For example, the copy on write (COW) mechanism of unix where memory pages are mapped to the same location until a forked process writes to one, in which case the virtual memory manager makes a mutable copy.
There's also Redux and Closure's immutable state tree that makes copies under a similar mechanism to COW but through code, since they run at a level of abstraction above C++ or Rust.
My feeling is that these techniques run within a few percent of the speed of hand-optimization. But in the real world, I've seen very little human code remain optimized over the long term. Someone invariably comes along who doesn't understand the principles behind the code and inadvertently does a manual copy somewhere or breaks the O() speed of the algorithm by using the wrong abstractions. Meanwhile immutable code using mechanisms like COW avoids these pitfalls because the code is small and obvious.
I feel that the things that Rust is trying to do were solved long ago under FP, so I don't think it's the language for me. That's also why I moved away from C#, Java, C++, etc. Better languages might be Elixer or Clojure/ClojureScript, although they still have ugly syntax from a mainstream perspective compared to say Javascript or Python. I love that Rust exists as a formal spec of a mature imperative programming language. I think it's still useful in kernels and the embedded space. But I'm concerned that it's borrowing ideas like async/await that trade determinism for performance.
