directed-visit

Implement the visitor pattern with interchangable implementations for both the visit algorithm and node object traversal. It is also possible to dynamically create temporary 'virtual' nodes during traversal.

An object traversal is comprised of 3 parts:
The input to traverse, which can start at any node type. This can be an external type too, there are no required derives or trait impls.
The director navigates between node objects. It must implement Direct<_, N> for each node type N in the object graph, determining the sub-nodes.
The visitor performs the desired algorithm, implementing Visit<N> for each node type N in the object graph.

fn my_visit(input: &MyTree) -> usize {
    let mut my_director = MyDirector::new();
    let mut my_visitor = MyVisitor::new();
    directed_visit::visit(
        &mut my_director,
        &mut my_visitor,
        input,
    );

    my_visitor.result_value()
}

DirectMut and VisitMut traits are also provided. Note that if your DirectMut implementation dynamically creates temporary nodes, you must convert those temporary nodes back to their original form after visiting the node, or the changes will be lost.

syn

The crate includes a replacement for syn::visit::Visit/syn::visit_mut::VisitMut if the syn feature is enabled. Implement directed_visit::syn::visit::Full as you would syn::visit::Visit.
For your director, directed_visit::syn::direct::FullDefault traverses as syn::visit does, or you can customize the behavior by implementing directed_visit::syn::direct::Full.
In addition to the existing syn AST, two nodes have been added to the tree to represent when generic parameters become in and out of scope.
The derive feature subset of full is not yet supported.

Limitations

Because the director can dynamically create new nodes to visit, the visitor cannot hold references to the node graph (i.e. there is no single 'ast lifetime for all nodes).