Add raw pointer interface

[ravern]

Closes #62. Aims to implement these methods:

fn into_raw(gc: Gc<T>) -> *const T { ... }
unsafe fn from_raw(ptr: *const T) -> Gc<T> { ... }

which are analogous to Rc::into_raw, Rc::from_raw.

Mostly just following the code from alloc/rc.rs.

[ravern]

Currently, I am unsure of how to implement Gc::from_raw.

Rc is defined as the following:

pub struct Rc<T: ?Sized> {
    ptr: NonNull<RcBox<T>>,
    phantom: PhantomData<T>,
}

Gc is defined as the following:

pub struct Gc<T: Trace + ?Sized + 'static> {
    ptr_root: Cell<NonNull<GcBox<T>>>,
    marker: PhantomData<Rc<T>>,
}

There is an additional Cell wrapping the box in Gc, does this affect the memory alignment such that the code in Rc::from_raw won't work?

Also, when attempting to copy over the code (with the appropriate changes) into Gc, I ran into the error where layout.padding_needed_for(align) is a nightly-only feature. Not sure how Rc uses this in stable.

[mystor]

There is an additional Cell wrapping the box in Gc, does this affect the memory alignment such that the code in Rc::from_raw won't work?

Nope, Cell shouldn't impact the alignment :-)

Also, when attempting to copy over the code (with the appropriate changes) into Gc, I ran into the error where layout.padding_needed_for(align) is a nightly-only feature. Not sure how Rc uses this in stable.

The standard library can use nightly-only features in stable code due to being part of the distribution. There are actually quite a few features which the stdlib uses which aren't available to the wider ecosystem.

The reason for that logic is to determine how to offset the pointer back from a T* to get a GcBox<T>*. In our case we can emulate padding_needed_for pretty easily.

fn padding_needed_for_gc(align: usize) -> usize {
    assert!(align.count_ones() == 1, "align must be a power-of-two!");

    // Determine how much our header is misaligned for `align`.
    // `align - 1` is a mask of all bits less than the alignment,
    // as `align` is a power-of-two.
    let misaligned = mem::size_of::<GcBox<()>>() & (align - 1);
    if misaligned > 0 { align - misaligned } else { 0 }
}

[ravern]

Oops, made a careless mistake with the doctest there 😅(821c1e2).

I have implemented Gc::into_raw and Gc::from_raw along with some documentation, mostly following std::rc::Rc.

[mystor]

I don't know if a double-free would necessarily occur. The more likely problem would be a use-after-free.

[mystor]

Given that this is a Gc<T>, that is not true :-). It won't be freed until after the collector is run. It should instead be said that the memory may be freed any time after x went out of scope above, so x_ptr may now be dangling.

[ravern]

Have corrected the documentation.

[mystor]

Thanks!

[andersk]

This Gc needs to be set as a root (#122).