Enum VisitMutAction 

enum VisitMutAction<T> {
    Enter(*mut T),
    Leave(*mut T),
}

Frames for mutable post-traversals, will be kept in a stack.

Notice that we use mutable pointers, because mutable post-traversal is unsafe in rust.

The core argument for correctness mirrors the tree-borrow correctness argument Rust’s borrow checker uses for the ordinary function call stack. Loosely:

• We split a mutable parent node into its children, and put them on the stack.
• We keep a mutable pointer to the parent node, but won’t touch it until we’re done with all children.
  • In the function call stack, this mutable pointer is the stack frame, safely inaccessible.
  • In our unsafe action stack, this mutable pointer is below all of the Enter actions, and we promise not to touch it until they complete.
• When we have processed all children, we can reassemble access to the parent from its parts.
  • In the function call stack, we do this on return from recursive calls.
  • In our unsafe action stack, we do this after popping all children.

Variants

Enter(*mut T)

Put on the stack when entering a node.

Causes us to push children.

Leave(*mut T)

Put on the stack when leaving a node, all children visited.

Causes us to do the post-traversal visit of the parent.