ch15-06: None is for 1ˢᵗ time

[deep-outcome]

Listing 15-29 has only 1 println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());. Thus None option shows in output just once by end — leaf parent = None — not “again”.

leaf strong = 1, weak = 0
branch strong = 1, weak = 1
leaf strong = 2, weak = 0
leaf parent = None
leaf strong = 1, weak = 0

[deep-outcome]

Also I noticed, in spite of obvious intent of concise sentencing, some maybe confusing formatting.

For istance

We use the borrow_mut method on the RefCell<Weak<Node>> in the parent field of leaf, and then we use the
Rc::downgrade function to create a Weak<Node> reference to branch from the Rc<Node> in branch.

Since code marked terms are recognized mainly as something from code (variable, keyword, type name, …) this sentence could be interpreted as if

reference to branch from the Rc<Node> in branch.

is not meant as reference to branch somewhere in heap but to branch residing at stack — that which is visually available in code sample.

Anoter one is

We know it can’t contain an Rc<T>, because that would create a reference cycle with leaf.parent pointing to branch and
branch.children pointing to leaf, which would cause their strong_count values to never be 0.

that could be seen in same manner as if branch and leaf denotes exact points in stack — not some places at heap.

While Rc<T> refers only to heap, it seems this okay but still I wonder if this is really intentional or if misuse is overseen due following concise writing. Once again I mean that

reference to branch from the Rc<Node> in branch.

uses 1ˢᵗ time branch to refer to heap allocated data and in turn 2ⁿᵈ time branch refers to variable at stack.

Again, I find this fine. Just wondering if it is intentional. I would fix it immediately if were sure about it. There is quite lot of these.

[chriskrycho]

As regards “None again”: we do actually show leaf parent = None, just a fair bit earlier in the section. 😅 As regards branch here, that’s also correct: it is intending to refer to the branch shown in the code. I think perhaps you were experiencing some confusion about the example code, and particularly about the heap/stack dynamics: every time branch appears here, it is an Rc, which means the data within it is on the heap, never on the stack. (I trust you probably got that sorted out long since, it having been a few years now!)

[deep-outcome]

just a fair bit earlier in the section
I see. The sense is carried on from there.

Regard highlighting. I don't see so much of it as of that time but I still recognize my intention.

There's variable on stack of size of pointer, e.g. RC in this case. This points to heap where the size is of at least of T. These 2 addresses are different but text threats them seemingly alike. My lector skills are not good.

So, no issue after all.