDom("DocGroup")
+
+        for d in dictionaries:
+            if len(d.members) > 0:
+                builder.addInMozillaDom(d.identifier.name + "Atoms", isStruct=True)
+            for t in getTypesFromDictionary(d):
+                builder.forwardDeclareForType(t, config)
+
+        for className, isStruct in additionalDeclarations:
+            builder.add(className, isStruct=isStruct)
+
+        CGWrapper.__init__(self, builder.build())
+
+
+def dependencySortDictionariesAndUnionsAndCallbacks(types):
+    def getDependenciesFromType(type):
+        if type.isDictionary():
+            return set([type.unroll().inner])
+        if type.isSequence():
+            return getDependenciesFromType(type.unroll())
+        if type.isUnion():
+            return set([type.unroll()])
+        if type.isRecord():
+            return set([type.unroll().inner])
+        if type.isCallback():
+            return set([type.unroll()])
+        return set()
+
+    def getDependencies(unionTypeOrDictionaryOrCallback):
+        if isinstance(unionTypeOrDictionaryOrCallback, IDLDictionary):
+            deps = set()
+            if unionTypeOrDictionaryOrCallback.parent:
+                deps.add(unionTypeOrDictionaryOrCallback.parent)
+            for member in unionTypeOrDictionaryOrCallback.members:
+                deps |= getDependenciesFromType(member.type)
+            return deps
+
+        if (
+            unionTypeOrDictionaryOrCallback.isType()
+            and unionTypeOrDictionaryOrCallback.isUnion()
+        ):
+            deps = set()
+            for member in unionTypeOrDictionaryOrCallback.flatMemberTypes:
+                deps |= getDependenciesFromType(member)
+            return deps
+
+        assert unionTypeOrDictionaryOrCallback.isCallback()
+        return set()
+
+    def getName(unionTypeOrDictionaryOrCallback):
+        if isinstance(unionTypeOrDictionaryOrCallback, IDLDictionary):
+            return unionTypeOrDictionaryOrCallback.identifier.name
+
+        if (
+            unionTypeOrDictionaryOrCallback.isType()
+            and unionTypeOrDictionaryOrCallback.isUnion()
+        ):
+            return unionTypeOrDictionaryOrCallback.name
+
+        assert unionTypeOrDictionaryOrCallback.isCallback()
+        return unionTypeOrDictionaryOrCallback.identifier.name
+
+    return dependencySortObjects(types, getDependencies, getName)
+
+
+class CGBindingRoot(CGThing):
+    """
+    Root codegen class for binding generation. Instantiate the class, and call
+    declare or define to generate header or cpp code (respectively).
+    """
+
+    def __init__(self, config, prefix, webIDLFile):
+        bindingHeaders = dict.fromkeys(
+            ("mozilla/dom/NonRefcountedDOMObject.h", "MainThreadUtils.h"), True
+        )
+        bindingDeclareHeaders = dict.fromkeys(
+            (
+                "mozilla/dom/BindingDeclarations.h",
+                "mozilla/dom/Nullable.h",
+            ),
+            True,
+        )
+
+        descriptors = config.getDescriptors(
+            webIDLFile=webIDLFile, hasInterfaceOrInterfacePrototypeObject=True
+        )
+
+        unionTypes = UnionsForFile(config, webIDLFile)
+
+        (
+            unionHeaders,
+            unionImplheaders,
+            unionDeclarations,
+            traverseMethods,
+            unlinkMethods,
+            unionStructs,
+        ) = UnionTypes(unionTypes, config)
+
+        bindingDeclareHeaders.update(dict.fromkeys(unionHeaders, True))
+        bindingHeaders.update(dict.fromkeys(unionImplheaders, True))
+        bindingDeclareHeaders["mozilla/dom/UnionMember.h"] = len(unionStructs) > 0
+        bindingDeclareHeaders["mozilla/dom/FakeString.h"] = len(unionStructs) > 0
+        # BindingUtils.h is only needed for SetToObject.
+        # If it stops being inlined or stops calling CallerSubsumes
+        # both this bit and the bit in UnionTypes can be removed.
+        bindingDeclareHeaders["mozilla/dom/BindingUtils.h"] = any(
+            d.isObject() for t in unionTypes for d in t.flatMemberTypes
+        )
+        bindingHeaders["mozilla/dom/IterableIterator.h"] = any(
+            (
+                d.interface.isIteratorInterface()
+                and d.interface.maplikeOrSetlikeOrIterable.isPairIterator()
+            )
+            or d.interface.isAsyncIteratorInterface()
+            or d.interface.isIterable()
+            or d.interface.isAsyncIterable()
+            for d in descriptors
+        )
+
+        def memberNeedsSubjectPrincipal(d, m):
+            if m.isAttr():
+                return (
+                    "needsSubjectPrincipal" in d.getExtendedAttributes(m, getter=True)
+                ) or (
+                    not m.readonly
+                    and "needsSubjectPrincipal"
+                    in d.getExtendedAttributes(m, setter=True)
+                )
+            return m.isMethod() and "needsSubjectPrincipal" in d.getExtendedAttributes(
+                m
+            )
+
+        if any(
+            memberNeedsSubjectPrincipal(d, m)
+            for d in descriptors
+            for m in d.interface.members
+        ):
+            bindingHeaders["mozilla/BasePrincipal.h"] = True
+            bindingHeaders["nsJSPrincipals.h"] = True
+
+        # The conditions for which we generate profiler labels are fairly
+        # complicated. The check below is a little imprecise to make it simple.
+        # It includes the profiler header in all cases where it is necessary and
+        # generates only a few false positives.
+        bindingHeaders["mozilla/ProfilerLabels.h"] = any(
+            # constructor profiler label
+            d.interface.legacyFactoryFunctions
+            or (d.interface.hasInterfaceObject() and d.interface.ctor())
+            or any(
+                # getter/setter profiler labels
+                m.isAttr()
+                # method profiler label
+                or m.isMethod()
+                for m in d.interface.members
+            )
+            for d in descriptors
+        )
+
+        def descriptorHasCrossOriginProperties(desc):
+            def hasCrossOriginProperty(m):
+                props = memberProperties(m, desc)
+                return (
+                    props.isCrossOriginMethod
+                    or props.isCrossOriginGetter
+                    or props.isCrossOriginSetter
+                )
+
+            return any(hasCrossOriginProperty(m) for m in desc.interface.members)
+
+        def descriptorHasObservableArrayTypes(desc):
+            def hasObservableArrayTypes(m):
+                return m.isAttr() and m.type.isObservableArray()
+
+            return any(hasObservableArrayTypes(m) for m in desc.interface.members)
+
+        bindingDeclareHeaders["mozilla/dom/RemoteObjectProxy.h"] = any(
+            descriptorHasCrossOriginProperties(d) for d in descriptors
+        )
+        bindingDeclareHeaders["jsapi.h"] = any(
+            descriptorHasCrossOriginProperties(d)
+            or descriptorHasObservableArrayTypes(d)
+            for d in descriptors
+        )
+        bindingDeclareHeaders["js/TypeDecls.h"] = not bindingDeclareHeaders["jsapi.h"]
+        bindingDeclareHeaders["js/RootingAPI.h"] = not bindingDeclareHeaders["jsapi.h"]
+
+        # JS::IsCallable
+        bindingDeclareHeaders["js/CallAndConstruct.h"] = True
+
+        def descriptorHasIteratorAlias(desc):
+            def hasIteratorAlias(m):
+                return m.isMethod() and (
+                    ("@@iterator" in m.aliases) or ("@@asyncIterator" in m.aliases)
+                )
+
+            return any(hasIteratorAlias(m) for m in desc.interface.members)
+
+        bindingHeaders["js/Symbol.h"] = any(
+            descriptorHasIteratorAlias(d) for d in descriptors
+        )
+
+        bindingHeaders["js/shadow/Object.h"] = any(
+            d.interface.hasMembersInSlots() for d in descriptors
+        )
+
+        # The symbols supplied by this header are used so ubiquitously it's not
+        # worth the effort delineating the exact dependency, if it can't be done
+        # *at* the places where their definitions are required.
+        bindingHeaders["js/experimental/JitInfo.h"] = True
+
+        # JS::GetClass, JS::GetCompartment, JS::GetReservedSlot, and
+        # JS::SetReservedSlot are also used too many places to restate
+        # dependency logic.
+        bindingHeaders["js/Object.h"] = True
+
+        # JS::IsCallable, JS::Call, JS::Construct
+        bindingHeaders["js/CallAndConstruct.h"] = True
+
+        # JS_IsExceptionPending
+        bindingHeaders["js/Exception.h"] = True
+
+        # JS::Map{Clear, Delete, Has, Get, Set}
+        bindingHeaders["js/MapAndSet.h"] = True
+
+        # JS_DefineElement, JS_DefineProperty, JS_DefinePropertyById,
+        # JS_DefineUCProperty, JS_ForwardGetPropertyTo, JS_GetProperty,
+        # JS_GetPropertyById, JS_HasPropertyById, JS_SetProperty,
+        # JS_SetPropertyById
+        bindingHeaders["js/PropertyAndElement.h"] = True
+
+        # JS_GetOwnPropertyDescriptorById
+        bindingHeaders["js/PropertyDescriptor.h"] = True
+
+        def descriptorDeprecated(desc):
+            iface = desc.interface
+            return any(
+                m.getExtendedAttribute("Deprecated") for m in iface.members + [iface]
+            )
+
+        bindingHeaders["mozilla/dom/Document.h"] = any(
+            descriptorDeprecated(d) for d in descriptors
+        )
+
+        bindingHeaders["mozilla/dom/DOMJSProxyHandler.h"] = any(
+            d.concrete and d.proxy for d in descriptors
+        )
+
+        bindingHeaders["mozilla/dom/ProxyHandlerUtils.h"] = any(
+            d.concrete and d.proxy for d in descriptors
+        )
+
+        bindingHeaders["js/String.h"] = any(
+            d.needsMissingPropUseCounters for d in descriptors
+        )
+
+        hasCrossOriginObjects = any(
+            d.concrete and d.isMaybeCrossOriginObject() for d in descriptors
+        )
+        bindingHeaders["mozilla/dom/MaybeCrossOriginObject.h"] = hasCrossOriginObjects
+        bindingHeaders["AccessCheck.h"] = hasCrossOriginObjects
+        hasCEReactions = any(d.hasCEReactions() for d in descriptors)
+        bindingHeaders["mozilla/dom/CustomElementRegistry.h"] = hasCEReactions
+        bindingHeaders["mozilla/dom/DocGroup.h"] = hasCEReactions
+
+        def descriptorHasChromeOnly(desc):
+            ctor = desc.interface.ctor()
+
+            return (
+                any(
+                    isChromeOnly(a) or needsContainsHack(a) or needsCallerType(a)
+                    for a in desc.interface.members
+                )
+                or desc.interface.getExtendedAttribute("ChromeOnly") is not None
+                or
+                # JS-implemented interfaces with an interface object get a
+                # chromeonly _create method.  And interfaces with an
+                # interface object might have a ChromeOnly constructor.
+                (
+                    desc.interface.hasInterfaceObject()
+                    and (
+                        desc.interface.isJSImplemented()
+                        or (ctor and isChromeOnly(ctor))
+                    )
+                )
+            )
+
+        # XXXkhuey ugly hack but this is going away soon.
+        bindingHeaders["xpcprivate.h"] = webIDLFile.endswith("EventTarget.webidl")
+
+        hasThreadChecks = any(d.hasThreadChecks() for d in descriptors)
+        bindingHeaders["nsThreadUtils.h"] = hasThreadChecks
+
+        dictionaries = config.getDictionaries(webIDLFile)
+
+        def dictionaryHasChromeOnly(dictionary):
+            while dictionary:
+                if any(isChromeOnly(m) for m in dictionary.members):
+                    return True
+                dictionary = dictionary.parent
+            return False
+
+        def needsNonSystemPrincipal(member):
+            return (
+                member.getExtendedAttribute("NeedsSubjectPrincipal") == ["NonSystem"]
+                or member.getExtendedAttribute("SetterNeedsSubjectPrincipal")
+                == ["NonSystem"]
+                or member.getExtendedAttribute("GetterNeedsSubjectPrincipal")
+                == ["NonSystem"]
+            )
+
+        def descriptorNeedsNonSystemPrincipal(d):
+            return any(needsNonSystemPrincipal(m) for m in d.interface.members)
+
+        def descriptorHasPrefDisabler(desc):
+            iface = desc.interface
+            return any(
+                PropertyDefiner.getControllingCondition(m, desc).hasDisablers()
+                for m in iface.members
+                if (m.isMethod() or m.isAttr() or m.isConst())
+            )
+
+        def addPrefHeaderForObject(bindingHeaders, obj):
+            """
+            obj might be a dictionary member or an interface.
+            """
+            if obj is not None:
+                pref = PropertyDefiner.getStringAttr(obj, "Pref")
+                if pref:
+                    bindingHeaders[prefHeader(pref)] = True
+
+        def addPrefHeadersForDictionary(bindingHeaders, dictionary):
+            while dictionary:
+                for m in dictionary.members:
+                    addPrefHeaderForObject(bindingHeaders, m)
+                dictionary = dictionary.parent
+
+        for d in dictionaries:
+            addPrefHeadersForDictionary(bindingHeaders, d)
+        for d in descriptors:
+            interface = d.interface
+            addPrefHeaderForObject(bindingHeaders, interface)
+            addPrefHeaderForObject(bindingHeaders, interface.ctor())
+
+        bindingHeaders["mozilla/dom/WebIDLPrefs.h"] = any(
+            descriptorHasPrefDisabler(d) for d in descriptors
+        )
+        bindingHeaders["nsContentUtils.h"] = (
+            any(descriptorHasChromeOnly(d) for d in descriptors)
+            or any(descriptorNeedsNonSystemPrincipal(d) for d in descriptors)
+            or any(dictionaryHasChromeOnly(d) for d in dictionaries)
+        )
+        hasNonEmptyDictionaries = any(len(dict.members) > 0 for dict in dictionaries)
+        callbacks = config.getCallbacks(webIDLFile)
+        callbackDescriptors = config.getDescriptors(
+            webIDLFile=webIDLFile, isCallback=True
+        )
+        jsImplemented = config.getDescriptors(
+            webIDLFile=webIDLFile, isJSImplemented=True
+        )
+        bindingDeclareHeaders["nsWeakReference.h"] = jsImplemented
+        bindingDeclareHeaders["mozilla/dom/PrototypeList.h"] = descriptors
+        bindingHeaders["nsIGlobalObject.h"] = jsImplemented
+        bindingHeaders["AtomList.h"] = (
+            hasNonEmptyDictionaries or jsImplemented or callbackDescriptors
+        )
+
+        if callbackDescriptors:
+            bindingDeclareHeaders["mozilla/ErrorResult.h"] = True
+
+        def descriptorClearsPropsInSlots(descriptor):
+            if not descriptor.wrapperCache:
+                return False
+            return any(
+                m.isAttr() and m.getExtendedAttribute("StoreInSlot")
+                for m in descriptor.interface.members
+            )
+
+        bindingHeaders["nsJSUtils.h"] = any(
+            descriptorClearsPropsInSlots(d) for d in descriptors
+        )
+
+        # Make sure we can sanely use binding_detail in generated code.
+        cgthings = [
+            CGGeneric(
+                dedent(
+                    """
+            namespace binding_detail {}; // Just to make sure it's known as a namespace
+            using namespace mozilla::dom::binding_detail;
+            """
+                )
+            )
+        ]
+
+        # Do codegen for all the enums
+        enums = config.getEnums(webIDLFile)
+        cgthings.extend(CGEnum(e) for e in enums)
+
+        bindingDeclareHeaders["mozilla/Span.h"] = enums
+        bindingDeclareHeaders["mozilla/ArrayUtils.h"] = enums
+
+        hasCode = descriptors or callbackDescriptors or dictionaries or callbacks
+        bindingHeaders["mozilla/dom/BindingUtils.h"] = hasCode
+        bindingHeaders["mozilla/OwningNonNull.h"] = hasCode
+        bindingHeaders["<type_traits>"] = hasCode
+        bindingHeaders["mozilla/dom/BindingDeclarations.h"] = not hasCode and enums
+
+        bindingHeaders["WrapperFactory.h"] = descriptors
+        bindingHeaders["mozilla/dom/DOMJSClass.h"] = descriptors
+        bindingHeaders["mozilla/dom/ScriptSettings.h"] = dictionaries  # AutoJSAPI
+        # Ensure we see our enums in the generated .cpp file, for the ToJSValue
+        # method body.  Also ensure that we see jsapi.h.
+        if enums:
+            bindingHeaders[CGHeaders.getDeclarationFilename(enums[0])] = True
+            bindingHeaders["jsapi.h"] = True
+
+        # For things that have [UseCounter] or [InstrumentedProps] or [Trial]
+        for d in descriptors:
+            if d.concrete:
+                if d.instrumentedProps:
+                    bindingHeaders["mozilla/UseCounter.h"] = True
+                if d.needsMissingPropUseCounters:
+                    bindingHeaders[prefHeader(MISSING_PROP_PREF)] = True
+            if d.interface.isSerializable():
+                bindingHeaders["mozilla/dom/StructuredCloneTags.h"] = True
+            if d.wantsXrays:
+                bindingHeaders["mozilla/Atomics.h"] = True
+                bindingHeaders["mozilla/dom/XrayExpandoClass.h"] = True
+                if d.wantsXrayExpandoClass:
+                    bindingHeaders["XrayWrapper.h"] = True
+            for m in d.interface.members:
+                if m.getExtendedAttribute("UseCounter"):
+                    bindingHeaders["mozilla/UseCounter.h"] = True
+                if m.getExtendedAttribute("Trial"):
+                    bindingHeaders["mozilla/OriginTrials.h"] = True
+
+        bindingHeaders["mozilla/dom/SimpleGlobalObject.h"] = any(
+            CGDictionary.dictionarySafeToJSONify(d) for d in dictionaries
+        )
+
+        for ancestor in (findAncestorWithInstrumentedProps(d) for d in descriptors):
+            if not ancestor:
+                continue
+            bindingHeaders[CGHeaders.getDeclarationFilename(ancestor)] = True
+
+        cgthings.extend(traverseMethods)
+        cgthings.extend(unlinkMethods)
+
+        # Do codegen for all the dictionaries.  We have to be a bit careful
+        # here, because we have to generate these in order from least derived
+        # to most derived so that class inheritance works out.  We also have to
+        # generate members before the dictionary that contains them.
+
+        for t in dependencySortDictionariesAndUnionsAndCallbacks(
+            dictionaries + unionStructs + callbacks
+        ):
+            if t.isDictionary():
+                cgthings.append(CGDictionary(t, config))
+            elif t.isUnion():
+                cgthings.append(CGUnionStruct(t, config))
+                cgthings.append(CGUnionStruct(t, config, True))
+            else:
+                assert t.isCallback()
+                cgthings.append(CGCallbackFunction(t, config))
+                cgthings.append(CGNamespace("binding_detail", CGFastCallback(t)))
+
+        # Do codegen for all the descriptors
+        cgthings.extend([CGDescriptor(x) for x in descriptors])
+
+        # Do codegen for all the callback interfaces.
+        cgthings.extend([CGCallbackInterface(x) for x in callbackDescriptors])
+
+        cgthings.extend(
+            [
+                CGNamespace("binding_detail", CGFastCallback(x.interface))
+                for x in callbackDescriptors
+            ]
+        )
+
+        # Do codegen for JS implemented classes
+        def getParentDescriptor(desc):
+            if not desc.interface.parent:
+                return set()
+            return {desc.getDescriptor(desc.interface.parent.identifier.name)}
+
+        for x in dependencySortObjects(
+            jsImplemented, getParentDescriptor, lambda d: d.interface.identifier.name
+        ):
+            cgthings.append(
+                CGCallbackInterface(x, spiderMonkeyInterfacesAreStructs=True)
+            )
+            cgthings.append(CGJSImplClass(x))
+
+        # And make sure we have the right number of newlines at the end
+        curr = CGWrapper(CGList(cgthings, "\n\n"), post="\n\n")
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(curr, pre="\n"))
+
+        curr = CGList(
+            [
+                CGForwardDeclarations(
+                    config,
+                    descriptors,
+                    callbacks,
+                    dictionaries,
+                    callbackDescriptors + jsImplemented,
+                    additionalDeclarations=unionDeclarations,
+                ),
+                curr,
+            ],
+            "\n",
+        )
+
+        # Add header includes.
+        bindingHeaders = [
+            header for header, include in six.iteritems(bindingHeaders) if include
+        ]
+        bindingDeclareHeaders = [
+            header
+            for header, include in six.iteritems(bindingDeclareHeaders)
+            if include
+        ]
+
+        curr = CGHeaders(
+            descriptors,
+            dictionaries,
+            callbacks,
+            callbackDescriptors,
+            bindingDeclareHeaders,
+            bindingHeaders,
+            prefix,
+            curr,
+            config,
+            jsImplemented,
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard(prefix, curr)
+
+        # Add the auto-generated comment.
+        curr = CGWrapper(
+            curr,
+            pre=(
+                AUTOGENERATED_WITH_SOURCE_WARNING_COMMENT % os.path.basename(webIDLFile)
+            ),
+        )
+
+        # Store the final result.
+        self.root = curr
+
+    def declare(self):
+        return stripTrailingWhitespace(self.root.declare())
+
+    def define(self):
+        return stripTrailingWhitespace(self.root.define())
+
+    def deps(self):
+        return self.root.deps()
+
+
+class CGNativeMember(ClassMethod):
+    def __init__(
+        self,
+        descriptorProvider,
+        member,
+        name,
+        signature,
+        extendedAttrs,
+        breakAfter=True,
+        passJSBitsAsNeeded=True,
+        visibility="public",
+        spiderMonkeyInterfacesAreStructs=True,
+        variadicIsSequence=False,
+        resultNotAddRefed=False,
+        virtual=False,
+        override=False,
+        canRunScript=False,
+    ):
+        """
+        If spiderMonkeyInterfacesAreStructs is false, SpiderMonkey interfaces
+        will be passed as JS::Handle<JSObject*>.  If it's true they will be
+        passed as one of the dom::SpiderMonkeyInterfaceObjectStorage subclasses.
+
+        If passJSBitsAsNeeded is false, we don't automatically pass in a
+        JSContext* or a JSObject* based on the return and argument types.  We
+        can still pass it based on 'implicitJSContext' annotations.
+        """
+        self.descriptorProvider = descriptorProvider
+        self.member = member
+        self.extendedAttrs = extendedAttrs
+        self.resultAlreadyAddRefed = not resultNotAddRefed
+        self.passJSBitsAsNeeded = passJSBitsAsNeeded
+        self.spiderMonkeyInterfacesAreStructs = spiderMonkeyInterfacesAreStructs
+        self.variadicIsSequence = variadicIsSequence
+        breakAfterSelf = "\n" if breakAfter else ""
+        ClassMethod.__init__(
+            self,
+            name,
+            self.getReturnType(signature[0], False),
+            self.getArgs(signature[0], signature[1]),
+            static=member.isStatic(),
+            # Mark our getters, which are attrs that
+            # have a non-void return type, as const.
+            const=(
+                not member.isStatic()
+                and member.isAttr()
+                and not signature[0].isUndefined()
+            ),
+            breakAfterReturnDecl=" ",
+            breakAfterSelf=breakAfterSelf,
+            visibility=visibility,
+            virtual=virtual,
+            override=override,
+            canRunScript=canRunScript,
+        )
+
+    def getReturnType(self, type, isMember):
+        return self.getRetvalInfo(type, isMember)[0]
+
+    def getRetvalInfo(self, type, isMember):
+        """
+        Returns a tuple:
+
+        The first element is the type declaration for the retval
+
+        The second element is a default value that can be used on error returns.
+        For cases whose behavior depends on isMember, the second element will be
+        None if isMember is true.
+
+        The third element is a template for actually returning a value stored in
+        "${declName}" and "${holderName}".  This means actually returning it if
+        we're not outparam, else assigning to the "retval" outparam.  If
+        isMember is true, this can be None, since in that case the caller will
+        never examine this value.
