Unsoundness of QObjectPinned borrowing

[farmaazon]

When I dived into the crate's code I found the definition of QObjectPinned::borrow in qmetaobject.rs, which returns just the dereferenced pointer:

    /// Borrow the object
    // FIXME: there are too many cases for which we want reentrance after borrowing
    //pub fn borrow(&self) -> std::cell::Ref<T> { self.0.borrow() }
    #[cfg_attr(feature = "cargo-clippy", allow(clippy::should_implement_trait))]
    pub fn borrow(&self) -> &T {
        unsafe { &*self.0.as_ptr() }
    }

This is a part of the public API, so any user can use it any way: they are able to make and keep one mutable and many immutable reference to same object at the same time, without making any block unsafe, so it perfectly fits the unsoundness definition.

I think the better solution would be having the "commented" version of borrow method (which actually borrows the RefCell content), and as_ptr function to use in places when it is needed (according to the FIXME) - but such places should mark it with proper unsafe section.

[ogoffart]

Thanks for diving in the code.
You are right that this is one of the probably many unsoundness problems that should be addressed.
I wasn't yet really familiar wit rust at the time i wrote this.

The whole API should actually be revised now that Pin is stable.

Also see #11