[kibwen]

Ownership and the borrow checker two distinct things; putting these concepts together is the premier novelty of Rust. "Ownership" is this: an analysis pass that enforces single-ownership of values (call it "affine types" if you want to be fancy, but it's an extremely simple analysis), along with a mechanism to allow types to opt-out of single-ownership and allow multiple ownership/implicit copying (what Rust calls the `Copy` trait). That's all it is, and it automatically gives you Rust's trick of "automatic static memory management". It's much simpler than a borrow checker, which would also require a notion of generics and subtyping, to say nothing of lifetimes (or a control flow graph, which is what you want if you want a good borrow checker). Such a system of ownership without a borrow checker could even be memory-safe, if your language doesn't allow unmanaged pointers (though it wouldn't be as efficient as Rust, and would involve more copying, and makes for slightly more annoying APIs).

[bbkane]

Would this simple system efficiently allow for larger data structures to be referenced instead of copied?

[kibwen]

References are the purview of the borrow checker, naturally, but (assuming that you care about memory safety) you might be able to get away with a system that doesn't let you store arbitrary references, but conceivably you could have some kind of simple-ish scope-based analysis that might allow the compiler to transparently elide copies when passing owned values to functions deeper in the call stack. A mechanism to reify the notion of strictly-scoped values in the language (like Python's `with` statement) could probably go quite far here, allowing your compiler to know that a piece of data is anchored to a given scope which strictly outlives some child scopes, allowing a reference to that data to remain valid for those child scopes. You'd still have to have Rust's notion of aliasing XOR mutation, but if references are first-class values this might be tractable, because you get much of this for free from an ownership system (mutable references are owned, immutable references are copyable), although Rust has several other ways that it bends over backwards to make this work nicely (e.g. compiler-inserted reborrowing). If you wanted to avoid this complexity you could start by just sticking to only having immutable references that allow copying the inner data if you want to mutate it.

[bbkane]

Sounds interesting - I'm not entirely convinced it would work, but I'm certainly less of an expert than you on this :D