Documentation for Heap::handle is unclear

[jdm]

https://github.com/servo/mozjs/blob/d6c3bd9a9a86a53c627355241b015fa7768ed688/mozjs-sys/src/jsgc.rs#L333-L348

I no longer remember when "if you know what you're doing applies", but we have a lot of usages in Servo which makes me nervous.

[sagudev]

IIUC this is only safe when done on RootedGuard<Heap<T>> (or similar, something that ensures that heap is rooted), so we could just do

impl RootedGuard<Heap<T>> {
pub fn handle(&self) -> JS::Handle<T> { 
         JS::Handle::from_marked_location(self.ptr.get() as *const _) 
     } 
}

[redfire75369]

From my understanding, it's also safe if you have any Heap<T>, as long as it's traced, eg via a custom trace op or a global tracer.