This is the third post in a three-post series. In the first post we developed a stack-safe, ergonomic, and concise method for working with recursive data structures (using a simple expression language as an example). In the second post we made it fully generic, providing a set of generic tools for expanding and collapsing any recursive data structure in Rust.
In this post we will see how to combine these two things - expanding a structure and collapsing it at the same time, performing both operations in a single pass. In the process, we will gain the ability to write arbitrary recursive functions over traditional boxed-pointer recursive structures (instead of the novel RecursiveTree type introduced in my previous post) while retaining stack safety.