+        """
+        if type.isUndefined():
+            return "void", "", ""
+        if type.isPrimitive() and type.tag() in builtinNames:
+            result = CGGeneric(builtinNames[type.tag()])
+            defaultReturnArg = "0"
+            if type.nullable():
+                result = CGTemplatedType("Nullable", result)
+                defaultReturnArg = ""
+            return (
+                result.define(),
+                "%s(%s)" % (result.define(), defaultReturnArg),
+                "return ${declName};\n",
+            )
+        if type.isJSString():
+            if isMember:
+                raise TypeError("JSString not supported as return type member")
+            # Outparam
+            return "void", "", "aRetVal.set(${declName});\n"
+        if type.isDOMString() or type.isUSVString():
+            if isMember:
+                # No need for a third element in the isMember case
+                return "nsString", None, None
+            # Outparam
+            return "void", "", "aRetVal = ${declName};\n"
+        if type.isByteString() or type.isUTF8String():
+            if isMember:
+                # No need for a third element in the isMember case
+                return "nsCString", None, None
+            # Outparam
+            return "void", "", "aRetVal = ${declName};\n"
+        if type.isEnum():
+            enumName = type.unroll().inner.identifier.name
+            if type.nullable():
+                enumName = CGTemplatedType("Nullable", CGGeneric(enumName)).define()
+                defaultValue = "%s()" % enumName
+            else:
+                defaultValue = "%s(0)" % enumName
+            return enumName, defaultValue, "return ${declName};\n"
+        if type.isGeckoInterface() or type.isPromise():
+            if type.isGeckoInterface():
+                iface = type.unroll().inner
+                result = CGGeneric(
+                    self.descriptorProvider.getDescriptor(
+                        iface.identifier.name
+                    ).prettyNativeType
+                )
+            else:
+                result = CGGeneric("Promise")
+            if self.resultAlreadyAddRefed:
+                if isMember:
+                    holder = "RefPtr"
+                else:
+                    holder = "already_AddRefed"
+                if memberReturnsNewObject(self.member) or isMember:
+                    warning = ""
+                else:
+                    warning = "// Return a raw pointer here to avoid refcounting, but make sure it's safe (the object should be kept alive by the callee).\n"
+                result = CGWrapper(result, pre=("%s%s<" % (warning, holder)), post=">")
+            else:
+                result = CGWrapper(result, post="*")
+            # Since we always force an owning type for callback return values,
+            # our ${declName} is an OwningNonNull or RefPtr.  So we can just
+            # .forget() to get our already_AddRefed.
+            return result.define(), "nullptr", "return ${declName}.forget();\n"
+        if type.isCallback():
+            return (
+                "already_AddRefed<%s>" % type.unroll().callback.identifier.name,
+                "nullptr",
+                "return ${declName}.forget();\n",
+            )
+        if type.isAny():
+            if isMember:
+                # No need for a third element in the isMember case
+                return "JS::Value", None, None
+            # Outparam
+            return "void", "", "aRetVal.set(${declName});\n"
+
+        if type.isObject():
+            if isMember:
+                # No need for a third element in the isMember case
+                return "JSObject*", None, None
+            return "void", "", "aRetVal.set(${declName});\n"
+        if type.isSpiderMonkeyInterface():
+            if isMember:
+                # No need for a third element in the isMember case
+                return "JSObject*", None, None
+            if type.nullable():
+                returnCode = (
+                    "${declName}.IsNull() ? nullptr : ${declName}.Value().Obj()"
+                )
+            else:
+                returnCode = "${declName}.Obj()"
+            return "void", "", "aRetVal.set(%s);\n" % returnCode
+        if type.isSequence():
+            # If we want to handle sequence-of-sequences return values, we're
+            # going to need to fix example codegen to not produce nsTArray<void>
+            # for the relevant argument...
+            assert not isMember
+            # Outparam.
+            if type.nullable():
+                returnCode = dedent(
+                    """
+                    if (${declName}.IsNull()) {
+                      aRetVal.SetNull();
+                    } else {
+                      aRetVal.SetValue() = std::move(${declName}.Value());
+                    }
+                    """
+                )
+            else:
+                returnCode = "aRetVal = std::move(${declName});\n"
+            return "void", "", returnCode
+        if type.isRecord():
+            # If we want to handle record-of-record return values, we're
+            # going to need to fix example codegen to not produce record<void>
+            # for the relevant argument...
+            assert not isMember
+            # In this case we convert directly into our outparam to start with
+            return "void", "", ""
+        if type.isDictionary():
+            if isMember:
+                # Only the first member of the tuple matters here, but return
+                # bogus values for the others in case someone decides to use
+                # them.
+                return CGDictionary.makeDictionaryName(type.inner), None, None
+            # In this case we convert directly into our outparam to start with
+            return "void", "", ""
+        if type.isUnion():
+            if isMember:
+                # Only the first member of the tuple matters here, but return
+                # bogus values for the others in case someone decides to use
+                # them.
+                return CGUnionStruct.unionTypeDecl(type, True), None, None
+            # In this case we convert directly into our outparam to start with
+            return "void", "", ""
+
+        raise TypeError("Don't know how to declare return value for %s" % type)
+
+    def getArgs(self, returnType, argList):
+        args = [self.getArg(arg) for arg in argList]
+        # Now the outparams
+        if returnType.isJSString():
+            args.append(Argument("JS::MutableHandle<JSString*>", "aRetVal"))
+        elif returnType.isDOMString() or returnType.isUSVString():
+            args.append(Argument("nsString&", "aRetVal"))
+        elif returnType.isByteString() or returnType.isUTF8String():
+            args.append(Argument("nsCString&", "aRetVal"))
+        elif returnType.isSequence():
+            nullable = returnType.nullable()
+            if nullable:
+                returnType = returnType.inner
+            # And now the actual underlying type
+            elementDecl = self.getReturnType(returnType.inner, True)
+            type = CGTemplatedType("nsTArray", CGGeneric(elementDecl))
+            if nullable:
+                type = CGTemplatedType("Nullable", type)
+            args.append(Argument("%s&" % type.define(), "aRetVal"))
+        elif returnType.isRecord():
+            nullable = returnType.nullable()
+            if nullable:
+                returnType = returnType.inner
+            # And now the actual underlying type
+            elementDecl = self.getReturnType(returnType.inner, True)
+            type = CGTemplatedType(
+                "Record", [recordKeyDeclType(returnType), CGGeneric(elementDecl)]
+            )
+            if nullable:
+                type = CGTemplatedType("Nullable", type)
+            args.append(Argument("%s&" % type.define(), "aRetVal"))
+        elif returnType.isDictionary():
+            nullable = returnType.nullable()
+            if nullable:
+                returnType = returnType.inner
+            dictType = CGGeneric(CGDictionary.makeDictionaryName(returnType.inner))
+            if nullable:
+                dictType = CGTemplatedType("Nullable", dictType)
+            args.append(Argument("%s&" % dictType.define(), "aRetVal"))
+        elif returnType.isUnion():
+            args.append(
+                Argument(
+                    "%s&" % CGUnionStruct.unionTypeDecl(returnType, True), "aRetVal"
+                )
+            )
+        elif returnType.isAny():
+            args.append(Argument("JS::MutableHandle<JS::Value>", "aRetVal"))
+        elif returnType.isObject() or returnType.isSpiderMonkeyInterface():
+            args.append(Argument("JS::MutableHandle<JSObject*>", "aRetVal"))
+
+        # And the nsIPrincipal
+        if "needsSubjectPrincipal" in self.extendedAttrs:
+            # Cheat and assume self.descriptorProvider is a descriptor
+            if self.descriptorProvider.interface.isExposedInAnyWorker():
+                args.append(Argument("Maybe<nsIPrincipal*>", "aSubjectPrincipal"))
+            elif "needsNonSystemSubjectPrincipal" in self.extendedAttrs:
+                args.append(Argument("nsIPrincipal*", "aPrincipal"))
+            else:
+                args.append(Argument("nsIPrincipal&", "aPrincipal"))
+        # And the caller type, if desired.
+        if needsCallerType(self.member):
+            args.append(Argument("CallerType", "aCallerType"))
+        # And the ErrorResult or OOMReporter
+        if "needsErrorResult" in self.extendedAttrs:
+            # Use aRv so it won't conflict with local vars named "rv"
+            args.append(Argument("ErrorResult&", "aRv"))
+        elif "canOOM" in self.extendedAttrs:
+            args.append(Argument("OOMReporter&", "aRv"))
+
+        # The legacycaller thisval
+        if self.member.isMethod() and self.member.isLegacycaller():
+            # If it has an identifier, we can't deal with it yet
+            assert self.member.isIdentifierLess()
+            args.insert(0, Argument("const JS::Value&", "aThisVal"))
+        # And jscontext bits.
+        if needCx(
+            returnType,
+            argList,
+            self.extendedAttrs,
+            self.passJSBitsAsNeeded,
+            self.member.isStatic(),
+        ):
+            args.insert(0, Argument("JSContext*", "cx"))
+            if needScopeObject(
+                returnType,
+                argList,
+                self.extendedAttrs,
+                self.descriptorProvider.wrapperCache,
+                self.passJSBitsAsNeeded,
+                self.member.getExtendedAttribute("StoreInSlot"),
+            ):
+                args.insert(1, Argument("JS::Handle<JSObject*>", "obj"))
+        # And if we're static, a global
+        if self.member.isStatic():
+            args.insert(0, Argument("const GlobalObject&", "global"))
+        return args
+
+    def doGetArgType(self, type, optional, isMember):
+        """
+        The main work of getArgType.  Returns a string type decl, whether this
+        is a const ref, as well as whether the type should be wrapped in
+        Nullable as needed.
+
+        isMember can be false or one of the strings "Sequence", "Variadic",
+                 "Record"
+        """
+        if type.isSequence():
+            nullable = type.nullable()
+            if nullable:
+                type = type.inner
+            elementType = type.inner
+            argType = self.getArgType(elementType, False, "Sequence")[0]
+            decl = CGTemplatedType("Sequence", argType)
+            return decl.define(), True, True
+
+        if type.isRecord():
+            nullable = type.nullable()
+            if nullable:
+                type = type.inner
+            elementType = type.inner
+            argType = self.getArgType(elementType, False, "Record")[0]
+            decl = CGTemplatedType("Record", [recordKeyDeclType(type), argType])
+            return decl.define(), True, True
+
+        if type.isUnion():
+            # unionTypeDecl will handle nullable types, so return False for
+            # auto-wrapping in Nullable
+            return CGUnionStruct.unionTypeDecl(type, isMember), True, False
+
+        if type.isPromise():
+            assert not type.nullable()
+            if optional or isMember:
+                typeDecl = "OwningNonNull<Promise>"
+            else:
+                typeDecl = "Promise&"
+            return (typeDecl, False, False)
+
+        if type.isGeckoInterface() and not type.isCallbackInterface():
+            iface = type.unroll().inner
+            if iface.identifier.name == "WindowProxy":
+                return "WindowProxyHolder", True, False
+
+            argIsPointer = type.nullable() or iface.isExternal()
+            forceOwningType = iface.isCallback() or isMember
+            if argIsPointer:
+                if (optional or isMember) and forceOwningType:
+                    typeDecl = "RefPtr<%s>"
+                else:
+                    typeDecl = "%s*"
+            else:
+                if optional or isMember:
+                    if forceOwningType:
+                        typeDecl = "OwningNonNull<%s>"
+                    else:
+                        typeDecl = "NonNull<%s>"
+                else:
+                    typeDecl = "%s&"
+            return (
+                (
+                    typeDecl
+                    % self.descriptorProvider.getDescriptor(
+                        iface.identifier.name
+                    ).prettyNativeType
+                ),
+                False,
+                False,
+            )
+
+        if type.isSpiderMonkeyInterface():
+            if not self.spiderMonkeyInterfacesAreStructs:
+                return "JS::Handle<JSObject*>", False, False
+
+            # Unroll for the name, in case we're nullable.
+            return type.unroll().name, True, True
+
+        if type.isJSString():
+            if isMember:
+                raise TypeError("JSString not supported as member")
+            return "JS::Handle<JSString*>", False, False
+
+        if type.isDOMString() or type.isUSVString():
+            if isMember:
+                declType = "nsString"
+            else:
+                declType = "nsAString"
+            return declType, True, False
+
+        if type.isByteString() or type.isUTF8String():
+            # TODO(emilio): Maybe bytestrings could benefit from nsAutoCString
+            # or such too.
+            if type.isUTF8String() and not isMember:
+                declType = "nsACString"
+            else:
+                declType = "nsCString"
+            return declType, True, False
+
+        if type.isEnum():
+            return type.unroll().inner.identifier.name, False, True
+
+        if type.isCallback() or type.isCallbackInterface():
+            forceOwningType = optional or isMember
+            if type.nullable():
+                if forceOwningType:
+                    declType = "RefPtr<%s>"
+                else:
+                    declType = "%s*"
+            else:
+                if forceOwningType:
+                    declType = "OwningNonNull<%s>"
+                else:
+                    declType = "%s&"
+            if type.isCallback():
+                name = type.unroll().callback.identifier.name
+            else:
+                name = type.unroll().inner.identifier.name
+            return declType % name, False, False
+
+        if type.isAny():
+            # Don't do the rooting stuff for variadics for now
+            if isMember:
+                declType = "JS::Value"
+            else:
+                declType = "JS::Handle<JS::Value>"
+            return declType, False, False
+
+        if type.isObject():
+            if isMember:
+                declType = "JSObject*"
+            else:
+                declType = "JS::Handle<JSObject*>"
+            return declType, False, False
+
+        if type.isDictionary():
+            typeName = CGDictionary.makeDictionaryName(type.inner)
+            return typeName, True, True
+
+        assert type.isPrimitive()
+
+        return builtinNames[type.tag()], False, True
+
+    def getArgType(self, type, optional, isMember):
+        """
+        Get the type of an argument declaration.  Returns the type CGThing, and
+        whether this should be a const ref.
+
+        isMember can be False, "Sequence", or "Variadic"
+        """
+        decl, ref, handleNullable = self.doGetArgType(type, optional, isMember)
+        decl = CGGeneric(decl)
+        if handleNullable and type.nullable():
+            decl = CGTemplatedType("Nullable", decl)
+            ref = True
+        if isMember == "Variadic":
+            arrayType = "Sequence" if self.variadicIsSequence else "nsTArray"
+            decl = CGTemplatedType(arrayType, decl)
+            ref = True
+        elif optional:
+            # Note: All variadic args claim to be optional, but we can just use
+            # empty arrays to represent them not being present.
+            decl = CGTemplatedType("Optional", decl)
+            ref = True
+        return (decl, ref)
+
+    def getArg(self, arg):
+        """
+        Get the full argument declaration for an argument
+        """
+        decl, ref = self.getArgType(
+            arg.type, arg.canHaveMissingValue(), "Variadic" if arg.variadic else False
+        )
+        if ref:
+            decl = CGWrapper(decl, pre="const ", post="&")
+
+        return Argument(decl.define(), arg.identifier.name)
+
+    def arguments(self):
+        return self.member.signatures()[0][1]
+
+
+class CGExampleMethod(CGNativeMember):
+    def __init__(self, descriptor, method, signature, isConstructor, breakAfter=True):
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            method,
+            CGSpecializedMethod.makeNativeName(descriptor, method),
+            signature,
+            descriptor.getExtendedAttributes(method),
+            breakAfter=breakAfter,
+            variadicIsSequence=True,
+        )
+
+    def declare(self, cgClass):
+        assert self.member.isMethod()
+        # We skip declaring ourselves if this is a maplike/setlike/iterable
+        # method, because those get implemented automatically by the binding
+        # machinery, so the implementor of the interface doesn't have to worry
+        # about it.
+        if self.member.isMaplikeOrSetlikeOrIterableMethod():
+            return ""
+        return CGNativeMember.declare(self, cgClass)
+
+    def define(self, cgClass):
+        return ""
+
+
+class CGExampleGetter(CGNativeMember):
+    def __init__(self, descriptor, attr):
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            attr,
+            CGSpecializedGetter.makeNativeName(descriptor, attr),
+            (attr.type, []),
+            descriptor.getExtendedAttributes(attr, getter=True),
+        )
+
+    def declare(self, cgClass):
+        assert self.member.isAttr()
+        # We skip declaring ourselves if this is a maplike/setlike attr (in
+        # practice, "size"), because those get implemented automatically by the
+        # binding machinery, so the implementor of the interface doesn't have to
+        # worry about it.
+        if self.member.isMaplikeOrSetlikeAttr():
+            return ""
+        return CGNativeMember.declare(self, cgClass)
+
+    def define(self, cgClass):
+        return ""
+
+
+class CGExampleSetter(CGNativeMember):
+    def __init__(self, descriptor, attr):
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            attr,
+            CGSpecializedSetter.makeNativeName(descriptor, attr),
+            (
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [FakeArgument(attr.type)],
+            ),
+            descriptor.getExtendedAttributes(attr, setter=True),
+        )
+
+    def define(self, cgClass):
+        return ""
+
+
+class CGBindingImplClass(CGClass):
+    """
+    Common codegen for generating a C++ implementation of a WebIDL interface
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        cgMethod,
+        cgGetter,
+        cgSetter,
+        wantGetParent=True,
+        wrapMethodName="WrapObject",
+        skipStaticMethods=False,
+    ):
+        """
+        cgMethod, cgGetter and cgSetter are classes used to codegen methods,
+        getters and setters.
+        """
+        self.descriptor = descriptor
+        self._deps = descriptor.interface.getDeps()
+
+        iface = descriptor.interface
+
+        self.methodDecls = []
+
+        def appendMethod(m, isConstructor=False):
+            sigs = m.signatures()
+            for s in sigs[:-1]:
+                # Don't put a blank line after overloads, until we
+                # get to the last one.
+                self.methodDecls.append(
+                    cgMethod(descriptor, m, s, isConstructor, breakAfter=False)
+                )
+            self.methodDecls.append(cgMethod(descriptor, m, sigs[-1], isConstructor))
+
+        if iface.ctor():
+            appendMethod(iface.ctor(), isConstructor=True)
+        for n in iface.legacyFactoryFunctions:
+            appendMethod(n, isConstructor=True)
+        for m in iface.members:
+            if m.isMethod():
+                if m.isIdentifierLess():
+                    continue
+                if m.isMaplikeOrSetlikeOrIterableMethod():
+                    # Handled by generated code already
+                    continue
+                if not m.isStatic() or not skipStaticMethods:
+                    appendMethod(m)
+            elif m.isAttr():
+                if m.isMaplikeOrSetlikeAttr() or m.type.isObservableArray():
+                    # Handled by generated code already
+                    continue
+                self.methodDecls.append(cgGetter(descriptor, m))
+                if not m.readonly:
+                    self.methodDecls.append(cgSetter(descriptor, m))
+
+        # Now do the special operations
+        def appendSpecialOperation(name, op):
+            if op is None:
+                return
+            assert len(op.signatures()) == 1
+            returnType, args = op.signatures()[0]
+            # Make a copy of the args, since we plan to modify them.
+            args = list(args)
+            if op.isGetter() or op.isDeleter():
+                # This is a total hack.  The '&' belongs with the
+                # type, not the name!  But it works, and is simpler
+                # than trying to somehow make this pretty.
+                args.append(
+                    FakeArgument(
+                        BuiltinTypes[IDLBuiltinType.Types.boolean], name="&found"
+                    )
+                )
+            if name == "Stringifier":
+                if op.isIdentifierLess():
+                    # XXXbz I wish we were consistent about our renaming here.
+                    name = "Stringify"
+                else:
+                    # We already added this method
+                    return
+            if name == "LegacyCaller":
+                if op.isIdentifierLess():
+                    # XXXbz I wish we were consistent about our renaming here.
+                    name = "LegacyCall"
+                else:
+                    # We already added this method
+                    return
+            self.methodDecls.append(
+                CGNativeMember(
+                    descriptor,
+                    op,
+                    name,
+                    (returnType, args),
+                    descriptor.getExtendedAttributes(op),
+                )
+            )
+
+        # Sort things by name so we get stable ordering in the output.
+        ops = sorted(descriptor.operations.items(), key=lambda x: x[0])
+        for name, op in ops:
+            appendSpecialOperation(name, op)
+        # If we support indexed properties, then we need a Length()
+        # method so we know which indices are supported.
+        if descriptor.supportsIndexedProperties():
+            # But we don't need it if we already have an infallible
+            # "length" attribute, which we often do.
+            haveLengthAttr = any(
+                m
+                for m in iface.members
+                if m.isAttr()
+                and CGSpecializedGetter.makeNativeName(descriptor, m) == "Length"
+            )
+            if not haveLengthAttr:
+                self.methodDecls.append(
+                    CGNativeMember(
+                        descriptor,
+                        FakeMember(),
+                        "Length",
+                        (BuiltinTypes[IDLBuiltinType.Types.unsigned_long], []),
+                        [],
+                    ),
+                )
+        # And if we support named properties we need to be able to
+        # enumerate the supported names.
+        if descriptor.supportsNamedProperties():
+            self.methodDecls.append(
+                CGNativeMember(
+                    descriptor,
+                    FakeMember(),
+                    "GetSupportedNames",
+                    (
+                        IDLSequenceType(
+                            None, BuiltinTypes[IDLBuiltinType.Types.domstring]
+                        ),
+                        [],
+                    ),
+                    [],
+                )
+            )
+
+        if descriptor.concrete:
+            wrapArgs = [
+                Argument("JSContext*", "aCx"),
+                Argument("JS::Handle<JSObject*>", "aGivenProto"),
+            ]
+            if not descriptor.wrapperCache:
+                wrapReturnType = "bool"
+                wrapArgs.append(Argument("JS::MutableHandle<JSObject*>", "aReflector"))
+            else:
+                wrapReturnType = "JSObject*"
+            self.methodDecls.insert(
+                0,
+                ClassMethod(
+                    wrapMethodName,
+                    wrapReturnType,
+                    wrapArgs,
+                    virtual=descriptor.wrapperCache,
+                    breakAfterReturnDecl=" ",
+                    override=descriptor.wrapperCache,
+                    body=self.getWrapObjectBody(),
+                ),
+            )
+        if descriptor.hasCEReactions():
+            self.methodDecls.insert(
+                0,
+                ClassMethod(
+                    "GetDocGroup",
+                    "DocGroup*",
+                    [],
+                    const=True,
+                    breakAfterReturnDecl=" ",
+                    body=self.getGetDocGroupBody(),
+                ),
+            )
+        if wantGetParent:
+            self.methodDecls.insert(
+                0,
+                ClassMethod(
+                    "GetParentObject",
+                    self.getGetParentObjectReturnType(),
+                    [],
+                    const=True,
+                    breakAfterReturnDecl=" ",
+                    body=self.getGetParentObjectBody(),
+                ),
+            )
+
+        # Invoke  CGClass.__init__ in any subclasses afterwards to do the actual codegen.
+
+    def getWrapObjectBody(self):
+        return None
+
+    def getGetParentObjectReturnType(self):
+        # The lack of newline before the end of the string is on purpose.
+        return dedent(
+            """
+            // This should return something that eventually allows finding a
+            // path to the global this object is associated with.  Most simply,
+            // returning an actual global works.
+            nsIGlobalObject*"""
+        )
+
+    def getGetParentObjectBody(self):
+        return None
+
+    def getGetDocGroupBody(self):
+        return None
+
+    def deps(self):
+        return self._deps
+
+
+class CGExampleObservableArrayCallback(CGNativeMember):
+    def __init__(self, descriptor, attr, callbackName):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            attr,
+            self.makeNativeName(attr, callbackName),
+            (
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [
+                    FakeArgument(attr.type.inner, "aValue"),
+                    FakeArgument(
+                        BuiltinTypes[IDLBuiltinType.Types.unsigned_long], "aIndex"
+                    ),
+                ],
+            ),
+            ["needsErrorResult"],
+        )
+
+    def declare(self, cgClass):
+        assert self.member.isAttr()
+        assert self.member.type.isObservableArray()
+        return CGNativeMember.declare(self, cgClass)
+
+    def define(self, cgClass):
+        return ""
+
+    @staticmethod
+    def makeNativeName(attr, callbackName):
+        assert attr.isAttr()
+        nativeName = MakeNativeName(attr.identifier.name)
+        return "On" + callbackName + nativeName
+
+
+class CGExampleClass(CGBindingImplClass):
+    """
+    Codegen for the actual example class implementation for this descriptor
+    """
+
+    def __init__(self, descriptor):
+        CGBindingImplClass.__init__(
+            self,
+            descriptor,
+            CGExampleMethod,
+            CGExampleGetter,
+            CGExampleSetter,
+            wantGetParent=descriptor.wrapperCache,
+        )
+
+        self.parentIface = descriptor.interface.parent
+        if self.parentIface:
+            self.parentDesc = descriptor.getDescriptor(self.parentIface.identifier.name)
+            bases = [ClassBase(self.nativeLeafName(self.parentDesc))]
+        else:
+            bases = [
+                ClassBase(
+                    "nsISupports /* or NonRefcountedDOMObject if this is a non-refcounted object */"
+                )
+            ]
+            if descriptor.wrapperCache:
+                bases.append(
+                    ClassBase(
+                        "nsWrapperCache /* Change wrapperCache in the binding configuration if you don't want this */"
+                    )
+                )
+
+        destructorVisibility = "protected"
+        if self.parentIface:
+            extradeclarations = (
+                "public:\n"
+                "  NS_DECL_ISUPPORTS_INHERITED\n"
+                "  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(%s, %s)\n"
+                "\n"
+                % (
+                    self.nativeLeafName(descriptor),
+                    self.nativeLeafName(self.parentDesc),
+                )
+            )
+        else:
+            extradeclarations = (
+                "public:\n"
+                "  NS_DECL_CYCLE_COLLECTING_ISUPPORTS\n"
+                "  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(%s)\n"
+                "\n" % self.nativeLeafName(descriptor)
+            )
+
+        if descriptor.interface.hasChildInterfaces():
+            decorators = ""
+        else:
+            decorators = "final"
+
+        for m in descriptor.interface.members:
+            if m.isAttr() and m.type.isObservableArray():
+                self.methodDecls.append(
+                    CGExampleObservableArrayCallback(descriptor, m, "Set")
+                )
+                self.methodDecls.append(
+                    CGExampleObservableArrayCallback(descriptor, m, "Delete")
+                )
+
+        CGClass.__init__(
+            self,
+            self.nativeLeafName(descriptor),
+            bases=bases,
+            constructors=[ClassConstructor([], visibility="public")],
+            destructor=ClassDestructor(visibility=destructorVisibility),
+            methods=self.methodDecls,
+            decorators=decorators,
+            extradeclarations=extradeclarations,
+        )
+
+    def define(self):
+        # Just override CGClass and do our own thing
+        nativeType = self.nativeLeafName(self.descriptor)
+
+        ctordtor = fill(
+            """
+            ${nativeType}::${nativeType}()
+            {
+                // Add |MOZ_COUNT_CTOR(${nativeType});| for a non-refcounted object.
