Maybe unsound in CustomAutoRooterGuard::new

[lwz23]

Hello, thank you for your contribution in this project, I am scanning the unsoundness problem in rust project. I notice the following code:

pub struct CustomAutoRooterGuard<'a, T: 'a + CustomTrace> {
    rooter: &'a mut CustomAutoRooter<T>,
}

impl<'a, T: 'a + CustomTrace> CustomAutoRooterGuard<'a, T> {
    pub fn new(cx: *mut JSContext, rooter: &'a mut CustomAutoRooter<T>) -> Self {
        unsafe {
            rooter.add_to_root_stack(cx);
        }
        CustomAutoRooterGuard { rooter }
    }
...........................

Considering that pub mod gc, and new is also a pub function. I assume that users can directly call this function. This potential situation could result in rooter.add_to_root_stack(cx) operating on a null pointer, and directly dereferencing it might trigger undefined behavior (UB). For safety reasons, I felt it necessary to report this issue. If you have performed checks elsewhere that ensure this is safe, please don’t take offense at my raising this issue. I suggest Several possible fixes:

1. If there is no external usage for CustomAutoRooterGuard or new, they should not marked as pub, at least its new should not marked as pub
2. new method should add additional check for null pointer.
3. mark new method as unsafe and proper doc to let users know that they should provide valid Pointers.

[lwz23]

same to https://github.com/servo/mozjs/blob/e299b24d2d48e451a47994d7ad1283ea56ed653d/mozjs/src/gc/root.rs#L22

[jdm]

Agree: any method that accepts a raw pointer argument should be marked unsafe.