"Named constructor without 'new'" WPT failures

[pshaughn]

Servo's implementations of named constructors, for Audio and Image (and Option after #25393), aren't throwing an exception when they're called like normal functions; for comparison try typing just Image() without 'new' into the Firefox console. https://html.spec.whatwg.org/#dom-image specifies "When invoked, the constructor must perform the following steps"; what it seems to actually mean here is when invoked /as a constructor/, via new, not just with a regular function call. Within Javascript itself this distinction is detectable via the special expression "new.target", as described in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new.target ; I don't know what the mozjs equivalent is.

[pshaughn]

The test is here:

servo/tests/wpt/web-platform-tests/resources/idlharness.js

Lines 1877 to 1890 in 8002c6b

	subsetTestByKey(this.name, test, function()
	{
	// This function tests WebIDL as of 2019-01-14.
	// "1. Let steps be the following steps:
	// " 1. If NewTarget is undefined, then throw a TypeError."
	var name = constructor.rhs.value;
	var args = constructor.arguments.map(function(arg) {
	return create_suitable_object(arg.idlType);
	});
	assert_throws(new TypeError(), function() {
	self[name](...args);
	}.bind(this));
	}.bind(this), this.name + " interface: named constructor without 'new'");

https://heycam.github.io/webidl/#named-constructors 1.1 seems to be the verbiage referred to; the substeps of 1 here are the steps for the constructor function itself.

[jdm]

We're missing the equivalent code for https://searchfox.org/mozilla-central/rev/c79fc6e32a5b561205a71fcfef9112d34b4037ae/dom/bindings/Codegen.py#1875-1895 in

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

Lines 5741 to 5746 in 8002c6b

	else:
	name = self.constructor.identifier.name
	nativeName = MakeNativeName(self.descriptor.binaryNameFor(name))
	constructorCall = CGMethodCall(["&global"], nativeName, True,
	self.descriptor, self.constructor)
	return CGList([preamble, constructorCall])

.

[jdm]

That's https://doc.servo.org/mozjs/jsapi/struct.CallArgs.html#method.constructing_ from the args variable that's already present in the generated constructor code.

[pshaughn]

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

Lines 5678 to 5682 in 8002c6b

	rooted!(in(*cx) let new_target = UnwrapObjectDynamic(args.new_target().to_object(), *cx, 1));
	if new_target.is_null() {
	throw_dom_exception(cx, global.upcast::<GlobalScope>(), Error::Type("new.target is null".to_owned()));
	return false;
	}

looks like it should already be making the required check.

[jdm]

That code only runs in HTML element constructors, though:

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

Line 5669 in 8002c6b

if self.constructor.isHTMLConstructor():

. That will be more clear when #25395 is fixed.