+            }
+
+            ${nativeType}::~${nativeType}()
+            {
+                // Add |MOZ_COUNT_DTOR(${nativeType});| for a non-refcounted object.
+            }
+            """,
+            nativeType=nativeType,
+        )
+
+        if self.parentIface:
+            ccImpl = fill(
+                """
+
+                // Only needed for refcounted objects.
+                # error "If you don't have members that need cycle collection,
+                # then remove all the cycle collection bits from this
+                # implementation and the corresponding header.  If you do, you
+                # want NS_IMPL_CYCLE_COLLECTION_INHERITED(${nativeType},
+                # ${parentType}, your, members, here)"
+                NS_IMPL_ADDREF_INHERITED(${nativeType}, ${parentType})
+                NS_IMPL_RELEASE_INHERITED(${nativeType}, ${parentType})
+                NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(${nativeType})
+                NS_INTERFACE_MAP_END_INHERITING(${parentType})
+
+                """,
+                nativeType=nativeType,
+                parentType=self.nativeLeafName(self.parentDesc),
+            )
+        else:
+            ccImpl = fill(
+                """
+
+                // Only needed for refcounted objects.
+                NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(${nativeType})
+                NS_IMPL_CYCLE_COLLECTING_ADDREF(${nativeType})
+                NS_IMPL_CYCLE_COLLECTING_RELEASE(${nativeType})
+                NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(${nativeType})
+                  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+                  NS_INTERFACE_MAP_ENTRY(nsISupports)
+                NS_INTERFACE_MAP_END
+
+                """,
+                nativeType=nativeType,
+            )
+
+        classImpl = ccImpl + ctordtor + "\n"
+        if self.descriptor.concrete:
+            if self.descriptor.wrapperCache:
+                reflectorArg = ""
+                reflectorPassArg = ""
+                returnType = "JSObject*"
+            else:
+                reflectorArg = ", JS::MutableHandle<JSObject*> aReflector"
+                reflectorPassArg = ", aReflector"
+                returnType = "bool"
+            classImpl += fill(
+                """
+                ${returnType}
+                ${nativeType}::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto${reflectorArg})
+                {
+                  return ${ifaceName}_Binding::Wrap(aCx, this, aGivenProto${reflectorPassArg});
+                }
+
+                """,
+                returnType=returnType,
+                nativeType=nativeType,
+                reflectorArg=reflectorArg,
+                ifaceName=self.descriptor.name,
+                reflectorPassArg=reflectorPassArg,
+            )
+
+        return classImpl
+
+    @staticmethod
+    def nativeLeafName(descriptor):
+        return descriptor.nativeType.split("::")[-1]
+
+
+class CGExampleRoot(CGThing):
+    """
+    Root codegen class for example implementation generation.  Instantiate the
+    class and call declare or define to generate header or cpp code,
+    respectively.
+    """
+
+    def __init__(self, config, interfaceName):
+        descriptor = config.getDescriptor(interfaceName)
+
+        self.root = CGWrapper(CGExampleClass(descriptor), pre="\n", post="\n")
+
+        self.root = CGNamespace.build(["mozilla", "dom"], self.root)
+
+        builder = ForwardDeclarationBuilder()
+        if descriptor.hasCEReactions():
+            builder.addInMozillaDom("DocGroup")
+        for member in descriptor.interface.members:
+            if not member.isAttr() and not member.isMethod():
+                continue
+            if member.isStatic():
+                builder.addInMozillaDom("GlobalObject")
+            if member.isAttr():
+                if not member.isMaplikeOrSetlikeAttr():
+                    builder.forwardDeclareForType(member.type, config)
+            else:
+                assert member.isMethod()
+                if not member.isMaplikeOrSetlikeOrIterableMethod():
+                    for sig in member.signatures():
+                        builder.forwardDeclareForType(sig[0], config)
+                        for arg in sig[1]:
+                            builder.forwardDeclareForType(arg.type, config)
+
+        self.root = CGList([builder.build(), self.root], "\n")
+
+        # Throw in our #includes
+        self.root = CGHeaders(
+            [],
+            [],
+            [],
+            [],
+            [
+                "nsWrapperCache.h",
+                "nsCycleCollectionParticipant.h",
+                "mozilla/Attributes.h",
+                "mozilla/ErrorResult.h",
+                "mozilla/dom/BindingDeclarations.h",
+                "js/TypeDecls.h",
+            ],
+            [
+                "mozilla/dom/%s.h" % interfaceName,
+                (
+                    "mozilla/dom/%s"
+                    % CGHeaders.getDeclarationFilename(descriptor.interface)
+                ),
+            ],
+            "",
+            self.root,
+        )
+
+        # And now some include guards
+        self.root = CGIncludeGuard(interfaceName, self.root)
+
+        # And our license block comes before everything else
+        self.root = CGWrapper(
+            self.root,
+            pre=dedent(
+                """
+            /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+            /* vim:set ts=2 sw=2 sts=2 et cindent: */
+            /* This Source Code Form is subject to the terms of the Mozilla Public
+             * License, v. 2.0. If a copy of the MPL was not distributed with this
+             * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+            """
+            ),
+        )
+
+    def declare(self):
+        return self.root.declare()
+
+    def define(self):
+        return self.root.define()
+
+
+def jsImplName(name):
+    return name + "JSImpl"
+
+
+class CGJSImplMember(CGNativeMember):
+    """
+    Base class for generating code for the members of the implementation class
+    for a JS-implemented WebIDL interface.
+    """
+
+    def __init__(
+        self,
+        descriptorProvider,
+        member,
+        name,
+        signature,
+        extendedAttrs,
+        breakAfter=True,
+        passJSBitsAsNeeded=True,
+        visibility="public",
+        variadicIsSequence=False,
+        virtual=False,
+        override=False,
+    ):
+        CGNativeMember.__init__(
+            self,
+            descriptorProvider,
+            member,
+            name,
+            signature,
+            extendedAttrs,
+            breakAfter=breakAfter,
+            passJSBitsAsNeeded=passJSBitsAsNeeded,
+            visibility=visibility,
+            variadicIsSequence=variadicIsSequence,
+            virtual=virtual,
+            override=override,
+        )
+        self.body = self.getImpl()
+
+    def getArgs(self, returnType, argList):
+        args = CGNativeMember.getArgs(self, returnType, argList)
+        args.append(Argument("JS::Realm*", "aRealm", "nullptr"))
+        return args
+
+
+class CGJSImplMethod(CGJSImplMember):
+    """
+    Class for generating code for the methods for a JS-implemented WebIDL
+    interface.
+    """
+
+    def __init__(self, descriptor, method, signature, isConstructor, breakAfter=True):
+        self.signature = signature
+        self.descriptor = descriptor
+        self.isConstructor = isConstructor
+        CGJSImplMember.__init__(
+            self,
+            descriptor,
+            method,
+            CGSpecializedMethod.makeNativeName(descriptor, method),
+            signature,
+            descriptor.getExtendedAttributes(method),
+            breakAfter=breakAfter,
+            variadicIsSequence=True,
+            passJSBitsAsNeeded=False,
+        )
+
+    def getArgs(self, returnType, argList):
+        if self.isConstructor:
+            # Skip the JS::Compartment bits for constructors; it's handled
+            # manually in getImpl.  But we do need our aGivenProto argument.  We
+            # allow it to be omitted if the default proto is desired.
+            return CGNativeMember.getArgs(self, returnType, argList) + [
+                Argument("JS::Handle<JSObject*>", "aGivenProto", "nullptr")
+            ]
+        return CGJSImplMember.getArgs(self, returnType, argList)
+
+    def getImpl(self):
+        args = self.getArgs(self.signature[0], self.signature[1])
+        if not self.isConstructor:
+            return "return mImpl->%s(%s);\n" % (
+                self.name,
+                ", ".join(arg.name for arg in args),
+            )
+
+        assert self.descriptor.interface.isJSImplemented()
+        if self.name != "Constructor":
+            raise TypeError(
+                "Named constructors are not supported for JS implemented WebIDL. See bug 851287."
+            )
+        if len(self.signature[1]) != 0:
+            # The first two arguments to the constructor implementation are not
+            # arguments to the WebIDL constructor, so don't pass them to
+            # __Init().  The last argument is the prototype we're supposed to
+            # use, and shouldn't get passed to __Init() either.
+            assert args[0].argType == "const GlobalObject&"
+            assert args[1].argType == "JSContext*"
+            assert args[-1].argType == "JS::Handle<JSObject*>"
+            assert args[-1].name == "aGivenProto"
+            constructorArgs = [arg.name for arg in args[2:-1]]
+            constructorArgs.append("js::GetNonCCWObjectRealm(scopeObj)")
+            initCall = fill(
+                """
+                // Wrap the object before calling __Init so that __DOM_IMPL__ is available.
+                JS::Rooted<JSObject*> scopeObj(cx, global.Get());
+                MOZ_ASSERT(js::IsObjectInContextCompartment(scopeObj, cx));
+                JS::Rooted<JS::Value> wrappedVal(cx);
+                if (!GetOrCreateDOMReflector(cx, impl, &wrappedVal, aGivenProto)) {
+                  MOZ_ASSERT(JS_IsExceptionPending(cx));
+                  aRv.Throw(NS_ERROR_UNEXPECTED);
+                  return nullptr;
+                }
+                // Initialize the object with the constructor arguments.
+                impl->mImpl->__Init(${args});
+                if (aRv.Failed()) {
+                  return nullptr;
+                }
+                """,
+                args=", ".join(constructorArgs),
+            )
+        else:
+            initCall = ""
+        return fill(
+            """
+            RefPtr<${implClass}> impl =
+              ConstructJSImplementation<${implClass}>("${contractId}", global, aRv);
+            if (aRv.Failed()) {
+              return nullptr;
+            }
+            $*{initCall}
+            return impl.forget();
+            """,
+            contractId=self.descriptor.interface.getJSImplementation(),
+            implClass=self.descriptor.name,
+            initCall=initCall,
+        )
+
+
+# We're always fallible
+def callbackGetterName(attr, descriptor):
+    return "Get" + MakeNativeName(
+        descriptor.binaryNameFor(attr.identifier.name, attr.isStatic())
+    )
+
+
+def callbackSetterName(attr, descriptor):
+    return "Set" + MakeNativeName(
+        descriptor.binaryNameFor(attr.identifier.name, attr.isStatic())
+    )
+
+
+class CGJSImplGetter(CGJSImplMember):
+    """
+    Class for generating code for the getters of attributes for a JS-implemented
+    WebIDL interface.
+    """
+
+    def __init__(self, descriptor, attr):
+        CGJSImplMember.__init__(
+            self,
+            descriptor,
+            attr,
+            CGSpecializedGetter.makeNativeName(descriptor, attr),
+            (attr.type, []),
+            descriptor.getExtendedAttributes(attr, getter=True),
+            passJSBitsAsNeeded=False,
+        )
+
+    def getImpl(self):
+        callbackArgs = [arg.name for arg in self.getArgs(self.member.type, [])]
+        return "return mImpl->%s(%s);\n" % (
+            callbackGetterName(self.member, self.descriptorProvider),
+            ", ".join(callbackArgs),
+        )
+
+
+class CGJSImplSetter(CGJSImplMember):
+    """
+    Class for generating code for the setters of attributes for a JS-implemented
+    WebIDL interface.
+    """
+
+    def __init__(self, descriptor, attr):
+        CGJSImplMember.__init__(
+            self,
+            descriptor,
+            attr,
+            CGSpecializedSetter.makeNativeName(descriptor, attr),
+            (
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [FakeArgument(attr.type)],
+            ),
+            descriptor.getExtendedAttributes(attr, setter=True),
+            passJSBitsAsNeeded=False,
+        )
+
+    def getImpl(self):
+        callbackArgs = [
+            arg.name
+            for arg in self.getArgs(
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [FakeArgument(self.member.type)],
+            )
+        ]
+        return "mImpl->%s(%s);\n" % (
+            callbackSetterName(self.member, self.descriptorProvider),
+            ", ".join(callbackArgs),
+        )
+
+
+class CGJSImplClass(CGBindingImplClass):
+    def __init__(self, descriptor):
+        CGBindingImplClass.__init__(
+            self,
+            descriptor,
+            CGJSImplMethod,
+            CGJSImplGetter,
+            CGJSImplSetter,
+            skipStaticMethods=True,
+        )
+
+        if descriptor.interface.parent:
+            parentClass = descriptor.getDescriptor(
+                descriptor.interface.parent.identifier.name
+            ).jsImplParent
+            baseClasses = [ClassBase(parentClass)]
+            isupportsDecl = "NS_DECL_ISUPPORTS_INHERITED\n"
+            ccDecl = "NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(%s, %s)\n" % (
+                descriptor.name,
+                parentClass,
+            )
+            extradefinitions = fill(
+                """
+                NS_IMPL_CYCLE_COLLECTION_INHERITED(${ifaceName}, ${parentClass}, mImpl, mParent)
+                NS_IMPL_ADDREF_INHERITED(${ifaceName}, ${parentClass})
+                NS_IMPL_RELEASE_INHERITED(${ifaceName}, ${parentClass})
+                NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(${ifaceName})
+                NS_INTERFACE_MAP_END_INHERITING(${parentClass})
+                """,
+                ifaceName=self.descriptor.name,
+                parentClass=parentClass,
+            )
+        else:
+            baseClasses = [
+                ClassBase("nsSupportsWeakReference"),
+                ClassBase("nsWrapperCache"),
+            ]
+            isupportsDecl = "NS_DECL_CYCLE_COLLECTING_ISUPPORTS\n"
+            ccDecl = (
+                "NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(%s)\n" % descriptor.name
+            )
+            extradefinitions = fill(
+                """
+                NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(${ifaceName})
+                NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(${ifaceName})
+                  NS_IMPL_CYCLE_COLLECTION_UNLINK(mImpl)
+                  NS_IMPL_CYCLE_COLLECTION_UNLINK(mParent)
+                  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+                  tmp->ClearWeakReferences();
+                NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+                NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(${ifaceName})
+                  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mImpl)
+                  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParent)
+                NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+                NS_IMPL_CYCLE_COLLECTING_ADDREF(${ifaceName})
+                NS_IMPL_CYCLE_COLLECTING_RELEASE(${ifaceName})
+                NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(${ifaceName})
+                  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+                  NS_INTERFACE_MAP_ENTRY(nsISupports)
+                  NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
+                NS_INTERFACE_MAP_END
+                """,
+                ifaceName=self.descriptor.name,
+            )
+
+        extradeclarations = fill(
+            """
+            public:
+              $*{isupportsDecl}
+              $*{ccDecl}
+
+            private:
+              RefPtr<${jsImplName}> mImpl;
+              nsCOMPtr<nsIGlobalObject> mParent;
+
+            """,
+            isupportsDecl=isupportsDecl,
+            ccDecl=ccDecl,
+            jsImplName=jsImplName(descriptor.name),
+        )
+
+        if descriptor.interface.getExtendedAttribute("WantsEventListenerHooks"):
+            # No need to do too much sanity checking here; the
+            # generated code will fail to compile if the methods we
+            # try to overrid aren't on a superclass.
+            self.methodDecls.extend(
+                self.getEventHookMethod(parentClass, "EventListenerAdded")
+            )
+            self.methodDecls.extend(
+                self.getEventHookMethod(parentClass, "EventListenerRemoved")
+            )
+
+        if descriptor.interface.hasChildInterfaces():
+            decorators = ""
+            # We need a protected virtual destructor our subclasses can use
+            destructor = ClassDestructor(virtual=True, visibility="protected")
+        else:
+            decorators = "final"
+            destructor = ClassDestructor(virtual=False, visibility="private")
+
+        baseConstructors = [
+            (
+                "mImpl(new %s(nullptr, aJSImplObject, aJSImplGlobal, /* aIncumbentGlobal = */ nullptr))"
+                % jsImplName(descriptor.name)
+            ),
+            "mParent(aParent)",
+        ]
+        parentInterface = descriptor.interface.parent
+        while parentInterface:
+            if parentInterface.isJSImplemented():
+                baseConstructors.insert(
+                    0, "%s(aJSImplObject, aJSImplGlobal, aParent)" % parentClass
+                )
+                break
+            parentInterface = parentInterface.parent
+        if not parentInterface and descriptor.interface.parent:
+            # We only have C++ ancestors, so only pass along the window
+            baseConstructors.insert(0, "%s(aParent)" % parentClass)
+
+        constructor = ClassConstructor(
+            [
+                Argument("JS::Handle<JSObject*>", "aJSImplObject"),
+                Argument("JS::Handle<JSObject*>", "aJSImplGlobal"),
+                Argument("nsIGlobalObject*", "aParent"),
+            ],
+            visibility="public",
+            baseConstructors=baseConstructors,
+        )
+
+        self.methodDecls.append(
+            ClassMethod(
+                "_Create",
+                "bool",
+                JSNativeArguments(),
+                static=True,
+                body=self.getCreateFromExistingBody(),
+            )
+        )
+
+        CGClass.__init__(
+            self,
+            descriptor.name,
+            bases=baseClasses,
+            constructors=[constructor],
+            destructor=destructor,
+            methods=self.methodDecls,
+            decorators=decorators,
+            extradeclarations=extradeclarations,
+            extradefinitions=extradefinitions,
+        )
+
+    def getWrapObjectBody(self):
+        return fill(
+            """
+            JS::Rooted<JSObject*> obj(aCx, ${name}_Binding::Wrap(aCx, this, aGivenProto));
+            if (!obj) {
+              return nullptr;
+            }
+
+            // Now define it on our chrome object
+            JSAutoRealm ar(aCx, mImpl->CallbackGlobalOrNull());
+            if (!JS_WrapObject(aCx, &obj)) {
+              return nullptr;
+            }
+            JS::Rooted<JSObject*> callback(aCx, mImpl->CallbackOrNull());
+            if (!JS_DefineProperty(aCx, callback, "__DOM_IMPL__", obj, 0)) {
+              return nullptr;
+            }
+            return obj;
+            """,
+            name=self.descriptor.name,
+        )
+
+    def getGetParentObjectReturnType(self):
+        return "nsISupports*"
+
+    def getGetParentObjectBody(self):
+        return "return mParent;\n"
+
+    def getGetDocGroupBody(self):
+        return dedent(
+            """
+            nsCOMPtr<nsPIDOMWindowInner> window = do_QueryInterface(mParent);
+            if (!window) {
+              return nullptr;
+            }
+            return window->GetDocGroup();
+            """
+        )
+
+    def getCreateFromExistingBody(self):
+        # XXXbz we could try to get parts of this (e.g. the argument
+        # conversions) auto-generated by somehow creating an IDLMethod and
+        # adding it to our interface, but we'd still need to special-case the
+        # implementation slightly to have it not try to forward to the JS
+        # object...
+        return fill(
+            """
+            JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+            if (!args.requireAtLeast(cx, "${ifaceName}._create", 2)) {
+              return false;
+            }
+            BindingCallContext callCx(cx, "${ifaceName}._create");
+            if (!args[0].isObject()) {
+              return callCx.ThrowErrorMessage<MSG_NOT_OBJECT>("Argument 1");
+            }
+            if (!args[1].isObject()) {
+              return callCx.ThrowErrorMessage<MSG_NOT_OBJECT>("Argument 2");
+            }
+
+            // GlobalObject will go through wrappers as needed for us, and
+            // is simpler than the right UnwrapArg incantation.
+            GlobalObject global(cx, &args[0].toObject());
+            if (global.Failed()) {
+              return false;
+            }
+            nsCOMPtr<nsIGlobalObject> globalHolder = do_QueryInterface(global.GetAsSupports());
+            MOZ_ASSERT(globalHolder);
+            JS::Rooted<JSObject*> arg(cx, &args[1].toObject());
+            JS::Rooted<JSObject*> argGlobal(cx, JS::CurrentGlobalOrNull(cx));
+            RefPtr<${implName}> impl = new ${implName}(arg, argGlobal, globalHolder);
+            MOZ_ASSERT(js::IsObjectInContextCompartment(arg, cx));
+            return GetOrCreateDOMReflector(cx, impl, args.rval());
+            """,
+            ifaceName=self.descriptor.interface.identifier.name,
+            implName=self.descriptor.name,
+        )
+
+    def getEventHookMethod(self, parentClass, methodName):
+        body = fill(
+            """
+            ${parentClass}::${methodName}(aType);
+            mImpl->${methodName}(Substring(nsDependentAtomString(aType), 2), IgnoreErrors());
+            """,
+            parentClass=parentClass,
+            methodName=methodName,
+        )
+        return [
+            ClassMethod(
+                methodName,
+                "void",
+                [Argument("nsAtom*", "aType")],
+                virtual=True,
+                override=True,
+                body=body,
+            ),
+            ClassUsingDeclaration(parentClass, methodName),
+        ]
+
+
+def isJSImplementedDescriptor(descriptorProvider):
+    return (
+        isinstance(descriptorProvider, Descriptor)
+        and descriptorProvider.interface.isJSImplemented()
+    )
+
+
+class CGCallback(CGClass):
+    def __init__(
+        self, idlObject, descriptorProvider, baseName, methods, getters=[], setters=[]
+    ):
+        self.baseName = baseName
+        self._deps = idlObject.getDeps()
+        self.idlObject = idlObject
+        self.name = idlObject.identifier.name
+        if isJSImplementedDescriptor(descriptorProvider):
+            self.name = jsImplName(self.name)
+        # For our public methods that needThisHandling we want most of the
+        # same args and the same return type as what CallbackMember
+        # generates.  So we want to take advantage of all its
+        # CGNativeMember infrastructure, but that infrastructure can't deal
+        # with templates and most especially template arguments.  So just
+        # cheat and have CallbackMember compute all those things for us.
+        realMethods = []
+        for method in methods:
+            if not isinstance(method, CallbackMember) or not method.needThisHandling:
+                realMethods.append(method)
+            else:
+                realMethods.extend(self.getMethodImpls(method))
+        realMethods.append(
+            ClassMethod(
+                "operator==",
+                "bool",
+                [Argument("const %s&" % self.name, "aOther")],
+                inline=True,
+                bodyInHeader=True,
+                const=True,
+                body=("return %s::operator==(aOther);\n" % baseName),
+            )
+        )
+        CGClass.__init__(
+            self,
+            self.name,
+            bases=[ClassBase(baseName)],
+            constructors=self.getConstructors(),
+            methods=realMethods + getters + setters,
+        )
+
+    def getConstructors(self):
+        if (
+            not self.idlObject.isInterface()
+            and not self.idlObject._treatNonObjectAsNull
+        ):
+            body = "MOZ_ASSERT(JS::IsCallable(mCallback));\n"
+        else:
+            # Not much we can assert about it, other than not being null, and
+            # CallbackObject does that already.
