Can't use `&mut self` in tree nodes

[ckaran]

See the comments at #54 (comment)_ but the gist is that as @RustyYato pointed out you can't use &mut self as I've been using it. Fix this up.

[RustyYato]

I opened a related issue on rust-lang/unsafe-code-guidelines#272, because it looks like it won't be possible to provide a safe API without allocating, which is unfortunate.

[ckaran]

Yup, just saw it pop up on my feed. I wish I could help in the decisions on this, but I don't (yet) know rust well enough to make that happen. I'll keep reading as new posts pop up in my feed though!

[RustyYato]

The conclusion to the discussion was that nodes that hold intrusive collection nodes should be guarded to prevent &mut _ access. One way is to do this is with a stack pin (Pin<&T>), something similar to pin_utils::pin_mut or a wrapper around Pin<Rc<T>>

[ckaran]

OK, I think that Pin<Rc<T>> might be the easiest to follow. I think I'm going to have to make both thread-safe and thread-unsafe versions, so there will also need to be a Pin<Arc<Mutex<T>>>. Right?

I'm going to admit that I'm learning as I go for some of this, the aliasing rules get weird for me...

[RustyYato]

I would postpone the thread-safe version. Rust aliasing rules mixed with thread safety is a nightmare. I think a Mutex would be easiest, but it basically makes a &mut _, so it might be unusable. I think ReentrantMutex (from parking_lot) will provide the guarantees that we want. But users of Link and DoubleLink can use those without issue.

For the tutorial, I would stick to Pin<Rc<Node>> if you want to show how to create a safe API, or just show the unsafe API.

[ckaran]

OK, I'll stick with Pin<Rc<Node>>. Thank you!