[jstimpfle]

I'm not worried about expensive copies. I'm worried about being able to understand my systems code. The solution isn't adding more abstractions (like move semantics on top). I don't want to move anything. I want to be clear about taking a reference or making an actual copy, these are deeply, semantically, different. This difference is important for single threaded code but also for concurrency -- not only with mutable data types but also with immutable ones.

Performance is mostly a consequence of clear and direct code. You mostly don't achieve performance by saving individual copies, but by being in control of the code and architecture.

[ibotty]

I don't think your run often into these things, because of Rust's ownership enforcement. But I might be misunderstanding you, because it's all pretty abstract and I might not have the whole context.