+            body = ""
+        return [
+            ClassConstructor(
+                [
+                    Argument("JSContext*", "aCx"),
+                    Argument("JS::Handle<JSObject*>", "aCallback"),
+                    Argument("JS::Handle<JSObject*>", "aCallbackGlobal"),
+                    Argument("nsIGlobalObject*", "aIncumbentGlobal"),
+                ],
+                bodyInHeader=True,
+                visibility="public",
+                explicit=True,
+                baseConstructors=[
+                    "%s(aCx, aCallback, aCallbackGlobal, aIncumbentGlobal)"
+                    % self.baseName,
+                ],
+                body=body,
+            ),
+            ClassConstructor(
+                [
+                    Argument("JSObject*", "aCallback"),
+                    Argument("JSObject*", "aCallbackGlobal"),
+                    Argument("const FastCallbackConstructor&", ""),
+                ],
+                bodyInHeader=True,
+                visibility="public",
+                explicit=True,
+                baseConstructors=[
+                    "%s(aCallback, aCallbackGlobal, FastCallbackConstructor())"
+                    % self.baseName,
+                ],
+                body=body,
+            ),
+            ClassConstructor(
+                [
+                    Argument("JSObject*", "aCallback"),
+                    Argument("JSObject*", "aCallbackGlobal"),
+                    Argument("JSObject*", "aAsyncStack"),
+                    Argument("nsIGlobalObject*", "aIncumbentGlobal"),
+                ],
+                bodyInHeader=True,
+                visibility="public",
+                explicit=True,
+                baseConstructors=[
+                    "%s(aCallback, aCallbackGlobal, aAsyncStack, aIncumbentGlobal)"
+                    % self.baseName,
+                ],
+                body=body,
+            ),
+        ]
+
+    def getMethodImpls(self, method):
+        assert method.needThisHandling
+        args = list(method.args)
+        # Strip out the BindingCallContext&/JSObject* args
+        # that got added.
+        assert args[0].name == "cx" and args[0].argType == "BindingCallContext&"
+        assert args[1].name == "aThisVal" and args[1].argType == "JS::Handle<JS::Value>"
+        args = args[2:]
+
+        # Now remember which index the ErrorResult argument is at;
+        # we'll need this below.
+        assert args[-1].name == "aRv" and args[-1].argType == "ErrorResult&"
+        rvIndex = len(args) - 1
+        assert rvIndex >= 0
+
+        # Record the names of all the arguments, so we can use them when we call
+        # the private method.
+        argnames = [arg.name for arg in args]
+        argnamesWithThis = ["s.GetCallContext()", "thisValJS"] + argnames
+        argnamesWithoutThis = [
+            "s.GetCallContext()",
+            "JS::UndefinedHandleValue",
+        ] + argnames
+        # Now that we've recorded the argnames for our call to our private
+        # method, insert our optional argument for the execution reason.
+        args.append(Argument("const char*", "aExecutionReason", "nullptr"))
+
+        # Make copies of the arg list for the two "without rv" overloads.  Note
+        # that those don't need aExceptionHandling or aRealm arguments because
+        # those would make not sense anyway: the only sane thing to do with
+        # exceptions in the "without rv" cases is to report them.
+        argsWithoutRv = list(args)
+        argsWithoutRv.pop(rvIndex)
+        argsWithoutThisAndRv = list(argsWithoutRv)
+
+        # Add the potional argument for deciding whether the CallSetup should
+        # re-throw exceptions on aRv.
+        args.append(
+            Argument("ExceptionHandling", "aExceptionHandling", "eReportExceptions")
+        )
+        # And the argument for communicating when exceptions should really be
+        # rethrown.  In particular, even when aExceptionHandling is
+        # eRethrowExceptions they won't get rethrown if aRealm is provided
+        # and its principal doesn't subsume either the callback or the
+        # exception.
+        args.append(Argument("JS::Realm*", "aRealm", "nullptr"))
+        # And now insert our template argument.
+        argsWithoutThis = list(args)
+        args.insert(0, Argument("const T&", "thisVal"))
+        argsWithoutRv.insert(0, Argument("const T&", "thisVal"))
+
+        argnamesWithoutThisAndRv = [arg.name for arg in argsWithoutThisAndRv]
+        argnamesWithoutThisAndRv.insert(rvIndex, "IgnoreErrors()")
+        # If we just leave things like that, and have no actual arguments in the
+        # IDL, we will end up trying to call the templated "without rv" overload
+        # with "rv" as the thisVal.  That's no good.  So explicitly append the
+        # aExceptionHandling and aRealm values we need to end up matching the
+        # signature of our non-templated "with rv" overload.
+        argnamesWithoutThisAndRv.extend(["eReportExceptions", "nullptr"])
+
+        argnamesWithoutRv = [arg.name for arg in argsWithoutRv]
+        # Note that we need to insert at rvIndex + 1, since we inserted a
+        # thisVal arg at the start.
+        argnamesWithoutRv.insert(rvIndex + 1, "IgnoreErrors()")
+
+        errorReturn = method.getDefaultRetval()
+
+        setupCall = fill(
+            """
+            MOZ_ASSERT(!aRv.Failed(), "Don't pass an already-failed ErrorResult to a callback!");
+            if (!aExecutionReason) {
+              aExecutionReason = "${executionReason}";
+            }
+            CallSetup s(this, aRv, aExecutionReason, aExceptionHandling, aRealm);
+            if (!s.GetContext()) {
+              MOZ_ASSERT(aRv.Failed());
+              return${errorReturn};
+            }
+            """,
+            errorReturn=errorReturn,
+            executionReason=method.getPrettyName(),
+        )
+
+        bodyWithThis = fill(
+            """
+            $*{setupCall}
+            JS::Rooted<JS::Value> thisValJS(s.GetContext());
+            if (!ToJSValue(s.GetContext(), thisVal, &thisValJS)) {
+              aRv.Throw(NS_ERROR_FAILURE);
+              return${errorReturn};
+            }
+            return ${methodName}(${callArgs});
+            """,
+            setupCall=setupCall,
+            errorReturn=errorReturn,
+            methodName=method.name,
+            callArgs=", ".join(argnamesWithThis),
+        )
+        bodyWithoutThis = fill(
+            """
+            $*{setupCall}
+            return ${methodName}(${callArgs});
+            """,
+            setupCall=setupCall,
+            errorReturn=errorReturn,
+            methodName=method.name,
+            callArgs=", ".join(argnamesWithoutThis),
+        )
+        bodyWithThisWithoutRv = fill(
+            """
+            return ${methodName}(${callArgs});
+            """,
+            methodName=method.name,
+            callArgs=", ".join(argnamesWithoutRv),
+        )
+        bodyWithoutThisAndRv = fill(
+            """
+            return ${methodName}(${callArgs});
+            """,
+            methodName=method.name,
+            callArgs=", ".join(argnamesWithoutThisAndRv),
+        )
+
+        return [
+            ClassMethod(
+                method.name,
+                method.returnType,
+                args,
+                bodyInHeader=True,
+                templateArgs=["typename T"],
+                body=bodyWithThis,
+                canRunScript=method.canRunScript,
+            ),
+            ClassMethod(
+                method.name,
+                method.returnType,
+                argsWithoutThis,
+                bodyInHeader=True,
+                body=bodyWithoutThis,
+                canRunScript=method.canRunScript,
+            ),
+            ClassMethod(
+                method.name,
+                method.returnType,
+                argsWithoutRv,
+                bodyInHeader=True,
+                templateArgs=["typename T"],
+                body=bodyWithThisWithoutRv,
+                canRunScript=method.canRunScript,
+            ),
+            ClassMethod(
+                method.name,
+                method.returnType,
+                argsWithoutThisAndRv,
+                bodyInHeader=True,
+                body=bodyWithoutThisAndRv,
+                canRunScript=method.canRunScript,
+            ),
+            method,
+        ]
+
+    def deps(self):
+        return self._deps
+
+
+class CGCallbackFunction(CGCallback):
+    def __init__(self, callback, descriptorProvider):
+        self.callback = callback
+        if callback.isConstructor():
+            methods = [ConstructCallback(callback, descriptorProvider)]
+        else:
+            methods = [CallCallback(callback, descriptorProvider)]
+        CGCallback.__init__(
+            self, callback, descriptorProvider, "CallbackFunction", methods
+        )
+
+    def getConstructors(self):
+        return CGCallback.getConstructors(self) + [
+            ClassConstructor(
+                [Argument("CallbackFunction*", "aOther")],
+                bodyInHeader=True,
+                visibility="public",
+                explicit=True,
+                baseConstructors=["CallbackFunction(aOther)"],
+            )
+        ]
+
+
+class CGFastCallback(CGClass):
+    def __init__(self, idlObject):
+        self._deps = idlObject.getDeps()
+        baseName = idlObject.identifier.name
+        constructor = ClassConstructor(
+            [
+                Argument("JSObject*", "aCallback"),
+                Argument("JSObject*", "aCallbackGlobal"),
+            ],
+            bodyInHeader=True,
+            visibility="public",
+            explicit=True,
+            baseConstructors=[
+                "%s(aCallback, aCallbackGlobal, FastCallbackConstructor())" % baseName,
+            ],
+            body="",
+        )
+
+        traceMethod = ClassMethod(
+            "Trace",
+            "void",
+            [Argument("JSTracer*", "aTracer")],
+            inline=True,
+            bodyInHeader=True,
+            visibility="public",
+            body="%s::Trace(aTracer);\n" % baseName,
+        )
+        holdMethod = ClassMethod(
+            "FinishSlowJSInitIfMoreThanOneOwner",
+            "void",
+            [Argument("JSContext*", "aCx")],
+            inline=True,
+            bodyInHeader=True,
+            visibility="public",
+            body=("%s::FinishSlowJSInitIfMoreThanOneOwner(aCx);\n" % baseName),
+        )
+
+        CGClass.__init__(
+            self,
+            "Fast%s" % baseName,
+            bases=[ClassBase(baseName)],
+            constructors=[constructor],
+            methods=[traceMethod, holdMethod],
+        )
+
+    def deps(self):
+        return self._deps
+
+
+class CGCallbackInterface(CGCallback):
+    def __init__(self, descriptor, spiderMonkeyInterfacesAreStructs=False):
+        iface = descriptor.interface
+        attrs = [
+            m
+            for m in iface.members
+            if (
+                m.isAttr()
+                and not m.isStatic()
+                and (not m.isMaplikeOrSetlikeAttr() or not iface.isJSImplemented())
+            )
+        ]
+        getters = [
+            CallbackGetter(a, descriptor, spiderMonkeyInterfacesAreStructs)
+            for a in attrs
+        ]
+        setters = [
+            CallbackSetter(a, descriptor, spiderMonkeyInterfacesAreStructs)
+            for a in attrs
+            if not a.readonly
+        ]
+        methods = [
+            m
+            for m in iface.members
+            if (
+                m.isMethod()
+                and not m.isStatic()
+                and not m.isIdentifierLess()
+                and (
+                    not m.isMaplikeOrSetlikeOrIterableMethod()
+                    or not iface.isJSImplemented()
+                )
+            )
+        ]
+        methods = [
+            CallbackOperation(m, sig, descriptor, spiderMonkeyInterfacesAreStructs)
+            for m in methods
+            for sig in m.signatures()
+        ]
+
+        needInitId = False
+        if iface.isJSImplemented() and iface.ctor():
+            sigs = descriptor.interface.ctor().signatures()
+            if len(sigs) != 1:
+                raise TypeError("We only handle one constructor.  See bug 869268.")
+            methods.append(CGJSImplInitOperation(sigs[0], descriptor))
+            needInitId = True
+
+        idlist = [
+            descriptor.binaryNameFor(m.identifier.name, m.isStatic())
+            for m in iface.members
+            if m.isAttr() or m.isMethod()
+        ]
+        if needInitId:
+            idlist.append("__init")
+
+        if iface.isJSImplemented() and iface.getExtendedAttribute(
+            "WantsEventListenerHooks"
+        ):
+            methods.append(CGJSImplEventHookOperation(descriptor, "eventListenerAdded"))
+            methods.append(
+                CGJSImplEventHookOperation(descriptor, "eventListenerRemoved")
+            )
+            idlist.append("eventListenerAdded")
+            idlist.append("eventListenerRemoved")
+
+        if len(idlist) != 0:
+            methods.append(initIdsClassMethod(idlist, iface.identifier.name + "Atoms"))
+        CGCallback.__init__(
+            self,
+            iface,
+            descriptor,
+            "CallbackInterface",
+            methods,
+            getters=getters,
+            setters=setters,
+        )
+
+
+class FakeMember:
+    def __init__(self, name=None):
+        if name is not None:
+            self.identifier = FakeIdentifier(name)
+
+    def isStatic(self):
+        return False
+
+    def isAttr(self):
+        return False
+
+    def isMethod(self):
+        return False
+
+    def getExtendedAttribute(self, name):
+        # Claim to be a [NewObject] so we can avoid the "return a raw pointer"
+        # comments CGNativeMember codegen would otherwise stick in.
+        if name == "NewObject":
+            return True
+        return None
+
+
+class CallbackMember(CGNativeMember):
+    # XXXbz It's OK to use CallbackKnownNotGray for wrapScope because
+    # CallSetup already handled the unmark-gray bits for us. we don't have
+    # anything better to use for 'obj', really...
+    def __init__(
+        self,
+        sig,
+        name,
+        descriptorProvider,
+        needThisHandling,
+        rethrowContentException=False,
+        spiderMonkeyInterfacesAreStructs=False,
+        wrapScope=None,
+        canRunScript=False,
+        passJSBitsAsNeeded=False,
+    ):
+        """
+        needThisHandling is True if we need to be able to accept a specified
+        thisObj, False otherwise.
+        """
+        assert not rethrowContentException or not needThisHandling
+
+        self.retvalType = sig[0]
+        self.originalSig = sig
+        args = sig[1]
+        self.argCount = len(args)
+        if self.argCount > 0:
+            # Check for variadic arguments
+            lastArg = args[self.argCount - 1]
+            if lastArg.variadic:
+                self.argCountStr = "(%d - 1) + %s.Length()" % (
+                    self.argCount,
+                    lastArg.identifier.name,
+                )
+            else:
+                self.argCountStr = "%d" % self.argCount
+        self.needThisHandling = needThisHandling
+        # If needThisHandling, we generate ourselves as private and the caller
+        # will handle generating public versions that handle the "this" stuff.
+        visibility = "private" if needThisHandling else "public"
+        self.rethrowContentException = rethrowContentException
+
+        self.wrapScope = wrapScope
+        # We don't care, for callback codegen, whether our original member was
+        # a method or attribute or whatnot.  Just always pass FakeMember()
+        # here.
+        CGNativeMember.__init__(
+            self,
+            descriptorProvider,
+            FakeMember(),
+            name,
+            (self.retvalType, args),
+            extendedAttrs=["needsErrorResult"],
+            passJSBitsAsNeeded=passJSBitsAsNeeded,
+            visibility=visibility,
+            spiderMonkeyInterfacesAreStructs=spiderMonkeyInterfacesAreStructs,
+            canRunScript=canRunScript,
+        )
+        # We have to do all the generation of our body now, because
+        # the caller relies on us throwing if we can't manage it.
+        self.body = self.getImpl()
+
+    def getImpl(self):
+        setupCall = self.getCallSetup()
+        declRval = self.getRvalDecl()
+        if self.argCount > 0:
+            argvDecl = fill(
+                """
+                JS::RootedVector<JS::Value> argv(cx);
+                if (!argv.resize(${argCount})) {
+                  $*{failureCode}
+                  return${errorReturn};
+                }
+                """,
+                argCount=self.argCountStr,
+                failureCode=self.getArgvDeclFailureCode(),
+                errorReturn=self.getDefaultRetval(),
+            )
+        else:
+            # Avoid weird 0-sized arrays
+            argvDecl = ""
+        convertArgs = self.getArgConversions()
+        doCall = self.getCall()
+        returnResult = self.getResultConversion()
+
+        body = declRval + argvDecl + convertArgs + doCall
+        if self.needsScopeBody():
+            body = "{\n" + indent(body) + "}\n"
+        return setupCall + body + returnResult
+
+    def needsScopeBody(self):
+        return False
+
+    def getArgvDeclFailureCode(self):
+        return dedent(
+            """
+            // That threw an exception on the JSContext, and our CallSetup will do
+            // the right thing with that.
+            """
+        )
+
+    def getExceptionCode(self, forResult):
+        return fill(
+            """
+            aRv.Throw(NS_ERROR_UNEXPECTED);
+            return${defaultRetval};
+            """,
+            defaultRetval=self.getDefaultRetval(),
+        )
+
+    def getResultConversion(
+        self, val="rval", failureCode=None, isDefinitelyObject=False, exceptionCode=None
+    ):
+        replacements = {
+            "val": val,
+            "holderName": "rvalHolder",
+            "declName": "rvalDecl",
+            # We actually want to pass in a null scope object here, because
+            # wrapping things into our current compartment (that of mCallback)
+            # is what we want.
+            "obj": "nullptr",
+            "passedToJSImpl": "false",
+        }
+
+        if isJSImplementedDescriptor(self.descriptorProvider):
+            isCallbackReturnValue = "JSImpl"
+        else:
+            isCallbackReturnValue = "Callback"
+        sourceDescription = "return value of %s" % self.getPrettyName()
+        convertType = instantiateJSToNativeConversion(
+            getJSToNativeConversionInfo(
+                self.retvalType,
+                self.descriptorProvider,
+                failureCode=failureCode,
+                isDefinitelyObject=isDefinitelyObject,
+                exceptionCode=exceptionCode or self.getExceptionCode(forResult=True),
+                isCallbackReturnValue=isCallbackReturnValue,
+                # Allow returning a callback type that
+                # allows non-callable objects.
+                allowTreatNonCallableAsNull=True,
+                sourceDescription=sourceDescription,
+            ),
+            replacements,
+        )
+        assignRetval = string.Template(
+            self.getRetvalInfo(self.retvalType, False)[2]
+        ).substitute(replacements)
+        type = convertType.define()
+        return type + assignRetval
+
+    def getArgConversions(self):
+        # Just reget the arglist from self.originalSig, because our superclasses
+        # just have way to many members they like to clobber, so I can't find a
+        # safe member name to store it in.
+        argConversions = [
+            self.getArgConversion(i, arg) for i, arg in enumerate(self.originalSig[1])
+        ]
+        if not argConversions:
+            return "\n"
+
+        # Do them back to front, so our argc modifications will work
+        # correctly, because we examine trailing arguments first.
+        argConversions.reverse()
+        # Wrap each one in a scope so that any locals it has don't leak out, and
+        # also so that we can just "break;" for our successCode.
+        argConversions = [
+            CGWrapper(CGIndenter(CGGeneric(c)), pre="do {\n", post="} while (false);\n")
+            for c in argConversions
+        ]
+        if self.argCount > 0:
+            argConversions.insert(0, self.getArgcDecl())
+        # And slap them together.
+        return CGList(argConversions, "\n").define() + "\n"
+
+    def getArgConversion(self, i, arg):
+        argval = arg.identifier.name
+
+        if arg.variadic:
+            argval = argval + "[idx]"
+            jsvalIndex = "%d + idx" % i
+        else:
+            jsvalIndex = "%d" % i
+            if arg.canHaveMissingValue():
+                argval += ".Value()"
+
+        if arg.type.isDOMString():
+            # XPConnect string-to-JS conversion wants to mutate the string.  So
+            # let's give it a string it can mutate
+            # XXXbz if we try to do a sequence of strings, this will kinda fail.
+            result = "mutableStr"
+            prepend = "nsString mutableStr(%s);\n" % argval
+        else:
+            result = argval
+            prepend = ""
+
+        wrapScope = self.wrapScope
+        if arg.type.isUnion() and wrapScope is None:
+            prepend += (
+                "JS::Rooted<JSObject*> callbackObj(cx, CallbackKnownNotGray());\n"
+            )
+            wrapScope = "callbackObj"
+
+        conversion = prepend + wrapForType(
+            arg.type,
+            self.descriptorProvider,
+            {
+                "result": result,
+                "successCode": "continue;\n" if arg.variadic else "break;\n",
+                "jsvalRef": "argv[%s]" % jsvalIndex,
+                "jsvalHandle": "argv[%s]" % jsvalIndex,
+                "obj": wrapScope,
+                "returnsNewObject": False,
+                "exceptionCode": self.getExceptionCode(forResult=False),
+                "spiderMonkeyInterfacesAreStructs": self.spiderMonkeyInterfacesAreStructs,
+            },
+        )
+
+        if arg.variadic:
+            conversion = fill(
+                """
+                for (uint32_t idx = 0; idx < ${arg}.Length(); ++idx) {
+                  $*{conversion}
+                }
+                break;
+                """,
+                arg=arg.identifier.name,
+                conversion=conversion,
+            )
+        elif arg.canHaveMissingValue():
+            conversion = fill(
+                """
+                if (${argName}.WasPassed()) {
+                  $*{conversion}
+                } else if (argc == ${iPlus1}) {
+                  // This is our current trailing argument; reduce argc
+                  --argc;
+                } else {
+                  argv[${i}].setUndefined();
+                }
+                """,
+                argName=arg.identifier.name,
+                conversion=conversion,
+                iPlus1=i + 1,
+                i=i,
+            )
+        return conversion
+
+    def getDefaultRetval(self):
+        default = self.getRetvalInfo(self.retvalType, False)[1]
+        if len(default) != 0:
+            default = " " + default
+        return default
+
+    def getArgs(self, returnType, argList):
+        args = CGNativeMember.getArgs(self, returnType, argList)
+        if not self.needThisHandling:
+            # Since we don't need this handling, we're the actual method that
+            # will be called, so we need an aRethrowExceptions argument.
+            if not self.rethrowContentException:
+                args.append(Argument("const char*", "aExecutionReason", "nullptr"))
+                args.append(
+                    Argument(
+                        "ExceptionHandling", "aExceptionHandling", "eReportExceptions"
+                    )
+                )
+            args.append(Argument("JS::Realm*", "aRealm", "nullptr"))
+            return args
+        # We want to allow the caller to pass in a "this" value, as
+        # well as a BindingCallContext.
+        return [
+            Argument("BindingCallContext&", "cx"),
+            Argument("JS::Handle<JS::Value>", "aThisVal"),
+        ] + args
+
+    def getCallSetup(self):
+        if self.needThisHandling:
+            # It's been done for us already
+            return ""
+        callSetup = "CallSetup s(this, aRv"
+        if self.rethrowContentException:
+            # getArgs doesn't add the aExceptionHandling argument but does add
+            # aRealm for us.
+            callSetup += (
+                ', "%s", eRethrowContentExceptions, aRealm, /* aIsJSImplementedWebIDL = */ '
+                % self.getPrettyName()
+            )
+            callSetup += toStringBool(
+                isJSImplementedDescriptor(self.descriptorProvider)
+            )
+        else:
+            callSetup += ', "%s", aExceptionHandling, aRealm' % self.getPrettyName()
+        callSetup += ");\n"
+        return fill(
+            """
+            $*{callSetup}
+            if (aRv.Failed()) {
+              return${errorReturn};
+            }
+            MOZ_ASSERT(s.GetContext());
+            BindingCallContext& cx = s.GetCallContext();
+
+            """,
+            callSetup=callSetup,
+            errorReturn=self.getDefaultRetval(),
+        )
+
+    def getArgcDecl(self):
+        return CGGeneric("unsigned argc = %s;\n" % self.argCountStr)
+
+    @staticmethod
+    def ensureASCIIName(idlObject):
+        type = "attribute" if idlObject.isAttr() else "operation"
+        if re.match("[^\x20-\x7E]", idlObject.identifier.name):
+            raise SyntaxError(
+                'Callback %s name "%s" contains non-ASCII '
+                "characters.  We can't handle that.  %s"
+                % (type, idlObject.identifier.name, idlObject.location)
+            )
+        if re.match('"', idlObject.identifier.name):
+            raise SyntaxError(
+                "Callback %s name '%s' contains "
+                "double-quote character.  We can't handle "
+                "that.  %s" % (type, idlObject.identifier.name, idlObject.location)
+            )
+
+
+class ConstructCallback(CallbackMember):
+    def __init__(self, callback, descriptorProvider):
+        self.callback = callback
+        CallbackMember.__init__(
+            self,
+            callback.signatures()[0],
+            "Construct",
+            descriptorProvider,
+            needThisHandling=False,
+            canRunScript=True,
+        )
+
+    def getRvalDecl(self):
+        # Box constructedObj for getJSToNativeConversionInfo().
