Adapton in Rust

A general-purpose Incremental Computation (IC) library for Rust.
available on crates.io

Based on:

Library Components:

The library exposes a small core interface.
See the Adapton trait.
The library uses rust macros to provide syntactic sugar.
See macros.rs.
The library implements this interface with an imperative data structure, and without garbage collection.
See engine.rs.
The library provides generic incremental data structures and algorithms.
See also: collection_traits.rs: Generic trees and lists.
And: collection_algo.rs: Simple algorithms over generic trees and lists.
The library provides interfaces to script interactions using generic DSLs for editing and querying
See also: collection_edit.rs:
Next:
- tries that represent sets, maps,
- generic fixed-point loop
- graphs, graph exploration algorithms (e.g., search)

Pure Function Caching:
Incremental computation via function caching
Bill Pugh and Tim Teitelbaum.
POPL 1989.
- hash-cons'd, purely-functional data structures
- memoized function calls (to pure computations)
Structural Adapton:
Adapton: Composable, Demand-Driven Incremental Computation.
Matthew A. Hammer, Yit Phang Khoo, Michael Hicks and Jeffrey S. Foster.
PLDI 2014.
- changeable input cells
- bidirectional DCG structure
- dirtying traversal; repair traversal.
Nominal Adapton:
Incremental Computation with Names
Matthew A. Hammer, Joshua Dunfield, Kyle Headley, Nicholas Labich, Jeffrey S. Foster, Michael Hicks, David Van Horn.
OOPSLA 2015.
- first-class names
- nominal memoization

Benchmarking based on tests:
- report time statistics
- report memory statistics

In engine.rs I wrote Producer::copy and the ShapeShifter trait. Both avoid returning a Self.
Is there a better way?
See also: 0255-object-safety
Do I need really need Rc<Box<Fn (_) -> _>> instead of Rc<Fn (_) -> _>?
Why?
Done:
- trait-objects-with-associated-types