Is it really safe for QObjectExt::set_parent to accept an immutable reference?

[jnbooth]

Currently, QObjectExt::set_parent accepts a parent: &QObject argument, which it then converts to a mutable pointer:

cxx-qt/crates/cxx-qt-lib/src/core/qobject.rs

Lines 94 to 99 in 511ec27

	fn set_parent(self: Pin<&mut Self>, parent: &Self) {
	unsafe {
	cast_pin(self)
	.set_parent(cast(parent) as *const QObjectExternal as *mut QObjectExternal);
	}
	}

Is this really safe? QObject::setParent() requires a mutable pointer on Qt's side, and its implementation mutates the parent by appending the object to its list of children. It also triggers the ChildAdded event on the parent, which may perform arbitrary code in its event handler.

I would have expected something more like this:

fn set_parent<T>(self: Pin<&mut Self>, parent: Option<Pin<&mut T>>)
where
    T: Upcast<QObject>,
{
    unsafe {
        let parent = match parent {
            Some(parent) => ptr::from_mut(parent.upcast_pin().get_unchecked_mut()).cast(),
            None => ptr::null_mut(),
        };
        cast_pin(self).set_parent(parent);
    }
}

[LeonMatthesKDAB]

Hi @jnbooth , I think you're right, and I like your suggested API.

@BenFordTytherington or was there a reason for the casting from &Self to *mut QObjectExternal?