+        return "JS::Rooted<JS::Value> rval(cx);\n"
+
+    def getCall(self):
+        if self.argCount > 0:
+            args = "JS::HandleValueArray::subarray(argv, 0, argc)"
+        else:
+            args = "JS::HandleValueArray::empty()"
+
+        return fill(
+            """
+            JS::Rooted<JS::Value> constructor(cx, JS::ObjectValue(*mCallback));
+            JS::Rooted<JSObject*> constructedObj(cx);
+            if (!JS::Construct(cx, constructor,
+                          ${args}, &constructedObj)) {
+              aRv.NoteJSContextException(cx);
+              return${errorReturn};
+            }
+            rval.setObject(*constructedObj);
+            """,
+            args=args,
+            errorReturn=self.getDefaultRetval(),
+        )
+
+    def getResultConversion(self):
+        return CallbackMember.getResultConversion(self, isDefinitelyObject=True)
+
+    def getPrettyName(self):
+        return self.callback.identifier.name
+
+
+class CallbackMethod(CallbackMember):
+    def __init__(
+        self,
+        sig,
+        name,
+        descriptorProvider,
+        needThisHandling,
+        rethrowContentException=False,
+        spiderMonkeyInterfacesAreStructs=False,
+        canRunScript=False,
+    ):
+        CallbackMember.__init__(
+            self,
+            sig,
+            name,
+            descriptorProvider,
+            needThisHandling,
+            rethrowContentException,
+            spiderMonkeyInterfacesAreStructs=spiderMonkeyInterfacesAreStructs,
+            canRunScript=canRunScript,
+        )
+
+    def getRvalDecl(self):
+        return "JS::Rooted<JS::Value> rval(cx);\n"
+
+    def getNoteCallFailed(self):
+        return fill(
+            """
+                aRv.NoteJSContextException(cx);
+                return${errorReturn};
+                """,
+            errorReturn=self.getDefaultRetval(),
+        )
+
+    def getCall(self):
+        if self.argCount > 0:
+            args = "JS::HandleValueArray::subarray(argv, 0, argc)"
+        else:
+            args = "JS::HandleValueArray::empty()"
+
+        return fill(
+            """
+            $*{declCallable}
+            $*{declThis}
+            if (${callGuard}!JS::Call(cx, ${thisVal}, callable,
+                          ${args}, &rval)) {
+              $*{noteError}
+            }
+            """,
+            declCallable=self.getCallableDecl(),
+            declThis=self.getThisDecl(),
+            callGuard=self.getCallGuard(),
+            thisVal=self.getThisVal(),
+            args=args,
+            noteError=self.getNoteCallFailed(),
+        )
+
+
+class CallCallback(CallbackMethod):
+    def __init__(self, callback, descriptorProvider):
+        self.callback = callback
+        CallbackMethod.__init__(
+            self,
+            callback.signatures()[0],
+            "Call",
+            descriptorProvider,
+            needThisHandling=True,
+            canRunScript=not callback.isRunScriptBoundary(),
+        )
+
+    def getNoteCallFailed(self):
+        if self.retvalType.isPromise():
+            return dedent(
+                """
+                // Convert exception to a rejected promise.
+                // See https://heycam.github.io/webidl/#call-a-user-objects-operation
+                // step 12 and step 15.5.
+                return CreateRejectedPromiseFromThrownException(cx, aRv);
+                """
+            )
+        return CallbackMethod.getNoteCallFailed(self)
+
+    def getExceptionCode(self, forResult):
+        # If the result value is a promise, and conversion
+        # to the promise throws an exception we shouldn't
+        # try to convert that exception to a promise again.
+        if self.retvalType.isPromise() and not forResult:
+            return dedent(
+                """
+                // Convert exception to a rejected promise.
+                // See https://heycam.github.io/webidl/#call-a-user-objects-operation
+                // step 10 and step 15.5.
+                return CreateRejectedPromiseFromThrownException(cx, aRv);
+                """
+            )
+        return CallbackMethod.getExceptionCode(self, forResult)
+
+    def getThisDecl(self):
+        return ""
+
+    def getThisVal(self):
+        return "aThisVal"
+
+    def getCallableDecl(self):
+        return "JS::Rooted<JS::Value> callable(cx, JS::ObjectValue(*mCallback));\n"
+
+    def getPrettyName(self):
+        return self.callback.identifier.name
+
+    def getCallGuard(self):
+        if self.callback._treatNonObjectAsNull:
+            return "JS::IsCallable(mCallback) && "
+        return ""
+
+
+class CallbackOperationBase(CallbackMethod):
+    """
+    Common class for implementing various callback operations.
+    """
+
+    def __init__(
+        self,
+        signature,
+        jsName,
+        nativeName,
+        descriptor,
+        singleOperation,
+        rethrowContentException=False,
+        spiderMonkeyInterfacesAreStructs=False,
+    ):
+        self.singleOperation = singleOperation
+        self.methodName = descriptor.binaryNameFor(jsName, False)
+        CallbackMethod.__init__(
+            self,
+            signature,
+            nativeName,
+            descriptor,
+            singleOperation,
+            rethrowContentException,
+            spiderMonkeyInterfacesAreStructs=spiderMonkeyInterfacesAreStructs,
+        )
+
+    def getThisDecl(self):
+        if not self.singleOperation:
+            return "JS::Rooted<JS::Value> thisValue(cx, JS::ObjectValue(*mCallback));\n"
+        # This relies on getCallableDecl declaring a boolean
+        # isCallable in the case when we're a single-operation
+        # interface.
+        return dedent(
+            """
+            JS::Rooted<JS::Value> thisValue(cx, isCallable ? aThisVal.get()
+                                                           : JS::ObjectValue(*mCallback));
+            """
+        )
+
+    def getThisVal(self):
+        return "thisValue"
+
+    def getCallableDecl(self):
+        getCallableFromProp = fill(
+            """
+            ${atomCacheName}* atomsCache = GetAtomCache<${atomCacheName}>(cx);
+            if ((reinterpret_cast<jsid*>(atomsCache)->isVoid() &&
+                 !InitIds(cx, atomsCache)) ||
+                !GetCallableProperty(cx, atomsCache->${methodAtomName}, &callable)) {
+              aRv.Throw(NS_ERROR_UNEXPECTED);
+              return${errorReturn};
+            }
+            """,
+            methodAtomName=CGDictionary.makeIdName(self.methodName),
+            atomCacheName=self.descriptorProvider.interface.identifier.name + "Atoms",
+            errorReturn=self.getDefaultRetval(),
+        )
+        if not self.singleOperation:
+            return "JS::Rooted<JS::Value> callable(cx);\n" + getCallableFromProp
+        return fill(
+            """
+            bool isCallable = JS::IsCallable(mCallback);
+            JS::Rooted<JS::Value> callable(cx);
+            if (isCallable) {
+              callable = JS::ObjectValue(*mCallback);
+            } else {
+              $*{getCallableFromProp}
+            }
+            """,
+            getCallableFromProp=getCallableFromProp,
+        )
+
+    def getCallGuard(self):
+        return ""
+
+
+class CallbackOperation(CallbackOperationBase):
+    """
+    Codegen actual WebIDL operations on callback interfaces.
+    """
+
+    def __init__(self, method, signature, descriptor, spiderMonkeyInterfacesAreStructs):
+        self.ensureASCIIName(method)
+        self.method = method
+        jsName = method.identifier.name
+        CallbackOperationBase.__init__(
+            self,
+            signature,
+            jsName,
+            MakeNativeName(descriptor.binaryNameFor(jsName, False)),
+            descriptor,
+            descriptor.interface.isSingleOperationInterface(),
+            rethrowContentException=descriptor.interface.isJSImplemented(),
+            spiderMonkeyInterfacesAreStructs=spiderMonkeyInterfacesAreStructs,
+        )
+
+    def getPrettyName(self):
+        return "%s.%s" % (
+            self.descriptorProvider.interface.identifier.name,
+            self.method.identifier.name,
+        )
+
+
+class CallbackAccessor(CallbackMember):
+    """
+    Shared superclass for CallbackGetter and CallbackSetter.
+    """
+
+    def __init__(self, attr, sig, name, descriptor, spiderMonkeyInterfacesAreStructs):
+        self.ensureASCIIName(attr)
+        self.attrName = attr.identifier.name
+        CallbackMember.__init__(
+            self,
+            sig,
+            name,
+            descriptor,
+            needThisHandling=False,
+            rethrowContentException=descriptor.interface.isJSImplemented(),
+            spiderMonkeyInterfacesAreStructs=spiderMonkeyInterfacesAreStructs,
+        )
+
+    def getPrettyName(self):
+        return "%s.%s" % (
+            self.descriptorProvider.interface.identifier.name,
+            self.attrName,
+        )
+
+
+class CallbackGetter(CallbackAccessor):
+    def __init__(self, attr, descriptor, spiderMonkeyInterfacesAreStructs):
+        CallbackAccessor.__init__(
+            self,
+            attr,
+            (attr.type, []),
+            callbackGetterName(attr, descriptor),
+            descriptor,
+            spiderMonkeyInterfacesAreStructs,
+        )
+
+    def getRvalDecl(self):
+        return "JS::Rooted<JS::Value> rval(cx);\n"
+
+    def getCall(self):
+        return fill(
+            """
+            JS::Rooted<JSObject *> callback(cx, mCallback);
+            ${atomCacheName}* atomsCache = GetAtomCache<${atomCacheName}>(cx);
+            if ((reinterpret_cast<jsid*>(atomsCache)->isVoid()
+                 && !InitIds(cx, atomsCache)) ||
+                !JS_GetPropertyById(cx, callback, atomsCache->${attrAtomName}, &rval)) {
+              aRv.Throw(NS_ERROR_UNEXPECTED);
+              return${errorReturn};
+            }
+            """,
+            atomCacheName=self.descriptorProvider.interface.identifier.name + "Atoms",
+            attrAtomName=CGDictionary.makeIdName(
+                self.descriptorProvider.binaryNameFor(self.attrName, False)
+            ),
+            errorReturn=self.getDefaultRetval(),
+        )
+
+
+class CallbackSetter(CallbackAccessor):
+    def __init__(self, attr, descriptor, spiderMonkeyInterfacesAreStructs):
+        CallbackAccessor.__init__(
+            self,
+            attr,
+            (
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [FakeArgument(attr.type)],
+            ),
+            callbackSetterName(attr, descriptor),
+            descriptor,
+            spiderMonkeyInterfacesAreStructs,
+        )
+
+    def getRvalDecl(self):
+        # We don't need an rval
+        return ""
+
+    def getCall(self):
+        return fill(
+            """
+            MOZ_ASSERT(argv.length() == 1);
+            JS::Rooted<JSObject*> callback(cx, CallbackKnownNotGray());
+            ${atomCacheName}* atomsCache = GetAtomCache<${atomCacheName}>(cx);
+            if ((reinterpret_cast<jsid*>(atomsCache)->isVoid() &&
+                 !InitIds(cx, atomsCache)) ||
+                !JS_SetPropertyById(cx, callback, atomsCache->${attrAtomName}, argv[0])) {
+              aRv.Throw(NS_ERROR_UNEXPECTED);
+              return${errorReturn};
+            }
+            """,
+            atomCacheName=self.descriptorProvider.interface.identifier.name + "Atoms",
+            attrAtomName=CGDictionary.makeIdName(
+                self.descriptorProvider.binaryNameFor(self.attrName, False)
+            ),
+            errorReturn=self.getDefaultRetval(),
+        )
+
+    def getArgcDecl(self):
+        return None
+
+
+class CGJSImplInitOperation(CallbackOperationBase):
+    """
+    Codegen the __Init() method used to pass along constructor arguments for JS-implemented WebIDL.
+    """
+
+    def __init__(self, sig, descriptor):
+        assert sig in descriptor.interface.ctor().signatures()
+        CallbackOperationBase.__init__(
+            self,
+            (BuiltinTypes[IDLBuiltinType.Types.undefined], sig[1]),
+            "__init",
+            "__Init",
+            descriptor,
+            singleOperation=False,
+            rethrowContentException=True,
+            spiderMonkeyInterfacesAreStructs=True,
+        )
+
+    def getPrettyName(self):
+        return "__init"
+
+
+class CGJSImplEventHookOperation(CallbackOperationBase):
+    """
+    Codegen the hooks on a JS impl for adding/removing event listeners.
+    """
+
+    def __init__(self, descriptor, name):
+        self.name = name
+
+        CallbackOperationBase.__init__(
+            self,
+            (
+                BuiltinTypes[IDLBuiltinType.Types.undefined],
+                [FakeArgument(BuiltinTypes[IDLBuiltinType.Types.domstring], "aType")],
+            ),
+            name,
+            MakeNativeName(name),
+            descriptor,
+            singleOperation=False,
+            rethrowContentException=False,
+            spiderMonkeyInterfacesAreStructs=True,
+        )
+
+    def getPrettyName(self):
+        return self.name
+
+
+def getMaplikeOrSetlikeErrorReturn(helperImpl):
+    """
+    Generate return values based on whether a maplike or setlike generated
+    method is an interface method (which returns bool) or a helper function
+    (which uses ErrorResult).
+    """
+    if helperImpl:
+        return dedent(
+            """
+            aRv.Throw(NS_ERROR_UNEXPECTED);
+            return%s;
+            """
+            % helperImpl.getDefaultRetval()
+        )
+    return "return false;\n"
+
+
+def getMaplikeOrSetlikeBackingObject(descriptor, maplikeOrSetlike, helperImpl=None):
+    """
+    Generate code to get/create a JS backing object for a maplike/setlike
+    declaration from the declaration slot.
+    """
+    func_prefix = maplikeOrSetlike.maplikeOrSetlikeOrIterableType.title()
+    ret = fill(
+        """
+        JS::Rooted<JSObject*> backingObj(cx);
+        bool created = false;
+        if (!Get${func_prefix}BackingObject(cx, obj, ${slot}, &backingObj, &created)) {
+          $*{errorReturn}
+        }
+        if (created) {
+          PreserveWrapper<${selfType}>(self);
+        }
+        """,
+        slot=memberReservedSlot(maplikeOrSetlike, descriptor),
+        func_prefix=func_prefix,
+        errorReturn=getMaplikeOrSetlikeErrorReturn(helperImpl),
+        selfType=descriptor.nativeType,
+    )
+    return ret
+
+
+def getMaplikeOrSetlikeSizeGetterBody(descriptor, attr):
+    """
+    Creates the body for the size getter method of maplike/setlike interfaces.
+    """
+    # We should only have one declaration attribute currently
+    assert attr.identifier.name == "size"
+    assert attr.isMaplikeOrSetlikeAttr()
+    return fill(
+        """
+        $*{getBackingObj}
+        uint32_t result = JS::${funcPrefix}Size(cx, backingObj);
+        MOZ_ASSERT(!JS_IsExceptionPending(cx));
+        args.rval().setNumber(result);
+        return true;
+        """,
+        getBackingObj=getMaplikeOrSetlikeBackingObject(
+            descriptor, attr.maplikeOrSetlike
+        ),
+        funcPrefix=attr.maplikeOrSetlike.prefix,
+    )
+
+
+class CGMaplikeOrSetlikeMethodGenerator(CGThing):
+    """
+    Creates methods for maplike/setlike interfaces. It is expected that all
+    methods will be have a maplike/setlike object attached. Unwrapping/wrapping
+    will be taken care of by the usual method generation machinery in
+    CGMethodCall/CGPerSignatureCall. Functionality is filled in here instead of
+    using CGCallGenerator.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        maplikeOrSetlike,
+        methodName,
+        needsValueTypeReturn=False,
+        helperImpl=None,
+    ):
+        CGThing.__init__(self)
+        # True if this will be the body of a C++ helper function.
+        self.helperImpl = helperImpl
+        self.descriptor = descriptor
+        self.maplikeOrSetlike = maplikeOrSetlike
+        self.cgRoot = CGList([])
+        impl_method_name = methodName
+        if impl_method_name[0] == "_":
+            # double underscore means this is a js-implemented chrome only rw
+            # function. Truncate the double underscore so calling the right
+            # underlying JSAPI function still works.
+            impl_method_name = impl_method_name[2:]
+        self.cgRoot.append(
+            CGGeneric(
+                getMaplikeOrSetlikeBackingObject(
+                    self.descriptor, self.maplikeOrSetlike, self.helperImpl
+                )
+            )
+        )
+        self.returnStmt = getMaplikeOrSetlikeErrorReturn(self.helperImpl)
+
+        # Generates required code for the method. Method descriptions included
+        # in definitions below. Throw if we don't have a method to fill in what
+        # we're looking for.
+        try:
+            methodGenerator = getattr(self, impl_method_name)
+        except AttributeError:
+            raise TypeError(
+                "Missing %s method definition '%s'"
+                % (self.maplikeOrSetlike.maplikeOrSetlikeType, methodName)
+            )
+        # Method generator returns tuple, containing:
+        #
+        # - a list of CGThings representing setup code for preparing to call
+        #   the JS API function
+        # - a list of arguments needed for the JS API function we're calling
+        # - list of code CGThings needed for return value conversion.
+        (setupCode, arguments, setResult) = methodGenerator()
+
+        # Create the actual method call, and then wrap it with the code to
+        # return the value if needed.
+        funcName = self.maplikeOrSetlike.prefix + MakeNativeName(impl_method_name)
+        # Append the list of setup code CGThings
+        self.cgRoot.append(CGList(setupCode))
+        # Create the JS API call
+        code = dedent(
+            """
+            if (!JS::${funcName}(${args})) {
+              $*{errorReturn}
+            }
+            """
+        )
+
+        if needsValueTypeReturn:
+            assert self.helperImpl and impl_method_name == "get"
+            code += fill(
+                """
+                if (result.isUndefined()) {
+                  aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+                  return${retval};
+                }
+                """,
+                retval=self.helperImpl.getDefaultRetval(),
+            )
+
+        self.cgRoot.append(
+            CGWrapper(
+                CGGeneric(
+                    fill(
+                        code,
+                        funcName=funcName,
+                        args=", ".join(["cx", "backingObj"] + arguments),
+                        errorReturn=self.returnStmt,
+                    )
+                )
+            )
+        )
+        # Append result conversion
+        self.cgRoot.append(CGList(setResult))
+
+    def mergeTuples(self, a, b):
+        """
+        Expecting to take 2 tuples were all elements are lists, append the lists in
+        the second tuple to the lists in the first.
+        """
+        return tuple([x + y for x, y in zip(a, b)])
+
+    def appendArgConversion(self, name):
+        """
+        Generate code to convert arguments to JS::Values, so they can be
+        passed into JSAPI functions.
+        """
+        return CGGeneric(
+            fill(
+                """
+            JS::Rooted<JS::Value> ${name}Val(cx);
+            if (!ToJSValue(cx, ${name}, &${name}Val)) {
+              $*{errorReturn}
+            }
+            """,
+                name=name,
+                errorReturn=self.returnStmt,
+            )
+        )
+
+    def appendKeyArgConversion(self):
+        """
+        Generates the key argument for methods. Helper functions will use
+        a RootedVector<JS::Value>, while interface methods have separate JS::Values.
+        """
+        if self.helperImpl:
+            return ([], ["argv[0]"], [])
+        return ([self.appendArgConversion("arg0")], ["arg0Val"], [])
+
+    def appendKeyAndValueArgConversion(self):
+        """
+        Generates arguments for methods that require a key and value. Helper
+        functions will use a RootedVector<JS::Value>, while interface methods have
+        separate JS::Values.
+        """
+        r = self.appendKeyArgConversion()
+        if self.helperImpl:
+            return self.mergeTuples(r, ([], ["argv[1]"], []))
+        return self.mergeTuples(
+            r, ([self.appendArgConversion("arg1")], ["arg1Val"], [])
+        )
+
+    def appendIteratorResult(self):
+        """
+        Generate code to output JSObject* return values, needed for functions that
+        return iterators. Iterators cannot currently be wrapped via Xrays. If
+        something that would return an iterator is called via Xray, fail early.
+        """
+        # TODO: Bug 1173651 - Remove check once bug 1023984 is fixed.
+        code = CGGeneric(
+            dedent(
+                """
+            // TODO (Bug 1173651): Xrays currently cannot wrap iterators. Change
+            // after bug 1023984 is fixed.
+            if (xpc::WrapperFactory::IsXrayWrapper(obj)) {
+              JS_ReportErrorASCII(cx, "Xray wrapping of iterators not supported.");
+              return false;
+            }
+            JS::Rooted<JSObject*> result(cx);
+            JS::Rooted<JS::Value> v(cx);
+            """
+            )
+        )
+        arguments = "&v"
+        setResult = CGGeneric(
+            dedent(
+                """
+            result = &v.toObject();
+            """
+            )
+        )
+        return ([code], [arguments], [setResult])
+
+    def appendSelfResult(self):
+        """
+        Generate code to return the interface object itself.
+        """
+        code = CGGeneric(
+            dedent(
+                """
+            JS::Rooted<JSObject*> result(cx);
+            """
+            )
+        )
+        setResult = CGGeneric(
+            dedent(
+                """
+            result = obj;
+            """
+            )
+        )
+        return ([code], [], [setResult])
+
+    def appendBoolResult(self):
+        if self.helperImpl:
+            return ([CGGeneric("bool retVal;\n")], ["&retVal"], [])
+        return ([CGGeneric("bool result;\n")], ["&result"], [])
+
+    def forEach(self):
+        """
+        void forEach(callback c, any thisval);
+
+        ForEach takes a callback, and a possible value to use as 'this'. The
+        callback needs to take value, key, and the interface object
+        implementing maplike/setlike. In order to make sure that the third arg
+        is our interface object instead of the map/set backing object, we
+        create a js function with the callback and original object in its
+        storage slots, then use a helper function in BindingUtils to make sure
+        the callback is called correctly.
+        """
+        assert not self.helperImpl
+        code = [
+            CGGeneric(
+                dedent(
+                    """
+            // Create a wrapper function.
+            JSFunction* func = js::NewFunctionWithReserved(cx, ForEachHandler, 3, 0, nullptr);
+            if (!func) {
+              return false;
+            }
+            JS::Rooted<JSObject*> funcObj(cx, JS_GetFunctionObject(func));
+            JS::Rooted<JS::Value> funcVal(cx, JS::ObjectValue(*funcObj));
+            js::SetFunctionNativeReserved(funcObj, FOREACH_CALLBACK_SLOT,
+                                          JS::ObjectValue(*arg0));
+            js::SetFunctionNativeReserved(funcObj, FOREACH_MAPLIKEORSETLIKEOBJ_SLOT,
+                                          JS::ObjectValue(*obj));
+            """
+                )
+            )
+        ]
+        arguments = ["funcVal", "arg1"]
+        return (code, arguments, [])
+
+    def set(self):
+        """
+        object set(key, value);
+
+        Maplike only function, takes key and sets value to it, returns
+        interface object unless being called from a C++ helper.
+        """
+        assert self.maplikeOrSetlike.isMaplike()
+        r = self.appendKeyAndValueArgConversion()
+        if self.helperImpl:
+            return r
+        return self.mergeTuples(r, self.appendSelfResult())
+
+    def add(self):
+        """
+        object add(value);
+
+        Setlike only function, adds value to set, returns interface object
+        unless being called from a C++ helper
+        """
+        assert self.maplikeOrSetlike.isSetlike()
+        r = self.appendKeyArgConversion()
+        if self.helperImpl:
+            return r
+        return self.mergeTuples(r, self.appendSelfResult())
+
+    def get(self):
+        """
+        type? get(key);
+
+        Retrieves a value from a backing object based on the key. Returns value
+        if key is in backing object, undefined otherwise.
+        """
+        assert self.maplikeOrSetlike.isMaplike()
+        r = self.appendKeyArgConversion()
+
+        code = []
+        # We don't need to create the result variable because it'll be created elsewhere
+        # for JSObject Get method
+        if not self.helperImpl or not self.helperImpl.needsScopeBody():
+            code = [
+                CGGeneric(
+                    dedent(
+                        """
+                        JS::Rooted<JS::Value> result(cx);
+                        """
+                    )
+                )
+            ]
+
+        arguments = ["&result"]
+        return self.mergeTuples(r, (code, arguments, []))
+
+    def has(self):
+        """
+        bool has(key);
+
+        Check if an entry exists in the backing object. Returns true if value
+        exists in backing object, false otherwise.
+        """
+        return self.mergeTuples(self.appendKeyArgConversion(), self.appendBoolResult())
+
+    def keys(self):
+        """
+        object keys();
+
+        Returns new object iterator with all keys from backing object.
+        """
+        return self.appendIteratorResult()
+
+    def values(self):
+        """
+        object values();
+
+        Returns new object iterator with all values from backing object.
+        """
+        return self.appendIteratorResult()
+
+    def entries(self):
+        """
+        object entries();
+
+        Returns new object iterator with all keys and values from backing
+        object. Keys will be null for set.
+        """
+        return self.appendIteratorResult()
+
+    def clear(self):
+        """
+        void clear();
+
+        Removes all entries from map/set.
+        """
+        return ([], [], [])
+
+    def delete(self):
+        """
+        bool delete(key);
+
+        Deletes an entry from the backing object. Returns true if value existed
+        in backing object, false otherwise.
