Rooted::new_unrooted can easily cause undefined behaviour

[simonwuelker]

Rooted::new_unrooted will zero-initialize the contained value (RootedGuard::new will usually be used to init it later)

mozjs/mozjs-sys/src/jsimpls.rs

Lines 431 to 440 in 0081fc4

	pub fn new_unrooted() -> JS::Rooted<T> {
	JS::Rooted {
	vtable: T::VTABLE,
	base: RootedBase {
	stack: ptr::null_mut(),
	prev: ptr::null_mut(),
	},
	ptr: unsafe { std::mem::zeroed() },
	}
	}

This causes immediate undefined behaviour (or a panic in debug mode) if T is a type that must not be zero, like std::ptr::NonNull<T>.

[simonwuelker]

I think we should wrap the value in std::mem::MaybeUninit.

[simonwuelker]

This is currently preventing me from implementing the :slotted selector and laying out slot elements in servo