+        """
+        return self.mergeTuples(self.appendKeyArgConversion(), self.appendBoolResult())
+
+    def define(self):
+        return self.cgRoot.define()
+
+
+class CGHelperFunctionGenerator(CallbackMember):
+    """
+    Generates code to allow C++ to perform operations. Gets a context from the
+    binding wrapper, turns arguments into JS::Values (via
+    CallbackMember/CGNativeMember argument conversion), then uses
+    getCall to generate the body for getting result, and maybe convert the
+    result into return type (via CallbackMember/CGNativeMember result
+    conversion)
+    """
+
+    class HelperFunction(CGAbstractMethod):
+        """
+        Generates context retrieval code and rooted JSObject for interface for
+        method generator to use
+        """
+
+        def __init__(self, descriptor, name, args, code, returnType):
+            self.code = code
+            CGAbstractMethod.__init__(self, descriptor, name, returnType, args)
+
+        def definition_body(self):
+            return self.code
+
+    def __init__(
+        self,
+        descriptor,
+        name,
+        args,
+        returnType=BuiltinTypes[IDLBuiltinType.Types.undefined],
+        needsResultConversion=True,
+    ):
+        assert returnType.isType()
+        self.needsResultConversion = needsResultConversion
+
+        # Run CallbackMember init function to generate argument conversion code.
+        # wrapScope is set to 'obj' when generating maplike or setlike helper
+        # functions, as we don't have access to the CallbackPreserveColor
+        # method.
+        CallbackMember.__init__(
+            self,
+            [returnType, args],
+            name,
+            descriptor,
+            False,
+            wrapScope="obj",
+            passJSBitsAsNeeded=typeNeedsCx(returnType),
+        )
+
+        # Wrap CallbackMember body code into a CGAbstractMethod to make
+        # generation easier.
+        self.implMethod = CGHelperFunctionGenerator.HelperFunction(
+            descriptor, name, self.args, self.body, self.returnType
+        )
+
+    def getCallSetup(self):
+        # If passJSBitsAsNeeded is true, it means the caller will provide a
+        # JSContext, so we don't need to create JSContext and enter
+        # UnprivilegedJunkScopeOrWorkerGlobal here.
+        code = "MOZ_ASSERT(self);\n"
+        if not self.passJSBitsAsNeeded:
+            code += dedent(
+                """
+                AutoJSAPI jsapi;
+                jsapi.Init();
+                JSContext* cx = jsapi.cx();
+                // It's safe to use UnprivilegedJunkScopeOrWorkerGlobal here because
+                // all we want is to wrap into _some_ scope and then unwrap to find
+                // the reflector, and wrapping has no side-effects.
+                JSObject* scope = UnprivilegedJunkScopeOrWorkerGlobal(fallible);
+                if (!scope) {
+                  aRv.Throw(NS_ERROR_UNEXPECTED);
+                  return%s;
+                }
+                JSAutoRealm tempRealm(cx, scope);
+                """
+                % self.getDefaultRetval()
+            )
+
+        code += dedent(
+            """
+            JS::Rooted<JS::Value> v(cx);
+            if(!ToJSValue(cx, self, &v)) {
+              aRv.Throw(NS_ERROR_UNEXPECTED);
+              return%s;
+            }
+            // This is a reflector, but due to trying to name things
+            // similarly across method generators, it's called obj here.
+            JS::Rooted<JSObject*> obj(cx);
+            obj = js::UncheckedUnwrap(&v.toObject(), /* stopAtWindowProxy = */ false);
+            """
+            % self.getDefaultRetval()
+        )
+
+        # We'd like wrap the inner code in a scope such that the code can use the
+        # same realm. So here we are creating the result variable outside of the
+        # scope.
+        if self.needsScopeBody():
+            code += "JS::Rooted<JS::Value> result(cx);\n"
+
+        return code
+
+    def getArgs(self, returnType, argList):
+        # We don't need the context or the value. We'll generate those instead.
+        args = CGNativeMember.getArgs(self, returnType, argList)
+        # Prepend a pointer to the binding object onto the arguments
+        return [Argument(self.descriptorProvider.nativeType + "*", "self")] + args
+
+    def needsScopeBody(self):
+        return self.passJSBitsAsNeeded
+
+    def getArgvDeclFailureCode(self):
+        return "aRv.Throw(NS_ERROR_UNEXPECTED);\n"
+
+    def getResultConversion(self):
+        if self.needsResultConversion:
+            code = ""
+            if self.needsScopeBody():
+                code = dedent(
+                    """
+                    if (!JS_WrapValue(cx, &result)) {
+                      aRv.NoteJSContextException(cx);
+                      return;
+                    }
+                    """
+                )
+
+            failureCode = dedent("aRv.Throw(NS_ERROR_UNEXPECTED);\nreturn nullptr;\n")
+
+            exceptionCode = None
+            if self.retvalType.isPrimitive():
+                exceptionCode = dedent(
+                    "aRv.NoteJSContextException(cx);\nreturn%s;\n"
+                    % self.getDefaultRetval()
+                )
+
+            return code + CallbackMember.getResultConversion(
+                self,
+                "result",
+                failureCode=failureCode,
+                isDefinitelyObject=True,
+                exceptionCode=exceptionCode,
+            )
+
+        assignRetval = string.Template(
+            self.getRetvalInfo(self.retvalType, False)[2]
+        ).substitute(
+            {
+                "declName": "retVal",
+            }
+        )
+        return assignRetval
+
+    def getRvalDecl(self):
+        # hack to make sure we put JSAutoRealm inside the body scope
+        return "JSAutoRealm reflectorRealm(cx, obj);\n"
+
+    def getArgcDecl(self):
+        # Don't need argc for anything.
+        return None
+
+    def getCall(self):
+        assert False  # Override me!
+
+    def getPrettyName(self):
+        return self.name
+
+    def declare(self):
+        return self.implMethod.declare()
+
+    def define(self):
+        return self.implMethod.define()
+
+
+class CGMaplikeOrSetlikeHelperFunctionGenerator(CGHelperFunctionGenerator):
+    """
+    Generates code to allow C++ to perform operations on backing objects. Gets
+    a context from the binding wrapper, turns arguments into JS::Values (via
+    CallbackMember/CGNativeMember argument conversion), then uses
+    CGMaplikeOrSetlikeMethodGenerator to generate the body.
+    """
+
+    def __init__(
+        self,
+        descriptor,
+        maplikeOrSetlike,
+        name,
+        needsKeyArg=False,
+        needsValueArg=False,
+        needsValueTypeReturn=False,
+        needsBoolReturn=False,
+        needsResultConversion=True,
+    ):
+        self.maplikeOrSetlike = maplikeOrSetlike
+        self.needsValueTypeReturn = needsValueTypeReturn
+
+        args = []
+        if needsKeyArg:
+            args.append(FakeArgument(maplikeOrSetlike.keyType, "aKey"))
+        if needsValueArg:
+            assert needsKeyArg
+            assert not needsValueTypeReturn
+            args.append(FakeArgument(maplikeOrSetlike.valueType, "aValue"))
+
+        returnType = BuiltinTypes[IDLBuiltinType.Types.undefined]
+        if needsBoolReturn:
+            returnType = BuiltinTypes[IDLBuiltinType.Types.boolean]
+        elif needsValueTypeReturn:
+            returnType = maplikeOrSetlike.valueType
+
+        CGHelperFunctionGenerator.__init__(
+            self,
+            descriptor,
+            name,
+            args,
+            returnType,
+            needsResultConversion,
+        )
+
+    def getCall(self):
+        return CGMaplikeOrSetlikeMethodGenerator(
+            self.descriptorProvider,
+            self.maplikeOrSetlike,
+            self.name.lower(),
+            self.needsValueTypeReturn,
+            helperImpl=self,
+        ).define()
+
+
+class CGMaplikeOrSetlikeHelperGenerator(CGNamespace):
+    """
+    Declares and defines convenience methods for accessing backing objects on
+    setlike/maplike interface. Generates function signatures, un/packs
+    backing objects from slot, etc.
+    """
+
+    def __init__(self, descriptor, maplikeOrSetlike):
+        self.descriptor = descriptor
+        # Since iterables are folded in with maplike/setlike, make sure we've
+        # got the right type here.
+        assert maplikeOrSetlike.isMaplike() or maplikeOrSetlike.isSetlike()
+        self.maplikeOrSetlike = maplikeOrSetlike
+        self.namespace = "%sHelpers" % (
+            self.maplikeOrSetlike.maplikeOrSetlikeOrIterableType.title()
+        )
+        self.helpers = [
+            CGMaplikeOrSetlikeHelperFunctionGenerator(
+                descriptor, maplikeOrSetlike, "Clear"
+            ),
+            CGMaplikeOrSetlikeHelperFunctionGenerator(
+                descriptor,
+                maplikeOrSetlike,
+                "Delete",
+                needsKeyArg=True,
+                needsBoolReturn=True,
+                needsResultConversion=False,
+            ),
+            CGMaplikeOrSetlikeHelperFunctionGenerator(
+                descriptor,
+                maplikeOrSetlike,
+                "Has",
+                needsKeyArg=True,
+                needsBoolReturn=True,
+                needsResultConversion=False,
+            ),
+        ]
+        if self.maplikeOrSetlike.isMaplike():
+            self.helpers.append(
+                CGMaplikeOrSetlikeHelperFunctionGenerator(
+                    descriptor,
+                    maplikeOrSetlike,
+                    "Set",
+                    needsKeyArg=True,
+                    needsValueArg=True,
+                )
+            )
+            self.helpers.append(
+                CGMaplikeOrSetlikeHelperFunctionGenerator(
+                    descriptor,
+                    maplikeOrSetlike,
+                    "Get",
+                    needsKeyArg=True,
+                    needsValueTypeReturn=True,
+                )
+            )
+        else:
+            assert self.maplikeOrSetlike.isSetlike()
+            self.helpers.append(
+                CGMaplikeOrSetlikeHelperFunctionGenerator(
+                    descriptor, maplikeOrSetlike, "Add", needsKeyArg=True
+                )
+            )
+        CGNamespace.__init__(self, self.namespace, CGList(self.helpers))
+
+
+class CGIterableMethodGenerator(CGGeneric):
+    """
+    Creates methods for iterable interfaces. Unwrapping/wrapping
+    will be taken care of by the usual method generation machinery in
+    CGMethodCall/CGPerSignatureCall. Functionality is filled in here instead of
+    using CGCallGenerator.
+    """
+
+    def __init__(self, descriptor, methodName, args):
+        if methodName == "forEach":
+            assert len(args) == 2
+
+            CGGeneric.__init__(
+                self,
+                fill(
+                    """
+                if (!JS::IsCallable(arg0)) {
+                  cx.ThrowErrorMessage<MSG_NOT_CALLABLE>("Argument 1");
+                  return false;
+                }
+                JS::RootedValueArray<3> callArgs(cx);
+                callArgs[2].setObject(*obj);
+                JS::Rooted<JS::Value> ignoredReturnVal(cx);
+                auto GetKeyAtIndex = &${selfType}::GetKeyAtIndex;
+                auto GetValueAtIndex = &${selfType}::GetValueAtIndex;
+                for (size_t i = 0; i < self->GetIterableLength(); ++i) {
+                  if (!CallIterableGetter(cx, GetValueAtIndex, self, i,
+                                          callArgs[0])) {
+                    return false;
+                  }
+                  if (!CallIterableGetter(cx, GetKeyAtIndex, self, i,
+                                          callArgs[1])) {
+                    return false;
+                  }
+                  if (!JS::Call(cx, arg1, arg0, JS::HandleValueArray(callArgs),
+                                &ignoredReturnVal)) {
+                    return false;
+                  }
+                }
+                """,
+                    ifaceName=descriptor.interface.identifier.name,
+                    selfType=descriptor.nativeType,
+                ),
+            )
+            return
+
+        if descriptor.interface.isIterable():
+            assert descriptor.interface.maplikeOrSetlikeOrIterable.isPairIterator()
+            assert len(args) == 0
+
+            wrap = f"{descriptor.interface.identifier.name}Iterator_Binding::Wrap"
+            iterClass = f"mozilla::dom::binding_detail::WrappableIterableIterator<{descriptor.nativeType}, &{wrap}>"
+        else:
+            needReturnMethod = toStringBool(
+                descriptor.interface.maplikeOrSetlikeOrIterable.getExtendedAttribute(
+                    "GenerateReturnMethod"
+                )
+                is not None
+            )
+            wrap = f"{descriptor.interface.identifier.name}AsyncIterator_Binding::Wrap"
+            iterClass = f"mozilla::dom::binding_detail::WrappableAsyncIterableIterator<{descriptor.nativeType}, {needReturnMethod}, &{wrap}>"
+
+        createIterator = fill(
+            """
+            typedef ${iterClass} itrType;
+            RefPtr<itrType> result(new itrType(self,
+                                               itrType::IteratorType::${itrMethod}));
+            """,
+            iterClass=iterClass,
+            itrMethod=methodName.title(),
+        )
+
+        if descriptor.interface.isAsyncIterable():
+            args.append("initError")
+            createIterator = fill(
+                """
+                $*{createIterator}
+                {
+                  ErrorResult initError;
+                  self->InitAsyncIteratorData(result->Data(), itrType::IteratorType::${itrMethod}, ${args});
+                  if (initError.MaybeSetPendingException(cx, "Asynchronous iterator initialization steps for ${ifaceName} failed")) {
+                    return false;
+                  }
+                }
+                """,
+                createIterator=createIterator,
+                itrMethod=methodName.title(),
+                args=", ".join(args),
+                ifaceName=descriptor.interface.identifier.name,
+            )
+
+        CGGeneric.__init__(self, createIterator)
+
+
+def getObservableArrayBackingObject(descriptor, attr, errorReturn="return false;\n"):
+    """
+    Generate code to get/create a JS backing list for an observableArray attribute
+    from the declaration slot.
+    """
+    assert attr.isAttr()
+    assert attr.type.isObservableArray()
+
+    # GetObservableArrayBackingObject may return a wrapped object for Xrays, so
+    # when we create it we need to unwrap it to store the interface in the
+    # reserved slot.
+    return fill(
+        """
+        JS::Rooted<JSObject*> backingObj(cx);
+        bool created = false;
+        if (!GetObservableArrayBackingObject(cx, obj, ${slot},
+                &backingObj, &created, ${namespace}::ObservableArrayProxyHandler::getInstance(),
+                self)) {
+          $*{errorReturn}
+        }
+        if (created) {
+          PreserveWrapper(self);
+        }
+        """,
+        namespace=toBindingNamespace(MakeNativeName(attr.identifier.name)),
+        slot=memberReservedSlot(attr, descriptor),
+        errorReturn=errorReturn,
+        selfType=descriptor.nativeType,
+    )
+
+
+def getObservableArrayGetterBody(descriptor, attr):
+    """
+    Creates the body for the getter method of an observableArray attribute.
+    """
+    assert attr.type.isObservableArray()
+    return fill(
+        """
+        $*{getBackingObj}
+        MOZ_ASSERT(!JS_IsExceptionPending(cx));
+        args.rval().setObject(*backingObj);
+        return true;
+        """,
+        getBackingObj=getObservableArrayBackingObject(descriptor, attr),
+    )
+
+
+class CGObservableArrayProxyHandler_callback(ClassMethod):
+    """
+    Base class for declaring and defining the hook methods for ObservableArrayProxyHandler
+    subclasses to get the interface native object from backing object and calls
+    its On{Set|Delete}* callback.
+
+     * 'callbackType': "Set" or "Delete".
+     * 'invalidTypeFatal' (optional): If True, we don't expect the type
+                                      conversion would fail, so generate the
+                                      assertion code if type conversion fails.
+    """
+
+    def __init__(
+        self, descriptor, attr, name, args, callbackType, invalidTypeFatal=False
+    ):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        assert callbackType in ["Set", "Delete"]
+        self.descriptor = descriptor
+        self.attr = attr
+        self.innertype = attr.type.inner
+        self.callbackType = callbackType
+        self.invalidTypeFatal = invalidTypeFatal
+        ClassMethod.__init__(
+            self,
+            name,
+            "bool",
+            args,
+            visibility="protected",
+            virtual=True,
+            override=True,
+            const=True,
+        )
+
+    def preConversion(self):
+        """
+        The code to run before the conversion steps.
+        """
+        return ""
+
+    def preCallback(self):
+        """
+        The code to run before calling the callback.
+        """
+        return ""
+
+    def postCallback(self):
+        """
+        The code to run after calling the callback, all subclasses should override
+        this to generate the return values.
+        """
+        assert False  # Override me!
+
+    def getBody(self):
+        exceptionCode = (
+            fill(
+                """
+                MOZ_ASSERT_UNREACHABLE("The item in ObservableArray backing list is not ${innertype}?");
+                return false;
+                """,
+                innertype=self.innertype,
+            )
+            if self.invalidTypeFatal
+            else None
+        )
+        convertType = instantiateJSToNativeConversion(
+            getJSToNativeConversionInfo(
+                self.innertype,
+                self.descriptor,
+                sourceDescription="Element in ObservableArray backing list",
+                exceptionCode=exceptionCode,
+            ),
+            {
+                "declName": "decl",
+                "holderName": "holder",
+                "val": "aValue",
+            },
+        )
+        callbackArgs = ["decl", "aIndex", "rv"]
+        if typeNeedsCx(self.innertype):
+            callbackArgs.insert(0, "cx")
+        return fill(
+            """
+            MOZ_ASSERT(IsObservableArrayProxy(aProxy));
+            $*{preConversion}
+
+            BindingCallContext cx(aCx, "ObservableArray ${name}");
+            $*{convertType}
+
+            $*{preCallback}
+            JS::Value val = js::GetProxyReservedSlot(aProxy, OBSERVABLE_ARRAY_DOM_INTERFACE_SLOT);
+            auto* interface = static_cast<${ifaceType}*>(val.toPrivate());
+            MOZ_ASSERT(interface);
+
+            ErrorResult rv;
+            MOZ_KnownLive(interface)->${methodName}(${callbackArgs});
+            $*{postCallback}
+            """,
+            preConversion=self.preConversion(),
+            name=self.name,
+            convertType=convertType.define(),
+            preCallback=self.preCallback(),
+            ifaceType=self.descriptor.nativeType,
+            methodName="On%s%s"
+            % (self.callbackType, MakeNativeName(self.attr.identifier.name)),
+            callbackArgs=", ".join(callbackArgs),
+            postCallback=self.postCallback(),
+        )
+
+
+class CGObservableArrayProxyHandler_OnDeleteItem(
+    CGObservableArrayProxyHandler_callback
+):
+    """
+    Declares and defines the hook methods for ObservableArrayProxyHandler
+    subclasses to get the interface native object from backing object and calls
+    its OnDelete* callback.
+    """
+
+    def __init__(self, descriptor, attr):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("JS::Handle<JSObject*>", "aProxy"),
+            Argument("JS::Handle<JS::Value>", "aValue"),
+            Argument("uint32_t", "aIndex"),
+        ]
+        CGObservableArrayProxyHandler_callback.__init__(
+            self,
+            descriptor,
+            attr,
+            "OnDeleteItem",
+            args,
+            "Delete",
+            True,
+        )
+
+    def postCallback(self):
+        return dedent(
+            """
+            return !rv.MaybeSetPendingException(cx);
+            """
+        )
+
+
+class CGObservableArrayProxyHandler_SetIndexedValue(
+    CGObservableArrayProxyHandler_callback
+):
+    """
+    Declares and defines the hook methods for ObservableArrayProxyHandler
+    subclasses to run the setting the indexed value steps.
+    """
+
+    def __init__(self, descriptor, attr):
+        args = [
+            Argument("JSContext*", "aCx"),
+            Argument("JS::Handle<JSObject*>", "aProxy"),
+            Argument("JS::Handle<JSObject*>", "aBackingList"),
+            Argument("uint32_t", "aIndex"),
+            Argument("JS::Handle<JS::Value>", "aValue"),
+            Argument("JS::ObjectOpResult&", "aResult"),
+        ]
+        CGObservableArrayProxyHandler_callback.__init__(
+            self,
+            descriptor,
+            attr,
+            "SetIndexedValue",
+            args,
+            "Set",
+        )
+
+    def preConversion(self):
+        return dedent(
+            """
+            uint32_t oldLen;
+            if (!JS::GetArrayLength(aCx, aBackingList, &oldLen)) {
+              return false;
+            }
+
+            if (aIndex > oldLen) {
+              return aResult.failBadIndex();
+            }
+            """
+        )
+
+    def preCallback(self):
+        return dedent(
+            """
+            if (aIndex < oldLen) {
+              JS::Rooted<JS::Value> value(aCx);
+              if (!JS_GetElement(aCx, aBackingList, aIndex, &value)) {
+                return false;
+              }
+
+              if (!OnDeleteItem(aCx, aProxy, value, aIndex)) {
+                return false;
+              }
+            }
+
+            """
+        )
+
+    def postCallback(self):
+        return dedent(
+            """
+            if (rv.MaybeSetPendingException(cx)) {
+              return false;
+            }
+
+            if (!JS_SetElement(aCx, aBackingList, aIndex, aValue)) {
+              return false;
+            }
+
+            return aResult.succeed();
+            """
+        )
+
+
+class CGObservableArrayProxyHandler(CGThing):
+    """
+    A class for declaring a ObservableArrayProxyHandler.
+    """
+
+    def __init__(self, descriptor, attr):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        methods = [
+            # The item stored in backing object should always be converted successfully.
+            CGObservableArrayProxyHandler_OnDeleteItem(descriptor, attr),
+            CGObservableArrayProxyHandler_SetIndexedValue(descriptor, attr),
+            CGJSProxyHandler_getInstance("ObservableArrayProxyHandler"),
+        ]
+        parentClass = "mozilla::dom::ObservableArrayProxyHandler"
+        self.proxyHandler = CGClass(
+            "ObservableArrayProxyHandler",
+            bases=[ClassBase(parentClass)],
+            constructors=[],
+            methods=methods,
+        )
+
+    def declare(self):
+        # Our class declaration should happen when we're defining
+        return ""
+
+    def define(self):
+        return self.proxyHandler.declare() + "\n" + self.proxyHandler.define()
+
+
+class CGObservableArrayProxyHandlerGenerator(CGNamespace):
+    """
+    Declares and defines convenience methods for accessing backing list objects
+    for observable array attribute. Generates function signatures, un/packs
+    backing list objects from slot, etc.
+    """
+
+    def __init__(self, descriptor, attr):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        namespace = toBindingNamespace(MakeNativeName(attr.identifier.name))
+        proxyHandler = CGObservableArrayProxyHandler(descriptor, attr)
+        CGNamespace.__init__(self, namespace, proxyHandler)
+
+
+class CGObservableArraySetterGenerator(CGGeneric):
+    """
+    Creates setter for an observableArray attributes.
+    """
+
+    def __init__(self, descriptor, attr):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        getBackingObject = getObservableArrayBackingObject(descriptor, attr)
+        setElement = dedent(
+            """
+            if (!JS_SetElement(cx, backingObj, i, val)) {
+              return false;
+            }
+            """
+        )
+        conversion = wrapForType(
+            attr.type.inner,
+            descriptor,
+            {
+                "result": "arg0.ElementAt(i)",
+                "successCode": setElement,
+                "jsvalRef": "val",
+                "jsvalHandle": "&val",
+            },
+        )
+        CGGeneric.__init__(
+            self,
+            fill(
+                """
+                if (xpc::WrapperFactory::IsXrayWrapper(obj)) {
+                  JS_ReportErrorASCII(cx, "Accessing from Xray wrapper is not supported.");
+                  return false;
+                }
+
+                ${getBackingObject}
+                const ObservableArrayProxyHandler* handler = GetObservableArrayProxyHandler(backingObj);
+                if (!handler->SetLength(cx, backingObj, 0)) {
+                  return false;
+                }
+
+                JS::Rooted<JS::Value> val(cx);
+                for (size_t i = 0; i < arg0.Length(); i++) {
+                  $*{conversion}
+                }
+                """,
+                conversion=conversion,
+                getBackingObject=getBackingObject,
+            ),
+        )
+
+
+class CGObservableArrayHelperFunctionGenerator(CGHelperFunctionGenerator):
+    """
+    Generates code to allow C++ to perform operations on backing objects. Gets
+    a context from the binding wrapper, turns arguments into JS::Values (via
+    CallbackMember/CGNativeMember argument conversion), then uses
+    MethodBodyGenerator to generate the body.
+    """
+
+    class MethodBodyGenerator(CGThing):
+        """
+        Creates methods body for observable array attribute. It is expected that all
+        methods will be have a maplike/setlike object attached. Unwrapping/wrapping
+        will be taken care of by the usual method generation machinery in
+        CGMethodCall/CGPerSignatureCall. Functionality is filled in here instead of
+        using CGCallGenerator.
+        """
+
+        def __init__(
+            self,
+            descriptor,
+            attr,
+            methodName,
+            helperGenerator,
+            needsIndexArg,
+        ):
+            assert attr.isAttr()
+            assert attr.type.isObservableArray()
+
+            CGThing.__init__(self)
+            self.helperGenerator = helperGenerator
+            self.cgRoot = CGList([])
+
+            self.cgRoot.append(
+                CGGeneric(
+                    getObservableArrayBackingObject(
+                        descriptor,
+                        attr,
+                        dedent(
+                            """
+                            aRv.Throw(NS_ERROR_UNEXPECTED);
+                            return%s;
+                            """
+                            % helperGenerator.getDefaultRetval()
+                        ),
+                    )
+                )
+            )
+
+            # Generates required code for the method. Method descriptions included
+            # in definitions below. Throw if we don't have a method to fill in what
+            # we're looking for.
+            try:
+                methodGenerator = getattr(self, methodName)
+            except AttributeError:
+                raise TypeError(
+                    "Missing observable array method definition '%s'" % methodName
+                )
+            # Method generator returns tuple, containing:
+            #
+            # - a list of CGThings representing setup code for preparing to call
+            #   the JS API function
+            # - JS API function name
+            # - a list of arguments needed for the JS API function we're calling
+            # - a list of CGThings representing code needed before return.
+            (setupCode, funcName, arguments, returnCode) = methodGenerator()
+
+            # Append the list of setup code CGThings
+            self.cgRoot.append(CGList(setupCode))
+            # Create the JS API call
+            if needsIndexArg:
+                arguments.insert(0, "aIndex")
+            self.cgRoot.append(
+                CGWrapper(
+                    CGGeneric(
+                        fill(
+                            """
+                            aRv.MightThrowJSException();
+                            if (!${funcName}(${args})) {
+                              aRv.StealExceptionFromJSContext(cx);
+                              return${retval};
+                            }
+                            """,
+                            funcName=funcName,
+                            args=", ".join(["cx", "backingObj"] + arguments),
+                            retval=helperGenerator.getDefaultRetval(),
+                        )
+                    )
+                )
+            )
+            # Append code before return
+            self.cgRoot.append(CGList(returnCode))
+
+        def elementat(self):
+            setupCode = []
+            if not self.helperGenerator.needsScopeBody():
+                setupCode.append(CGGeneric("JS::Rooted<JS::Value> result(cx);\n"))
+            returnCode = [
+                CGGeneric(
+                    fill(
+                        """
+                        if (result.isUndefined()) {
+                          aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+                          return${retval};
+                        }
+                        """,
+                        retval=self.helperGenerator.getDefaultRetval(),
+                    )
+                )
+            ]
+            return (setupCode, "JS_GetElement", ["&result"], returnCode)
+
+        def replaceelementat(self):
+            setupCode = [
+                CGGeneric(
+                    fill(
+                        """
+                        uint32_t length;
+                        aRv.MightThrowJSException();
+                        if (!JS::GetArrayLength(cx, backingObj, &length)) {
+                          aRv.StealExceptionFromJSContext(cx);
+                          return${retval};
+                        }
+                        if (aIndex > length) {
+                          aRv.ThrowRangeError("Invalid index");
+                          return${retval};
+                        }
+                        """,
+                        retval=self.helperGenerator.getDefaultRetval(),
+                    )
+                )
+            ]
+            return (setupCode, "JS_SetElement", ["argv[0]"], [])
+
+        def appendelement(self):
+            setupCode = [
+                CGGeneric(
+                    fill(
+                        """
+                        uint32_t length;
+                        aRv.MightThrowJSException();
+                        if (!JS::GetArrayLength(cx, backingObj, &length)) {
+                          aRv.StealExceptionFromJSContext(cx);
+                          return${retval};
+                        }
+                        """,
+                        retval=self.helperGenerator.getDefaultRetval(),
+                    )
+                )
+            ]
+            return (setupCode, "JS_SetElement", ["length", "argv[0]"], [])
+
+        def removelastelement(self):
+            setupCode = [
+                CGGeneric(
+                    fill(
+                        """
+                        uint32_t length;
+                        aRv.MightThrowJSException();
+                        if (!JS::GetArrayLength(cx, backingObj, &length)) {
+                          aRv.StealExceptionFromJSContext(cx);
+                          return${retval};
+                        }
+                        if (length == 0) {
+                          aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+                          return${retval};
+                        }
+                        """,
+                        retval=self.helperGenerator.getDefaultRetval(),
+                    )
+                )
+            ]
+            return (setupCode, "JS::SetArrayLength", ["length - 1"], [])
+
+        def length(self):
+            return (
+                [CGGeneric("uint32_t retVal;\n")],
+                "JS::GetArrayLength",
+                ["&retVal"],
+                [],
+            )
+
+        def define(self):
+            return self.cgRoot.define()
+
+    def __init__(
+        self,
+        descriptor,
+        attr,
+        name,
+        returnType=BuiltinTypes[IDLBuiltinType.Types.undefined],
+        needsResultConversion=True,
+        needsIndexArg=False,
+        needsValueArg=False,
+    ):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+        self.attr = attr
+        self.needsIndexArg = needsIndexArg
+
+        args = []
+        if needsValueArg:
+            args.append(FakeArgument(attr.type.inner, "aValue"))
+
+        CGHelperFunctionGenerator.__init__(
+            self,
+            descriptor,
+            name,
+            args,
+            returnType,
+            needsResultConversion,
+        )
+
+    def getArgs(self, returnType, argList):
+        if self.needsIndexArg:
+            argList = [
+                FakeArgument(BuiltinTypes[IDLBuiltinType.Types.unsigned_long], "aIndex")
+            ] + argList
+        return CGHelperFunctionGenerator.getArgs(self, returnType, argList)
+
+    def getCall(self):
+        return CGObservableArrayHelperFunctionGenerator.MethodBodyGenerator(
+            self.descriptorProvider,
+            self.attr,
+            self.name.lower(),
+            self,
+            self.needsIndexArg,
+        ).define()
+
+
+class CGObservableArrayHelperGenerator(CGNamespace):
+    """
+    Declares and defines convenience methods for accessing backing object for
+    observable array type. Generates function signatures, un/packs
+    backing objects from slot, etc.
+    """
+
+    def __init__(self, descriptor, attr):
+        assert attr.isAttr()
+        assert attr.type.isObservableArray()
+
+        namespace = "%sHelpers" % MakeNativeName(attr.identifier.name)
+        helpers = [
+            CGObservableArrayHelperFunctionGenerator(
+                descriptor,
+                attr,
+                "ElementAt",
+                returnType=attr.type.inner,
+                needsIndexArg=True,
+            ),
+            CGObservableArrayHelperFunctionGenerator(
+                descriptor,
+                attr,
+                "ReplaceElementAt",
+                needsIndexArg=True,
+                needsValueArg=True,
+            ),
+            CGObservableArrayHelperFunctionGenerator(
+                descriptor,
+                attr,
+                "AppendElement",
+                needsValueArg=True,
+            ),
+            CGObservableArrayHelperFunctionGenerator(
+                descriptor,
+                attr,
+                "RemoveLastElement",
+            ),
+            CGObservableArrayHelperFunctionGenerator(
+                descriptor,
+                attr,
+                "Length",
+                returnType=BuiltinTypes[IDLBuiltinType.Types.unsigned_long],
+                needsResultConversion=False,
+            ),
+        ]
+        CGNamespace.__init__(self, namespace, CGList(helpers, "\n"))
+
+
+class GlobalGenRoots:
+    """
+    Roots for global codegen.
+
+    To generate code, call the method associated with the target, and then
+    call the appropriate define/declare method.
+    """
+
+    @staticmethod
+    def GeneratedAtomList(config):
+        # Atom enum
+        dictionaries = config.dictionaries
+
+        structs = []
+
+        def memberToAtomCacheMember(binaryNameFor, m):
+            binaryMemberName = binaryNameFor(m)
+            return ClassMember(
+                CGDictionary.makeIdName(binaryMemberName),
+                "PinnedStringId",
+                visibility="public",
+            )
+
+        def buildAtomCacheStructure(idlobj, binaryNameFor, members):
+            classMembers = [memberToAtomCacheMember(binaryNameFor, m) for m in members]
+            structName = idlobj.identifier.name + "Atoms"
+            return (
+                structName,
+                CGWrapper(
+                    CGClass(
+                        structName, bases=None, isStruct=True, members=classMembers
+                    ),
+                    post="\n",
+                ),
+            )
+
+        for dict in dictionaries:
+            if len(dict.members) == 0:
+                continue
+
+            structs.append(
+                buildAtomCacheStructure(dict, lambda m: m.identifier.name, dict.members)
+            )
+
+        for d in config.getDescriptors(isJSImplemented=True) + config.getDescriptors(
+            isCallback=True
+        ):
+            members = [m for m in d.interface.members if m.isAttr() or m.isMethod()]
+            if d.interface.isJSImplemented() and d.interface.ctor():
+                # We'll have an __init() method.
+                members.append(FakeMember("__init"))
+            if d.interface.isJSImplemented() and d.interface.getExtendedAttribute(
+                "WantsEventListenerHooks"
+            ):
+                members.append(FakeMember("eventListenerAdded"))
+                members.append(FakeMember("eventListenerRemoved"))
+            if len(members) == 0:
+                continue
+
+            structs.append(
+                buildAtomCacheStructure(
+                    d.interface,
+                    lambda m: d.binaryNameFor(m.identifier.name, m.isStatic()),
+                    members,
+                )
+            )
+
+        structs.sort()
+        generatedStructs = [struct for structName, struct in structs]
+        structNames = [structName for structName, struct in structs]
+
+        mainStruct = CGWrapper(
+            CGClass(
+                "PerThreadAtomCache",
+                bases=[ClassBase(structName) for structName in structNames],
+                isStruct=True,
+            ),
+            post="\n",
+        )
+
+        structs = CGList(generatedStructs + [mainStruct])
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(structs, pre="\n"))
+        curr = CGWrapper(curr, post="\n")
+
+        # Add include statement for PinnedStringId.
+        declareIncludes = ["mozilla/dom/PinnedStringId.h"]
+        curr = CGHeaders([], [], [], [], declareIncludes, [], "GeneratedAtomList", curr)
+
+        # Add include guards.
+        curr = CGIncludeGuard("GeneratedAtomList", curr)
+
+        # Add the auto-generated comment.
+        curr = CGWrapper(curr, pre=AUTOGENERATED_WARNING_COMMENT)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def GeneratedEventList(config):
+        eventList = CGList([])
+        for generatedEvent in config.generatedEvents:
+            eventList.append(
+                CGGeneric(declare=("GENERATED_EVENT(%s)\n" % generatedEvent))
+            )
+        return eventList
+
+    @staticmethod
+    def PrototypeList(config):
+
+        # Prototype ID enum.
+        descriptorsWithPrototype = config.getDescriptors(
+            hasInterfacePrototypeObject=True
+        )
+        protos = [d.name for d in descriptorsWithPrototype]
+        idEnum = CGNamespacedEnum("id", "ID", ["_ID_Start"] + protos, [0, "_ID_Start"])
+        idEnum = CGList([idEnum])
+
+        def fieldSizeAssert(amount, jitInfoField, message):
+            maxFieldValue = (
+                "(uint64_t(1) << (sizeof(std::declval<JSJitInfo>().%s) * 8))"
+                % jitInfoField
+            )
+            return CGGeneric(
+                define='static_assert(%s < %s, "%s");\n\n'
+                % (amount, maxFieldValue, message)
+            )
+
+        idEnum.append(
+            fieldSizeAssert("id::_ID_Count", "protoID", "Too many prototypes!")
+        )
+
+        # Wrap all of that in our namespaces.
+        idEnum = CGNamespace.build(
+            ["mozilla", "dom", "prototypes"], CGWrapper(idEnum, pre="\n")
+        )
+        idEnum = CGWrapper(idEnum, post="\n")
+
+        curr = CGList(
+            [
+                CGGeneric(define="#include <stdint.h>\n"),
+                CGGeneric(define="#include <type_traits>\n\n"),
+                CGGeneric(define='#include "js/experimental/JitInfo.h"\n\n'),
+                CGGeneric(define='#include "mozilla/dom/BindingNames.h"\n\n'),
+                CGGeneric(define='#include "mozilla/dom/PrototypeList.h"\n\n'),
+                idEnum,
+            ]
+        )
+
+        # Let things know the maximum length of the prototype chain.
+        maxMacroName = "MAX_PROTOTYPE_CHAIN_LENGTH"
+        maxMacro = CGGeneric(
+            declare="#define " + maxMacroName + " " + str(config.maxProtoChainLength)
+        )
+        curr.append(CGWrapper(maxMacro, post="\n\n"))
+        curr.append(
+            fieldSizeAssert(
+                maxMacroName, "depth", "Some inheritance chain is too long!"
+            )
+        )
+
+        # Constructor ID enum.
+        constructors = [d.name for d in config.getDescriptors(hasInterfaceObject=True)]
+        idEnum = CGNamespacedEnum(
+            "id",
+            "ID",
+            ["_ID_Start"] + constructors,
+            ["prototypes::id::_ID_Count", "_ID_Start"],
+        )
+
+        # Wrap all of that in our namespaces.
+        idEnum = CGNamespace.build(
+            ["mozilla", "dom", "constructors"], CGWrapper(idEnum, pre="\n")
+        )
+        idEnum = CGWrapper(idEnum, post="\n")
+
+        curr.append(idEnum)
+
+        # Named properties object enum.
+        namedPropertiesObjects = [
+            d.name for d in config.getDescriptors(hasNamedPropertiesObject=True)
+        ]
+        idEnum = CGNamespacedEnum(
+            "id",
+            "ID",
+            ["_ID_Start"] + namedPropertiesObjects,
+            ["constructors::id::_ID_Count", "_ID_Start"],
+        )
+
+        # Wrap all of that in our namespaces.
+        idEnum = CGNamespace.build(
+            ["mozilla", "dom", "namedpropertiesobjects"], CGWrapper(idEnum, pre="\n")
+        )
+        idEnum = CGWrapper(idEnum, post="\n")
+
+        curr.append(idEnum)
+
+        traitsDecls = [
+            CGGeneric(
+                declare=dedent(
+                    """
+            template <prototypes::ID PrototypeID>
+            struct PrototypeTraits;
+            """
+                )
+            )
+        ]
+        traitsDecls.extend(CGPrototypeTraitsClass(d) for d in descriptorsWithPrototype)
+
+        ifaceNamesWithProto = [
+            d.interface.getClassName() for d in descriptorsWithPrototype
+        ]
+        traitsDecls.append(
+            CGStringTable("NamesOfInterfacesWithProtos", ifaceNamesWithProto)
+        )
+
+        traitsDecl = CGNamespace.build(["mozilla", "dom"], CGList(traitsDecls))
+
+        curr.append(traitsDecl)
+
+        # Add include guards.
+        curr = CGIncludeGuard("PrototypeList", curr)
+
+        # Add the auto-generated comment.
+        curr = CGWrapper(curr, pre=AUTOGENERATED_WARNING_COMMENT)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def BindingNames(config):
+        declare = fill(
+            """
+            enum class BindingNamesOffset : uint16_t {
+              $*{enumValues}
+            };
+
+            namespace binding_detail {
+            extern const char sBindingNames[];
+            }  // namespace binding_detail
+
+            MOZ_ALWAYS_INLINE const char* BindingName(BindingNamesOffset aOffset) {
+              return binding_detail::sBindingNames + static_cast<size_t>(aOffset);
+            }
+            """,
+            enumValues="".join(
+                "%s = %i,\n" % (BindingNamesOffsetEnum(n), o)
+                for (n, o) in config.namesStringOffsets
+            ),
+        )
+        define = fill(
+            """
+            namespace binding_detail {
+
+            const char sBindingNames[] = {
+              $*{namesString}
+            };
+
+            }  // namespace binding_detail
+
+            // Making this enum bigger than a uint16_t has consequences on the size
+            // of some structs (eg. WebIDLNameTableEntry) and tables. We should try
+            // to avoid that.
+            static_assert(EnumTypeFitsWithin<BindingNamesOffset, uint16_t>::value,
+                          "Size increase");
+            """,
+            namesString=' "\\0"\n'.join(
+                '/* %5i */ "%s"' % (o, n) for (n, o) in config.namesStringOffsets
+            )
+            + "\n",
+        )
+
+        curr = CGGeneric(declare=declare, define=define)
+        curr = CGWrapper(curr, pre="\n", post="\n")
+
+        curr = CGNamespace.build(["mozilla", "dom"], curr)
+        curr = CGWrapper(curr, post="\n")
+
+        curr = CGHeaders(
+            [],
+            [],
+            [],
+            [],
+            ["<stddef.h>", "<stdint.h>", "mozilla/Attributes.h"],
+            ["mozilla/dom/BindingNames.h", "mozilla/EnumTypeTraits.h"],
+            "BindingNames",
+            curr,
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("BindingNames", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def RegisterBindings(config):
+
+        curr = CGNamespace.build(
+            ["mozilla", "dom"], CGGlobalNames(config.windowGlobalNames)
+        )
+        curr = CGWrapper(curr, post="\n")
+
+        # Add the includes
+        defineIncludes = [
+            CGHeaders.getDeclarationFilename(desc.interface)
+            for desc in config.getDescriptors(
+                hasInterfaceObject=True, isExposedInWindow=True, register=True
+            )
+        ]
+        defineIncludes.append("mozilla/dom/BindingNames.h")
+        defineIncludes.append("mozilla/dom/WebIDLGlobalNameHash.h")
+        defineIncludes.append("mozilla/dom/PrototypeList.h")
+        defineIncludes.append("mozilla/PerfectHash.h")
+        defineIncludes.append("js/String.h")
+        curr = CGHeaders([], [], [], [], [], defineIncludes, "RegisterBindings", curr)
+
+        # Add include guards.
+        curr = CGIncludeGuard("RegisterBindings", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def RegisterWorkerBindings(config):
+
+        curr = CGRegisterWorkerBindings(config)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(curr, post="\n"))
+        curr = CGWrapper(curr, post="\n")
+
+        # Add the includes
+        defineIncludes = [
+            CGHeaders.getDeclarationFilename(desc.interface)
+            for desc in config.getDescriptors(
+                hasInterfaceObject=True, register=True, isExposedInAnyWorker=True
+            )
+        ]
+
+        curr = CGHeaders(
+            [], [], [], [], [], defineIncludes, "RegisterWorkerBindings", curr
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("RegisterWorkerBindings", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def RegisterWorkerDebuggerBindings(config):
+
+        curr = CGRegisterWorkerDebuggerBindings(config)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(curr, post="\n"))
+        curr = CGWrapper(curr, post="\n")
+
+        # Add the includes
+        defineIncludes = [
+            CGHeaders.getDeclarationFilename(desc.interface)
+            for desc in config.getDescriptors(
+                hasInterfaceObject=True, register=True, isExposedInWorkerDebugger=True
+            )
+        ]
+
+        curr = CGHeaders(
+            [], [], [], [], [], defineIncludes, "RegisterWorkerDebuggerBindings", curr
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("RegisterWorkerDebuggerBindings", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def RegisterWorkletBindings(config):
+
+        curr = CGRegisterWorkletBindings(config)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(curr, post="\n"))
+        curr = CGWrapper(curr, post="\n")
+
+        # Add the includes
+        defineIncludes = [
+            CGHeaders.getDeclarationFilename(desc.interface)
+            for desc in config.getDescriptors(
+                hasInterfaceObject=True, register=True, isExposedInAnyWorklet=True
+            )
+        ]
+
+        curr = CGHeaders(
+            [], [], [], [], [], defineIncludes, "RegisterWorkletBindings", curr
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("RegisterWorkletBindings", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def RegisterShadowRealmBindings(config):
+
+        curr = CGRegisterShadowRealmBindings(config)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], CGWrapper(curr, post="\n"))
+        curr = CGWrapper(curr, post="\n")
+
+        # Add the includes
+        defineIncludes = [
+            CGHeaders.getDeclarationFilename(desc.interface)
+            for desc in config.getDescriptors(
+                hasInterfaceObject=True, register=True, isExposedInShadowRealms=True
+            )
+        ]
+
+        curr = CGHeaders(
+            [], [], [], [], [], defineIncludes, "RegisterShadowRealmBindings", curr
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("RegisterShadowRealmBindings", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def UnionTypes(config):
+        unionTypes = UnionsForFile(config, None)
+        (
+            includes,
+            implincludes,
+            declarations,
+            traverseMethods,
+            unlinkMethods,
+            unionStructs,
+        ) = UnionTypes(unionTypes, config)
+
+        unionStructs = dependencySortDictionariesAndUnionsAndCallbacks(unionStructs)
+
+        unions = CGList(
+            traverseMethods
+            + unlinkMethods
+            + [CGUnionStruct(t, config) for t in unionStructs]
+            + [CGUnionStruct(t, config, True) for t in unionStructs],
+            "\n",
+        )
+
+        includes.add("mozilla/OwningNonNull.h")
+        includes.add("mozilla/dom/UnionMember.h")
+        includes.add("mozilla/dom/BindingDeclarations.h")
+        # BindingUtils.h is only needed for SetToObject.
+        # If it stops being inlined or stops calling CallerSubsumes
+        # both this bit and the bit in CGBindingRoot can be removed.
+        includes.add("mozilla/dom/BindingUtils.h")
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], unions)
+
+        curr = CGWrapper(curr, post="\n")
+
+        builder = ForwardDeclarationBuilder()
+        for className, isStruct in declarations:
+            builder.add(className, isStruct=isStruct)
+
+        curr = CGList([builder.build(), curr], "\n")
+
+        curr = CGHeaders([], [], [], [], includes, implincludes, "UnionTypes", curr)
+
+        # Add include guards.
+        curr = CGIncludeGuard("UnionTypes", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def WebIDLPrefs(config):
+        prefs = set()
+        headers = set(["mozilla/dom/WebIDLPrefs.h"])
+        for d in config.getDescriptors(hasInterfaceOrInterfacePrototypeObject=True):
+            for m in d.interface.members:
+                pref = PropertyDefiner.getStringAttr(m, "Pref")
+                if pref:
+                    headers.add(prefHeader(pref))
+                    prefs.add((pref, prefIdentifier(pref)))
+        prefs = sorted(prefs)
+        declare = fill(
+            """
+            enum class WebIDLPrefIndex : uint8_t {
+              NoPref,
+              $*{prefs}
+            };
+            typedef bool (*WebIDLPrefFunc)();
+            extern const WebIDLPrefFunc sWebIDLPrefs[${len}];
+            """,
+            prefs=",\n".join(map(lambda p: "// " + p[0] + "\n" + p[1], prefs)) + "\n",
+            len=len(prefs) + 1,
+        )
+        define = fill(
+            """
+            const WebIDLPrefFunc sWebIDLPrefs[] = {
+              nullptr,
+              $*{prefs}
+            };
+            """,
+            prefs=",\n".join(
+                map(lambda p: "// " + p[0] + "\nStaticPrefs::" + p[1], prefs)
+            )
+            + "\n",
+        )
+        prefFunctions = CGGeneric(declare=declare, define=define)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], prefFunctions)
+
+        curr = CGWrapper(curr, post="\n")
+
+        curr = CGHeaders([], [], [], [], [], headers, "WebIDLPrefs", curr)
+
+        # Add include guards.
+        curr = CGIncludeGuard("WebIDLPrefs", curr)
+
+        # Done.
+        return curr
+
+    @staticmethod
+    def WebIDLSerializable(config):
+        # We need a declaration of StructuredCloneTags in the header.
+        declareIncludes = set(
+            [
+                "mozilla/dom/DOMJSClass.h",
+                "mozilla/dom/StructuredCloneTags.h",
+                "js/TypeDecls.h",
+            ]
+        )
+        defineIncludes = set(
+            ["mozilla/dom/WebIDLSerializable.h", "mozilla/PerfectHash.h"]
+        )
+        names = list()
+        for d in config.getDescriptors(isSerializable=True):
+            names.append(d.name)
+            defineIncludes.add(CGHeaders.getDeclarationFilename(d.interface))
+
+        if len(names) == 0:
+            # We can't really create a PerfectHash out of this, but also there's
+            # not much point to this file if we have no [Serializable] objects.
+            # Just spit out an empty file.
+            return CGIncludeGuard("WebIDLSerializable", CGGeneric(""))
+
+        # If we had a lot of serializable things, it might be worth it to use a
+        # PerfectHash here, or an array ordered by sctag value and binary
+        # search.  But setting those up would require knowing in this python
+        # code the values of the various SCTAG_DOM_*.  We could hardcode them
+        # here and add static asserts that the values are right, or switch to
+        # code-generating StructuredCloneTags.h or something.  But in practice,
+        # there's a pretty small number of serializable interfaces, and just
+        # doing a linear walk is fine.  It's not obviously worse than the
+        # if-cascade we used to have.  Let's just make sure we notice if we do
+        # end up with a lot of serializable things here.
+        #
+        # Also, in practice it looks like compilers compile this linear walk to
+        # an out-of-bounds check followed by a direct index into an array, by
+        # basically making a second copy of this array ordered by tag, with the
+        # holes filled in.  Again, worth checking whether this still happens if
+        # we have too many serializable things.
+        if len(names) > 20:
+            raise TypeError(
+                "We now have %s serializable interfaces.  "
+                "Double-check that the compiler is still "
+                "generating a jump table." % len(names)
+            )
+
+        entries = list()
+        # Make sure we have stable ordering.
+        for name in sorted(names):
+            # Strip off trailing newline to make our formatting look right.
+            entries.append(
+                fill(
+                    """
+                {
+                  /* mTag */ ${tag},
+                  /* mDeserialize */ ${name}_Binding::Deserialize
+                }
+                """,
+                    tag=StructuredCloneTag(name),
+                    name=name,
+                )[:-1]
+            )
+
+        declare = dedent(
+            """
+            WebIDLDeserializer LookupDeserializer(StructuredCloneTags aTag);
+            """
+        )
+        define = fill(
+            """
+            struct WebIDLSerializableEntry {
+              StructuredCloneTags mTag;
+              WebIDLDeserializer mDeserialize;
+            };
+
+            static const WebIDLSerializableEntry sEntries[] = {
+              $*{entries}
+            };
+
+            WebIDLDeserializer LookupDeserializer(StructuredCloneTags aTag) {
+              for (auto& entry : sEntries) {
+                if (entry.mTag == aTag) {
+                  return entry.mDeserialize;
+                }
+              }
+              return nullptr;
+            }
+            """,
+            entries=",\n".join(entries) + "\n",
+        )
+
+        code = CGGeneric(declare=declare, define=define)
+
+        # Wrap all of that in our namespaces.
+        curr = CGNamespace.build(["mozilla", "dom"], code)
+
+        curr = CGWrapper(curr, post="\n")
+
+        curr = CGHeaders(
+            [], [], [], [], declareIncludes, defineIncludes, "WebIDLSerializable", curr
+        )
+
+        # Add include guards.
+        curr = CGIncludeGuard("WebIDLSerializable", curr)
+
+        # Done.
+        return curr
+
+
+# Code generator for simple events
+class CGEventGetter(CGNativeMember):
+    def __init__(self, descriptor, attr):
+        ea = descriptor.getExtendedAttributes(attr, getter=True)
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            attr,
+            CGSpecializedGetter.makeNativeName(descriptor, attr),
+            (attr.type, []),
+            ea,
+            resultNotAddRefed=not attr.type.isSequence(),
+        )
+        self.body = self.getMethodBody()
+
+    def getArgs(self, returnType, argList):
+        if "needsErrorResult" in self.extendedAttrs:
+            raise TypeError("Event code generator does not support [Throws]!")
+        if "canOOM" in self.extendedAttrs:
+            raise TypeError("Event code generator does not support [CanOOM]!")
+        if not self.member.isAttr():
+            raise TypeError("Event code generator does not support methods")
+        if self.member.isStatic():
+            raise TypeError("Event code generators does not support static attributes")
+        return CGNativeMember.getArgs(self, returnType, argList)
+
+    def getMethodBody(self):
+        type = self.member.type
+        memberName = CGDictionary.makeMemberName(self.member.identifier.name)
+        if (
+            (type.isPrimitive() and type.tag() in builtinNames)
+            or type.isEnum()
+            or type.isPromise()
+            or type.isGeckoInterface()
+        ):
+            return "return " + memberName + ";\n"
+        if type.isJSString():
+            # https://bugzilla.mozilla.org/show_bug.cgi?id=1580167
+            raise TypeError("JSString not supported as member of a generated event")
+        if (
+            type.isDOMString()
+            or type.isByteString()
+            or type.isUSVString()
+            or type.isUTF8String()
+        ):
+            return "aRetVal = " + memberName + ";\n"
+        if type.isSpiderMonkeyInterface() or type.isObject():
+            return fill(
+                """
+                if (${memberName}) {
+                  JS::ExposeObjectToActiveJS(${memberName});
+                }
+                aRetVal.set(${memberName});
+                return;
+                """,
+                memberName=memberName,
+            )
+        if type.isAny():
+            return fill(
+                """
+                ${selfName}(aRetVal);
+                """,
+                selfName=self.name,
+            )
+        if type.isUnion():
+            return "aRetVal = " + memberName + ";\n"
+        if type.isSequence():
+            if type.nullable():
+                return (
+                    "if ("
+                    + memberName
+                    + ".IsNull()) { aRetVal.SetNull(); } else { aRetVal.SetValue("
+                    + memberName
+                    + ".Value().Clone()); }\n"
+                )
+            else:
+                return "aRetVal = " + memberName + ".Clone();\n"
+        raise TypeError("Event code generator does not support this type!")
+
+    def declare(self, cgClass):
+        if (
+            getattr(self.member, "originatingInterface", cgClass.descriptor.interface)
+            != cgClass.descriptor.interface
+        ):
+            return ""
+        return CGNativeMember.declare(self, cgClass)
+
+    def define(self, cgClass):
+        if (
+            getattr(self.member, "originatingInterface", cgClass.descriptor.interface)
+            != cgClass.descriptor.interface
+        ):
+            return ""
+        return CGNativeMember.define(self, cgClass)
+
+
+class CGEventSetter(CGNativeMember):
+    def __init__(self):
+        raise TypeError("Event code generator does not support setters!")
+
+
+class CGEventMethod(CGNativeMember):
+    def __init__(self, descriptor, method, signature, isConstructor, breakAfter=True):
+        self.isInit = False
+
+        CGNativeMember.__init__(
+            self,
+            descriptor,
+            method,
+            CGSpecializedMethod.makeNativeName(descriptor, method),
+            signature,
+            descriptor.getExtendedAttributes(method),
+            breakAfter=breakAfter,
+            variadicIsSequence=True,
+        )
+        self.originalArgs = list(self.args)
+
+        iface = descriptor.interface
+        allowed = isConstructor
+        if not allowed and iface.getExtendedAttribute("LegacyEventInit"):
+            # Allow it, only if it fits the initFooEvent profile exactly
+            # We could check the arg types but it's not worth the effort.
+            if (
+                method.identifier.name == "init" + iface.identifier.name
+                and signature[1][0].type.isDOMString()
+                and signature[1][1].type.isBoolean()
+                and signature[1][2].type.isBoolean()
+                and
+                # -3 on the left to ignore the type, bubbles, and cancelable parameters
+                # -1 on the right to ignore the .trusted property which bleeds through
+                # here because it is [Unforgeable].
+                len(signature[1]) - 3
+                == len([x for x in iface.members if x.isAttr()]) - 1
+            ):
+                allowed = True
+                self.isInit = True
+
+        if not allowed:
+            raise TypeError("Event code generator does not support methods!")
+
+    def getArgs(self, returnType, argList):
+        args = [self.getArg(arg) for arg in argList]
+        return args
+
+    def getArg(self, arg):
+        decl, ref = self.getArgType(
+            arg.type, arg.canHaveMissingValue(), "Variadic" if arg.variadic else False
+        )
+        if ref:
+            decl = CGWrapper(decl, pre="const ", post="&")
+
+        name = arg.identifier.name
+        name = "a" + name[0].upper() + name[1:]
+        return Argument(decl.define(), name)
+
+    def declare(self, cgClass):
+        if self.isInit:
+            constructorForNativeCaller = ""
+        else:
+            self.args = list(self.originalArgs)
+            self.args.insert(0, Argument("mozilla::dom::EventTarget*", "aOwner"))
+            constructorForNativeCaller = CGNativeMember.declare(self, cgClass)
+
+        self.args = list(self.originalArgs)
+        if needCx(None, self.arguments(), [], considerTypes=True, static=True):
+            self.args.insert(0, Argument("JSContext*", "aCx"))
+        if not self.isInit:
+            self.args.insert(0, Argument("const GlobalObject&", "aGlobal"))
+
+        return constructorForNativeCaller + CGNativeMember.declare(self, cgClass)
+
+    def defineInit(self, cgClass):
+        iface = self.descriptorProvider.interface
+        members = ""
+        while iface.identifier.name != "Event":
+            i = 3  # Skip the boilerplate args: type, bubble,s cancelable.
+            for m in iface.members:
+                if m.isAttr():
+                    # We need to initialize all the member variables that do
+                    # not come from Event.
+                    if (
+                        getattr(m, "originatingInterface", iface).identifier.name
+                        == "Event"
+                    ):
+                        continue
+                    name = CGDictionary.makeMemberName(m.identifier.name)
+                    members += "%s = %s;\n" % (name, self.args[i].name)
+                    i += 1
+            iface = iface.parent
+
+        self.body = fill(
+            """
+            InitEvent(${typeArg}, ${bubblesArg}, ${cancelableArg});
+            ${members}
+            """,
+            typeArg=self.args[0].name,
+            bubblesArg=self.args[1].name,
+            cancelableArg=self.args[2].name,
+            members=members,
+        )
+
+        return CGNativeMember.define(self, cgClass)
+
+    def define(self, cgClass):
+        self.args = list(self.originalArgs)
+        if self.isInit:
+            return self.defineInit(cgClass)
+        members = ""
+        holdJS = ""
+        iface = self.descriptorProvider.interface
+        while iface.identifier.name != "Event":
+            for m in self.descriptorProvider.getDescriptor(
+                iface.identifier.name
+            ).interface.members:
+                if m.isAttr():
+                    # We initialize all the other member variables in the
+                    # Constructor except those ones coming from the Event.
+                    if (
+                        getattr(
+                            m, "originatingInterface", cgClass.descriptor.interface
+                        ).identifier.name
+                        == "Event"
+                    ):
+                        continue
+                    name = CGDictionary.makeMemberName(m.identifier.name)
+                    if m.type.isSequence():
+                        # For sequences we may not be able to do a simple
+                        # assignment because the underlying types may not match.
+                        # For example, the argument can be a
+                        # Sequence<OwningNonNull<SomeInterface>> while our
+                        # member is an nsTArray<RefPtr<SomeInterface>>.  So
+                        # use AppendElements, which is actually a template on
+                        # the incoming type on nsTArray and does the right thing
+                        # for this case.
+                        target = name
+                        source = "%s.%s" % (self.args[1].name, name)
+                        sequenceCopy = "e->%s.AppendElements(%s);\n"
+                        if m.type.nullable():
+                            sequenceCopy = CGIfWrapper(
+                                CGGeneric(sequenceCopy), "!%s.IsNull()" % source
+                            ).define()
+                            target += ".SetValue()"
+                            source += ".Value()"
+                        members += sequenceCopy % (target, source)
+                    elif m.type.isSpiderMonkeyInterface():
+                        srcname = "%s.%s" % (self.args[1].name, name)
+                        if m.type.nullable():
+                            members += fill(
+                                """
+                                if (${srcname}.IsNull()) {
+                                  e->${varname} = nullptr;
+                                } else {
+                                  e->${varname} = ${srcname}.Value().Obj();
+                                }
+                                """,
+                                varname=name,
+                                srcname=srcname,
+                            )
+                        else:
+                            members += fill(
+                                """
+                                e->${varname}.set(${srcname}.Obj());
+                                """,
+                                varname=name,
+                                srcname=srcname,
+                            )
+                    else:
+                        members += "e->%s = %s.%s;\n" % (name, self.args[1].name, name)
+                    if (
+                        m.type.isAny()
+                        or m.type.isObject()
+                        or m.type.isSpiderMonkeyInterface()
+                    ):
+                        holdJS = "mozilla::HoldJSObjects(e.get());\n"
+            iface = iface.parent
+
+        self.body = fill(
+            """
+            RefPtr<${nativeType}> e = new ${nativeType}(aOwner);
+            bool trusted = e->Init(aOwner);
+            e->InitEvent(${eventType}, ${eventInit}.mBubbles, ${eventInit}.mCancelable);
+            $*{members}
+            e->SetTrusted(trusted);
+            e->SetComposed(${eventInit}.mComposed);
+            $*{holdJS}
+            return e.forget();
+            """,
+            nativeType=self.descriptorProvider.nativeType.split("::")[-1],
+            eventType=self.args[0].name,
+            eventInit=self.args[1].name,
+            members=members,
+            holdJS=holdJS,
+        )
+
+        self.args.insert(0, Argument("mozilla::dom::EventTarget*", "aOwner"))
+        constructorForNativeCaller = CGNativeMember.define(self, cgClass) + "\n"
+        self.args = list(self.originalArgs)
+        self.body = fill(
+            """
+            nsCOMPtr<mozilla::dom::EventTarget> owner = do_QueryInterface(aGlobal.GetAsSupports());
+            return Constructor(owner, ${arg0}, ${arg1});
+            """,
+            arg0=self.args[0].name,
+            arg1=self.args[1].name,
+        )
+        if needCx(None, self.arguments(), [], considerTypes=True, static=True):
+            self.args.insert(0, Argument("JSContext*", "aCx"))
+        self.args.insert(0, Argument("const GlobalObject&", "aGlobal"))
+        return constructorForNativeCaller + CGNativeMember.define(self, cgClass)
+
+
+class CGEventClass(CGBindingImplClass):
+    """
+    Codegen for the actual Event class implementation for this descriptor
+    """
+
+    def __init__(self, descriptor):
+        CGBindingImplClass.__init__(
+            self,
+            descriptor,
+            CGEventMethod,
+            CGEventGetter,
+            CGEventSetter,
+            False,
+            "WrapObjectInternal",
+        )
+        members = []
+        extraMethods = []
+        self.membersNeedingCC = []
+        self.membersNeedingTrace = []
+
+        for m in descriptor.interface.members:
+            if (
+                getattr(m, "originatingInterface", descriptor.interface)
+                != descriptor.interface
+            ):
+                continue
+
+            if m.isAttr():
+                if m.type.isAny():
+                    self.membersNeedingTrace.append(m)
+                    # Add a getter that doesn't need a JSContext.  Note that we
+                    # don't need to do this if our originating interface is not
+                    # the descriptor's interface, because in that case we
+                    # wouldn't generate the getter that _does_ need a JSContext
+                    # either.
+                    extraMethods.append(
+                        ClassMethod(
+                            CGSpecializedGetter.makeNativeName(descriptor, m),
+                            "void",
+                            [Argument("JS::MutableHandle<JS::Value>", "aRetVal")],
+                            const=True,
+                            body=fill(
+                                """
+                                JS::ExposeValueToActiveJS(${memberName});
+                                aRetVal.set(${memberName});
+                                """,
+                                memberName=CGDictionary.makeMemberName(
+                                    m.identifier.name
+                                ),
+                            ),
+                        )
+                    )
+                elif m.type.isObject() or m.type.isSpiderMonkeyInterface():
+                    self.membersNeedingTrace.append(m)
+                elif typeNeedsRooting(m.type):
+                    raise TypeError(
+                        "Need to implement tracing for event member of type %s" % m.type
+                    )
+                elif idlTypeNeedsCycleCollection(m.type):
+                    self.membersNeedingCC.append(m)
+
+                nativeType = self.getNativeTypeForIDLType(m.type).define()
+                members.append(
+                    ClassMember(
+                        CGDictionary.makeMemberName(m.identifier.name),
+                        nativeType,
+                        visibility="private",
+                        body="body",
+                    )
+                )
+
+        parent = self.descriptor.interface.parent
+        self.parentType = self.descriptor.getDescriptor(
+            parent.identifier.name
+        ).nativeType.split("::")[-1]
+        self.nativeType = self.descriptor.nativeType.split("::")[-1]
+
+        if self.needCC():
+            isupportsDecl = fill(
+                """
+                NS_DECL_ISUPPORTS_INHERITED
+                NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(${nativeType}, ${parentType})
+                """,
+                nativeType=self.nativeType,
+                parentType=self.parentType,
+            )
+        else:
+            isupportsDecl = fill(
+                """
+                NS_INLINE_DECL_REFCOUNTING_INHERITED(${nativeType}, ${parentType})
+                """,
+                nativeType=self.nativeType,
+                parentType=self.parentType,
+            )
+
+        baseDeclarations = fill(
+            """
+            public:
+              $*{isupportsDecl}
+
+            protected:
+              virtual ~${nativeType}();
+              explicit ${nativeType}(mozilla::dom::EventTarget* aOwner);
+
+            """,
+            isupportsDecl=isupportsDecl,
+            nativeType=self.nativeType,
+            parentType=self.parentType,
+        )
+
+        className = self.nativeType
+        asConcreteTypeMethod = ClassMethod(
+            "As%s" % className,
+            "%s*" % className,
+            [],
+            virtual=True,
+            body="return this;\n",
+            breakAfterReturnDecl=" ",
+            override=True,
+        )
+        extraMethods.append(asConcreteTypeMethod)
+
+        CGClass.__init__(
+            self,
+            className,
+            bases=[ClassBase(self.parentType)],
+            methods=extraMethods + self.methodDecls,
+            members=members,
+            extradeclarations=baseDeclarations,
+        )
+
+    def getWrapObjectBody(self):
+        return (
+            "return %s_Binding::Wrap(aCx, this, aGivenProto);\n" % self.descriptor.name
+        )
+
+    def needCC(self):
+        return len(self.membersNeedingCC) != 0 or len(self.membersNeedingTrace) != 0
+
+    def implTraverse(self):
+        retVal = ""
+        for m in self.membersNeedingCC:
+            retVal += (
+                "  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(%s)\n"
+                % CGDictionary.makeMemberName(m.identifier.name)
+            )
+        return retVal
+
+    def implUnlink(self):
+        retVal = ""
+        for m in self.membersNeedingCC:
+            retVal += (
+                "  NS_IMPL_CYCLE_COLLECTION_UNLINK(%s)\n"
+                % CGDictionary.makeMemberName(m.identifier.name)
+            )
+        for m in self.membersNeedingTrace:
+            name = CGDictionary.makeMemberName(m.identifier.name)
+            if m.type.isAny():
+                retVal += "  tmp->" + name + ".setUndefined();\n"
+            elif m.type.isObject() or m.type.isSpiderMonkeyInterface():
+                retVal += "  tmp->" + name + " = nullptr;\n"
+            else:
+                raise TypeError("Unknown traceable member type %s" % m.type)
+        return retVal
+
+    def implTrace(self):
+        retVal = ""
+        for m in self.membersNeedingTrace:
+            retVal += (
+                "  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(%s)\n"
+                % CGDictionary.makeMemberName(m.identifier.name)
+            )
+        return retVal
+
+    def define(self):
+        for m in self.membersNeedingTrace:
+            if not (
+                m.type.isAny() or m.type.isObject() or m.type.isSpiderMonkeyInterface()
+            ):
+                raise TypeError("Unknown traceable member type %s" % m.type)
+
+        if len(self.membersNeedingTrace) > 0:
+            dropJS = "mozilla::DropJSObjects(this);\n"
+        else:
+            dropJS = ""
+        # Just override CGClass and do our own thing
+        ctorParams = (
+            "aOwner, nullptr, nullptr" if self.parentType == "Event" else "aOwner"
+        )
+
+        if self.needCC():
+            classImpl = fill(
+                """
+
+                NS_IMPL_CYCLE_COLLECTION_CLASS(${nativeType})
+
+                NS_IMPL_ADDREF_INHERITED(${nativeType}, ${parentType})
+                NS_IMPL_RELEASE_INHERITED(${nativeType}, ${parentType})
+
+                NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(${nativeType}, ${parentType})
+                $*{traverse}
+                NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+                NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(${nativeType}, ${parentType})
+                $*{trace}
+                NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+                NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(${nativeType}, ${parentType})
+                $*{unlink}
+                NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+                NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(${nativeType})
+                NS_INTERFACE_MAP_END_INHERITING(${parentType})
+                """,
+                nativeType=self.nativeType,
+                parentType=self.parentType,
+                traverse=self.implTraverse(),
+                unlink=self.implUnlink(),
+                trace=self.implTrace(),
+            )
+        else:
+            classImpl = ""
+
+        classImpl += fill(
+            """
+
+            ${nativeType}::${nativeType}(mozilla::dom::EventTarget* aOwner)
+              : ${parentType}(${ctorParams})
+            {
+            }
+
+            ${nativeType}::~${nativeType}()
+            {
+              $*{dropJS}
+            }
+
+            """,
+            nativeType=self.nativeType,
+            ctorParams=ctorParams,
+            parentType=self.parentType,
+            dropJS=dropJS,
+        )
+
+        return classImpl + CGBindingImplClass.define(self)
+
+    def getNativeTypeForIDLType(self, type):
+        if type.isPrimitive() and type.tag() in builtinNames:
+            nativeType = CGGeneric(builtinNames[type.tag()])
+            if type.nullable():
+                nativeType = CGTemplatedType("Nullable", nativeType)
+        elif type.isEnum():
+            nativeType = CGGeneric(type.unroll().inner.identifier.name)
+            if type.nullable():
+                nativeType = CGTemplatedType("Nullable", nativeType)
+        elif type.isJSString():
+            nativeType = CGGeneric("JS::Heap<JSString*>")
+        elif type.isDOMString() or type.isUSVString():
+            nativeType = CGGeneric("nsString")
+        elif type.isByteString() or type.isUTF8String():
+            nativeType = CGGeneric("nsCString")
+        elif type.isPromise():
+            nativeType = CGGeneric("RefPtr<Promise>")
+        elif type.isGeckoInterface():
+            iface = type.unroll().inner
+            nativeType = self.descriptor.getDescriptor(iface.identifier.name).nativeType
+            # Now trim off unnecessary namespaces
+            nativeType = nativeType.split("::")
+            if nativeType[0] == "mozilla":
+                nativeType.pop(0)
+                if nativeType[0] == "dom":
+                    nativeType.pop(0)
+            nativeType = CGWrapper(
+                CGGeneric("::".join(nativeType)), pre="RefPtr<", post=">"
+            )
+        elif type.isAny():
+            nativeType = CGGeneric("JS::Heap<JS::Value>")
+        elif type.isObject() or type.isSpiderMonkeyInterface():
+            nativeType = CGGeneric("JS::Heap<JSObject*>")
+        elif type.isUnion():
+            nativeType = CGGeneric(CGUnionStruct.unionTypeDecl(type, True))
+        elif type.isSequence():
+            if type.nullable():
+                innerType = type.inner.inner
+            else:
+                innerType = type.inner
+            if (
+                not innerType.isPrimitive()
+                and not innerType.isEnum()
+                and not innerType.isDOMString()
+                and not innerType.isByteString()
+                and not innerType.isUTF8String()
+                and not innerType.isPromise()
+                and not innerType.isGeckoInterface()
+            ):
+                raise TypeError(
+                    "Don't know how to properly manage GC/CC for "
+                    "event member of type %s" % type
+                )
+            nativeType = CGTemplatedType(
+                "nsTArray", self.getNativeTypeForIDLType(innerType)
+            )
+            if type.nullable():
+                nativeType = CGTemplatedType("Nullable", nativeType)
+        else:
+            raise TypeError("Don't know how to declare event member of type %s" % type)
+        return nativeType
+
+
+class CGEventRoot(CGThing):
+    def __init__(self, config, interfaceName):
+        descriptor = config.getDescriptor(interfaceName)
+
+        self.root = CGWrapper(CGEventClass(descriptor), pre="\n", post="\n")
+
+        self.root = CGNamespace.build(["mozilla", "dom"], self.root)
+
+        self.root = CGList(
+            [CGClassForwardDeclare("JSContext", isStruct=True), self.root]
+        )
+
+        parent = descriptor.interface.parent.identifier.name
+
+        # Throw in our #includes
+        self.root = CGHeaders(
+            [descriptor],
+            [],
+            [],
+            [],
+            [
+                config.getDescriptor(parent).headerFile,
+                "mozilla/Attributes.h",
+                "mozilla/dom/%sBinding.h" % interfaceName,
+                "mozilla/dom/BindingUtils.h",
+            ],
+            [
+                "%s.h" % interfaceName,
+                "js/GCAPI.h",
+                "mozilla/HoldDropJSObjects.h",
+                "mozilla/dom/Nullable.h",
+            ],
+            "",
+            self.root,
+            config,
+        )
+
+        # And now some include guards
+        self.root = CGIncludeGuard(interfaceName, self.root)
+
+        self.root = CGWrapper(
+            self.root,
+            pre=(
+                AUTOGENERATED_WITH_SOURCE_WARNING_COMMENT
+                % os.path.basename(descriptor.interface.filename())
+            ),
+        )
+
+        self.root = CGWrapper(
+            self.root,
+            pre=dedent(
+                """
+            /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+            /* vim:set ts=2 sw=2 sts=2 et cindent: */
+            /* This Source Code Form is subject to the terms of the Mozilla Public
+             * License, v. 2.0. If a copy of the MPL was not distributed with this
+             * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+            """
+            ),
+        )
+
+    def declare(self):
+        return self.root.declare()
+
+    def define(self):
+        return self.root.define()